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Re: [Mpi3-rma] Alternative RMA discussion
by Torsten Hoefler 24 Nov '09

24 Nov '09

Hi Jeff, > Is "The outcome of conﬂicting accesses to the same memory locations is > undefined" necessary because of the need to support cache incoherent > architectures? Also, but "conflicting" means that there is at least a single write involved. Read-only access never causes conflicts. Also remember, that even well-accepted cache-coherent CPU memory models don't have very strong consistency semantics for conflicting accesses (e.g., x86 writes are not atomic when the written data crosses a page or cache-line boundary -- ugly but true). > Most application codes that use GA/ARMCI require multiple gets > (read-only) and multiple accumulates within a single epoch. Heck, > merely allowing unlimited read-only access would solve most of my > problems. Yes, RO access will be fine, accumulates have stronger semantics (atomicity; also in the current spec). However, don't expect that you can read the result of an accumulate in the same epoch without strong memory semantics in the underlying implementation (cf. proposed MPI_RMA_query()). Get_accumulate could be used to emulate this on weaker architectures though ;). > Is it possible for the standard to allow processes to expose read-only > windows which cannot be modified within a epoch, but for which > unlimited local and remote gets are permitted? It is not possible to mark a window read-only as a protection mechanism. However, it is possible to issue unlimited local/remote gets in an epoch. The completion of those depends on the machine's memory model and the used synchronization model. Best, Torsten -- bash$ :(){ :|:&};: --------------------- http://www.unixer.de/ ----- Torsten Hoefler | Postdoctoral Fellow Open Systems Lab | Indiana University 150 S. Woodlawn Ave. | Bloomington, IN, 474045, USA Lindley Hall Room 135 | +01 (812) 856-0501

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Re: [Mpi3-rma] Alternative RMA discussion
by Jeff Hammond 24 Nov '09

24 Nov '09

Is "The outcome of conﬂicting accesses to the same memory locations is undefined" necessary because of the need to support cache incoherent architectures? Most application codes that use GA/ARMCI require multiple gets (read-only) and multiple accumulates within a single epoch. Heck, merely allowing unlimited read-only access would solve most of my problems. Is it possible for the standard to allow processes to expose read-only windows which cannot be modified within a epoch, but for which unlimited local and remote gets are permitted? Jeff On Sun, Nov 22, 2009 at 5:33 PM, Torsten Hoefler <htor(a)cs.indiana.edu> wrote: > Hello all, > > I think that the current RMA spec is not all that bad and that it's > possible to extend it slightly in order to meet our requirements and > maintain MPI's orthogonality and look&feel. I this sense, I follow > Jesper's and Hubert's proposals of small additions to the current spec > instead of a complete revamp. However, in any case, we need a much > better user-documentation because while I'm convinced that the standard > captures RMA semantics very well, I think that the complexity of the > topic is hard to grasp for an average user (who often thinks that > everything should be coherent etc.). That's one difference to ARMCI, > which just doesn't look as complex because it handles only a fraction of > the cases/platforms that MPI supports. > > The main issues that I address in the draft are: > > 1) we need local (point-to-point) windows > > 2) we want to perform all RMA ops on arbitrary memory > > 3) simultaneous accesses to the same memory should not be illegal but > rather undefined in their outcome (this is subtle but important) > > 4) we want test&set, test&accumulate, and compare&swap (and friends) > > 5) we want user-defined RMA operations > > 6) we want to take advantage of architectures that offer a stronger > memory models and progression (coherent&consistent) > > All those issues are orthogonal and can be discussed separately. I > propose changes in a draft chapter at > http://www.unixer.de/sec/one-side-2.pdf . This is of course the first > version and probably inconsistent, however, I think it solves all > problems discussed above. The semantics of the color-coding are: > > - green: removed text from MPI-2.2 > - red: added text > - blue: comments/discussion (not part of the chapter) > > The issues mentioned above are addressed on the following pages: > > 1) 4-5 > 2) 26 > 3) 3,6,33-34 > 4) 14-15 > 5) 13-15 > 6) 37-38 > > Right now, all proposed changes are source-compatible to MPI-2.2 (at a > small price of elegance though). > > I am sure there were reasons for all the restrictions on window/memory > access in MPI-2.0. However, I think that accepting undefined outcomes > should allow for efficient implementations. Error checking should be > left to tools on top of MPI (cf. other memory consistency models (who > knows omp_flush? ;) and threading). > > >From a performance-perspective, there are two open things to discuss: > > 1) function arguments get expensive when they spill on the stack > - RMA ops have horribly long argument lists > - the number of args could be reduced if we assume symmetric > count/datatype on both ends or if we send raw (MPI_BYTE) data (which > I think it not preferable but might be unavoidable to match SHMEM or > ARMCI performance) > 2) supporting full MPI semantics comes at some cost, however, one > could special-case for performance reasons (e.g., make contiguous > MPI_BYTE transfers a special case). This costs one or two branches in > each of the RMA calls (imho not too bad). We could even endorse this > parameter combination as some kind of fast-mode in the standard. > > >From a scalability standpoint, we might consider the following two > additions: > > 1) collective OP registration > - ops can be identified by numbers or memory addresses (function > pointers). However, if each process allocates them separately and > sends them around, \Omega(P) memory could be required in the worst > case. Collective allocation would reduce this to O(1) > 2) collective window/memory allocation > - potential \Omega(P) memory to store offsets; much discussed before, > the solution is simple: allocate memory collectively (gives the > library the chance to try to find "good addresses" (e.g., same local > addresses or aligned/strided global addresses). > > I prepared a quick and incomplete implementation of the local window > proposal (which uses ARMCI for RMA accesses and falls back to MPI for > collective windows). The code is at http://www.unixer.de/sec/rma.cpp . > > Please let me know what you think. I would not mind to drop this > completely if we decide that it's not a viable way to go (I only > invested two days so far). > > Sorry for the long mail & All the Best, > Torsten > > -- > bash$ :(){ :|:&};: --------------------- http://www.unixer.de/ ----- > Torsten Hoefler | Postdoctoral Fellow > Open Systems Lab | Indiana University > 150 S. Woodlawn Ave. | Bloomington, IN, 474045, USA > Lindley Hall Room 135 | +01 (812) 856-0501 > _______________________________________________ > mpi3-rma mailing list > mpi3-rma(a)lists.mpi-forum.org > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > -- Jeff Hammond Argonne Leadership Computing Facility jhammond(a)mcs.anl.gov / (630) 252-5381 http://www.linkedin.com/in/jeffhammond http://home.uchicago.edu/~jhammond/

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[Mpi3-rma] Alternative RMA discussion
by Torsten Hoefler 22 Nov '09

22 Nov '09

Hello all, I think that the current RMA spec is not all that bad and that it's possible to extend it slightly in order to meet our requirements and maintain MPI's orthogonality and look&feel. I this sense, I follow Jesper's and Hubert's proposals of small additions to the current spec instead of a complete revamp. However, in any case, we need a much better user-documentation because while I'm convinced that the standard captures RMA semantics very well, I think that the complexity of the topic is hard to grasp for an average user (who often thinks that everything should be coherent etc.). That's one difference to ARMCI, which just doesn't look as complex because it handles only a fraction of the cases/platforms that MPI supports. The main issues that I address in the draft are: 1) we need local (point-to-point) windows 2) we want to perform all RMA ops on arbitrary memory 3) simultaneous accesses to the same memory should not be illegal but rather undefined in their outcome (this is subtle but important) 4) we want test&set, test&accumulate, and compare&swap (and friends) 5) we want user-defined RMA operations 6) we want to take advantage of architectures that offer a stronger memory models and progression (coherent&consistent) All those issues are orthogonal and can be discussed separately. I propose changes in a draft chapter at http://www.unixer.de/sec/one-side-2.pdf . This is of course the first version and probably inconsistent, however, I think it solves all problems discussed above. The semantics of the color-coding are: - green: removed text from MPI-2.2 - red: added text - blue: comments/discussion (not part of the chapter) The issues mentioned above are addressed on the following pages: 1) 4-5 2) 26 3) 3,6,33-34 4) 14-15 5) 13-15 6) 37-38 Right now, all proposed changes are source-compatible to MPI-2.2 (at a small price of elegance though). I am sure there were reasons for all the restrictions on window/memory access in MPI-2.0. However, I think that accepting undefined outcomes should allow for efficient implementations. Error checking should be left to tools on top of MPI (cf. other memory consistency models (who knows omp_flush? ;) and threading). >From a performance-perspective, there are two open things to discuss: 1) function arguments get expensive when they spill on the stack - RMA ops have horribly long argument lists - the number of args could be reduced if we assume symmetric count/datatype on both ends or if we send raw (MPI_BYTE) data (which I think it not preferable but might be unavoidable to match SHMEM or ARMCI performance) 2) supporting full MPI semantics comes at some cost, however, one could special-case for performance reasons (e.g., make contiguous MPI_BYTE transfers a special case). This costs one or two branches in each of the RMA calls (imho not too bad). We could even endorse this parameter combination as some kind of fast-mode in the standard. >From a scalability standpoint, we might consider the following two additions: 1) collective OP registration - ops can be identified by numbers or memory addresses (function pointers). However, if each process allocates them separately and sends them around, \Omega(P) memory could be required in the worst case. Collective allocation would reduce this to O(1) 2) collective window/memory allocation - potential \Omega(P) memory to store offsets; much discussed before, the solution is simple: allocate memory collectively (gives the library the chance to try to find "good addresses" (e.g., same local addresses or aligned/strided global addresses). I prepared a quick and incomplete implementation of the local window proposal (which uses ARMCI for RMA accesses and falls back to MPI for collective windows). The code is at http://www.unixer.de/sec/rma.cpp . Please let me know what you think. I would not mind to drop this completely if we decide that it's not a viable way to go (I only invested two days so far). Sorry for the long mail & All the Best, Torsten -- bash$ :(){ :|:&};: --------------------- http://www.unixer.de/ ----- Torsten Hoefler | Postdoctoral Fellow Open Systems Lab | Indiana University 150 S. Woodlawn Ave. | Bloomington, IN, 474045, USA Lindley Hall Room 135 | +01 (812) 856-0501

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Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?)
by Underwood, Keith D 16 Sep '09

16 Sep '09

This is exactly why I think it is critical to think of the dichotomy as fast/slow instead of raw/complex. What you are proposing is a raw/complex dichotomy that goes faster when you use the raw interface than when you use the complex interface. But, the point of the raw mode is to offer significantly more speed to those who don't need the complex features. Thus, anything that doesn't exist in the raw mode interface will inherently be slower. If hardware adds a feature that isn't part of the raw mode interface, you have to either change MPI (not something we want to do a lot) or use the complex (aka slow) interface. As I pointed out earlier, the flip side of that coin is that if you put something "too hard" in the raw/fast interface, you risk making the fast interface slow. Keith > -----Original Message----- > From: mpi3-rma-bounces(a)lists.mpi-forum.org [mailto:mpi3-rma- > bounces(a)lists.mpi-forum.org] On Behalf Of Jeff Hammond > Sent: Wednesday, September 16, 2009 3:42 PM > To: MPI 3.0 Remote Memory Access working group > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > pack/unpack (?) > > Keith, > > I don't really think in terms of the fast/slow dichotomy, rather the > raw/complex one. This is really meant to align with the philosophical > dichotomy of MPI users - one group is using less than 10 functions and > are perfectly happy with the results, while another group's needs push > the limits of the standard, or at least require a non-trivial subset > of the MPI standard's calls. > > Raw_xfer should be simple and fast when that is all that is needed, > but should not be the only available option. I contend, as someone > who has not implemented nor will most likely not implement the MPI > standard, but only as a rather heavy user of GA/ARMCI, that excluding > Raw_xfer from the standard will prevent optimal performance on some > systems, particularly the ones at the bleeding-edge. > > I would love it if the standard branch-predicted future hardware and > defined RMA accordingly, but I think that the raw/complex dichotomy is > appropriate for what we know to be true now and that it in no way > handicaps the standard from responding to future hardware as new > capability is realized. The only way I would modify the Raw_xfer > based upon my optimistic vision of the future of hardware would be to > add a stride argument such that if someone develops hardware support > for strided transfer over primitive types, then they can seamlessly > become part of Raw_xfer. > > Dick, > > If you believe that the MPI implementations can do all the datatype > decoding required in the general purpose with ~very~ little overhead, > then Raw_xfer is not needed. However, I'm pessimistic that this will > be possible. Local completion of contiguous transfers on BlueGene/P > occurs in less than 10000 clock cycles. If the datatype cache is > large and is not prefetched to L1 cache, its a challenge to decode the > general case on that time-scale. I echo Keith's most recent post > along these lines. > > I appreciate Vinod's comments that BlueGene/P is a special case, but > few vendors are talking about clock speeds more than 5 times what > PPC450 runs at (0.85 GHz) and neither have I heard that we're going to > get to exascale with slower latency interconnects, so I hope 2-3 > microsecond latency becomes available on more machines than just > BlueGene/X varieties. Given that intra-node memory pipelines aren't > getting any faster, any MPI function call that has to go to a > database, i.e. for cached datatypes, is going to take a performance > hit now and into the future. > > Best, > > Jeff > > On Wed, Sep 16, 2009 at 3:43 PM, Underwood, Keith D > <keith.d.underwood(a)intel.com> wrote: > > It is a very fine line to walk. If you have "fast" and "slow" > functions that map to what is possible today, then you have a standard > which is nominally fast out of the box; however, you are then stuck > having to implement EVERYTHING in the "slow" function before you can > move that into the "fast" category for users. > > > > In contrast, if you branch predict (a little) what networks will be > able to do in a couple of years and push on the vendors a bit in the > definition of the "fast" interface, then the day the standard ships, it > may not be of optimal speed, but you have hope of getting a slightly > broader set of functionality in the fast category. > > > > Since I don't really expect that the new RMA ops will be available > the same day as the first printed standard, I tend to favor the latter > category. Let's define a subset that is a good subset, rather than > simply the subset that exists in DCMF and Verbs. > > > > Keith > > > >> -----Original Message----- > >> From: mpi3-rma-bounces(a)lists.mpi-forum.org [mailto:mpi3-rma- > >> bounces(a)lists.mpi-forum.org] On Behalf Of Jeff Hammond > >> Sent: Wednesday, September 16, 2009 2:37 PM > >> To: MPI 3.0 Remote Memory Access working group > >> Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > >> pack/unpack (?) > >> > >> I was having the same thoughts during my hideous commute today. > >> Clearly, future systems may have very smart NICs or OSs which > >> seamlessly manage communication co-processors to support, among > other > >> things, on-the-fly compression of non-contiguous buffers. However, > >> until some vendor starts taking seriously about this, I don't think > it > >> is good idea to ignore the present hardware support of RMA and > create > >> a standard which can perform well when the first MPI-3 libraries > >> become available. > >> > >> I'm not sure I understand all this talk about assertions upon > >> initialization, since I was hoping that Raw_xfer and general-purpose > >> xfer (Gen_xfer) would both always be available but that the former > >> would be much faster in certain contexts, since implementing the Raw > >> version close to hardware doesn't require much work and would not > >> interfere with how Gen_xfer operates. Am I missing something? > >> > >> When the day finally comes that NICs can do Gen_xfer for a complex > >> non-contiguous datatype as fast as Raw_xfer, would not the standard > >> still be sufficient, since implementers would just produce a faster > >> Gen_xfer and Raw_xfer would fade into the background? > >> > >> Raw_xfer is a pragmatic response to the present and near-future > state > >> of HPC systems rather than some sort of marquee feature to tape to > the > >> metaphorical refrigerator door. > >> > >> Best, > >> > >> Jeff > >> > >> On Wed, Sep 16, 2009 at 3:14 PM, Underwood, Keith D > >> <keith.d.underwood(a)intel.com> wrote: > >> > But we have to be very careful here. We don't want to overly > >> constrain what > >> > can be thought of as "fast". For example, I think it is perfectly > >> > reasonable to implement accumulate on a NIC. Just because it > doesn't > >> exist > >> > today doesn't mean that it shouldn't be part of the "fast" MPI > call. > >> > > >> > > >> > > >> > Now, datatype conversion. it is nominally possible that a NIC > could > >> do > >> > datatype conversion - just like it is nominally possible to for a > NIC > >> to be > >> > hooked to a Rube Goldberg device to implement MPI_Make_Breakfast > ;-) > >> > > >> > > >> > > >> > Anyway, the point is that we need to be forward looking in > defining > >> "fast" > >> > and "slow", not backward looking. > >> > > >> > > >> > > >> > Keith > >> > > >> > > >> > > >> > From: mpi3-rma-bounces(a)lists.mpi-forum.org > >> > [mailto:[email protected]] On Behalf Of Richard > >> Treumann > >> > Sent: Wednesday, September 16, 2009 2:06 PM > >> > To: MPI 3.0 Remote Memory Access working group > >> > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > >> > pack/unpack (?) > >> > > >> > > >> > > >> > BINGO Jeff > >> > > >> > We might also remove the datatype argument and twin count > arguments > >> from > >> > MPI_RMA_Raw_xfer just to eliminate the expectation that basic > put/get > >> do > >> > datatype conversions when origin and target are on heterogeneous > >> nodes. > >> > There would be a single "count" argument and it represents the > number > >> of > >> > contiguous bytes to be transferred. > >> > > >> > The assertion would be that there is no use of complex RMA. It > would > >> give > >> > the implementation the option to leave its software agent dormant. > >> Note that > >> > having this assertion as an option for MPI_Init_asserted does not > >> allow an > >> > MPI implementation to avoid having an agent available. An > application > >> that > >> > does not use the assertion can count on the agent being ready for > any > >> call > >> > to "full baked" RMA. > >> > > >> > Dick > >> > > >> > Dick Treumann - MPI Team > >> > IBM Systems & Technology Group > >> > Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 > >> > Tele (845) 433-7846 Fax (845) 433-8363 > >> > > >> > > >> > mpi3-rma-bounces(a)lists.mpi-forum.org wrote on 09/16/2009 03:43:15 > PM: > >> > > >> >> [image removed] > >> >> > >> >> Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > pack/unpack > >> (?) > >> >> > >> >> Jeff Hammond > >> >> > >> >> to: > >> >> > >> >> MPI 3.0 Remote Memory Access working group > >> >> > >> >> 09/16/2009 03:44 PM > >> >> > >> >> Sent by: > >> >> > >> >> mpi3-rma-bounces(a)lists.mpi-forum.org > >> >> > >> >> Please respond to "MPI 3.0 Remote Memory Access working group" > >> >> > >> >> I think that there is a need for two interfaces; one which is a > >> >> portable interface to the low-level truly one-sided bulk transfer > >> >> operation and another which is completely general and is > permitted > >> to > >> >> do operations which require remote agency. > >> >> > >> >> For example, I am aware of no NIC which can do accumulate on its > >> own, > >> >> hence RMA_ACC_SUM and related operations require remote agency, > and > >> >> thus this category of RMA operations are not truly one-sided. > >> >> > >> >> Thus the standard might support two xfer calls: > >> >> > >> >> MPI_RMA_Raw_xfer(origin_addr, origin_count, origin_datatype, > >> >> target_mem, target_disp, target_count , target_rank, request) > >> >> > >> >> which is exclusively for transferring contiguous bytes from one > >> place > >> >> to another, i.e. does raw put/get only, and the second, which has > >> been > >> >> described already, which handles the general case, including > >> >> accumulation, non-contiguous and other complex operations. > >> >> > >> >> The distinction over remote agency is extremely important from a > >> >> implementation perspective since contiguous put/get operations > can > >> be > >> >> performed in a fully asynchronous non-interrupting way with a > >> variety > >> >> of interconnects, and thus exposing this procedure in the MPI > >> standard > >> >> will allow for very efficient implementations on some systems. > It > >> >> should also encourage MPI users to think about their RMA needs > and > >> how > >> >> they might restructure their code to take advantage of the faster > >> >> flavor of xfer when doing so requires little modification. > >> >> > >> >> Jeff > >> >> > >> >> On Wed, Sep 16, 2009 at 1:49 PM, Vinod tipparaju > >> >> <tipparajuv(a)hotmail.com> wrote: > >> >> >>My argument is that any RMA depends on a call at the origin > being > >> able > >> >> >> to > >> >> >> trigger activity at the target. Modern RMA hardware has the > >> hooksto do > >> >> >> the > >> >> >> remote side of MPI_Fast_RMA_xfer() efficiently based on a call > at > >> the > >> >> >> origin. Because these hooks are in the hardware they are > simply > >> there. > >> >> >> They > >> >> >> do not use the CPU or hurt performance of things that do use > the > >> CPU. > >> >> > > >> >> > I read this as an argument that says two interfaces are not > >> necessary. > >> >> > Having application author promise (during init) it will not do > >> anything > >> >> > that > >> >> > needs an agent is certainly useful. Particularly when, as you > >> state, > >> >> > "having > >> >> > this agent standing by hurts general performance". > >> >> > The things that potentially cannot be done without an agent > >> >> > (technically, > >> >> > everything but atomics could be done with out need for any > >> agents)are > >> >> > users > >> >> > choice through explicit usage. Users choses these attributes > being > >> aware > >> >> > of > >> >> > their cost hence they can indicate that they will not use them > >> ahead of > >> >> > time > >> >> > when they don't use them. > >> >> > I have repeatedly considered dropping the atomicity attribute, > I > >> am > >> >> > unable > >> >> > to because it makes programming (and thinking) so much easier > for > >> many > >> >> > applications. > >> >> > Vinod. > >> >> > > >> >> > > >> >> > ________________________________ > >> >> > To: mpi3-rma(a)lists.mpi-forum.org > >> >> > From: treumann(a)us.ibm.com > >> >> > Date: Wed, 16 Sep 2009 14:18:15 -0400 > >> >> > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one- > sided > >> >> > pack/unpack (?) > >> >> > > >> >> > The assertion could then be: MPI_NO_SLOW_RMA (also a bit tongue > in > >> >> > cheek) > >> >> > > >> >> > My argument is that any RMA depends on a call at the origin > being > >> able > >> >> > to > >> >> > trigger activity at the target. Modern RMA hardware has the > hooks > >> to do > >> >> > the > >> >> > remote side of MPI_Fast_RMA_xfer() efficiently based on a call > at > >> the > >> >> > origin. Because these hooks are in the hardware they are simply > >> there. > >> >> > They > >> >> > do not use the CPU or hurt performance of things that do use > the > >> CPU. > >> >> > > >> >> > RMA hardware may not have the hooks to do the target side of > any > >> >> > arbitrary > >> >> > MPI_Slow_RMA_xfer(). As a result, support for the more complex > >> RMA_xfer > >> >> > may > >> >> > require a wake-able software agent (thread maybe) to be > standing > >> by at > >> >> > all > >> >> > tasks just because they may become target of a Slow_RMA_xfer. > >> >> > > >> >> > If having this agent standing by hurts general performance of > MPI > >> >> > applications that will never make a call to Slow_RMA_xfer, why > not > >> let > >> >> > the > >> >> > applications author promise up front "I have no need of this > >> agent." > >> >> > > >> >> > An MPI implementation that can support Slow_RMA_xfer with no > extra > >> costs > >> >> > (send/recv latency, memory, packet interrupts, CPU contention) > >> will > >> >> > simply > >> >> > ignore the assertion. > >> >> > > >> >> > BTW - I just took a look at the broad proposal and it may > contain > >> >> > several > >> >> > things that cannot be done without a wake-able remote software > >> agent. > >> >> > That > >> >> > argues for Keith's idea of an RMA operation which closely > matches > >> what > >> >> > RMA > >> >> > hardware does and a second one that brings along all the bells > >> >> > andwhistles. > >> >> > Maybe the assertion for an application that only uses the basic > >> RMA call > >> >> > or > >> >> > uses no RMA at all could be MPI_NO_KITCHEN_SINK (even more > tongue > >> in > >> >> > cheek). > >> >> > > >> >> > Dick > >> >> > > >> >> > > >> >> > Dick Treumann - MPI Team > >> >> > IBM Systems & Technology Group > >> >> > Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY > 12601 > >> >> > Tele (845) 433-7846 Fax (845) 433-8363 > >> >> > > >> >> > > >> >> > mpi3-rma-bounces(a)lists.mpi-forum.org wrote on 09/16/2009 > 01:08:51 > >> PM: > >> >> > > >> >> >> [image removed] > >> >> >> > >> >> >> Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > >> pack/unpack > >> >> >> (?) > >> >> >> > >> >> >> Underwood, Keith D > >> >> >> > >> >> >> to: > >> >> >> > >> >> >> MPI 3.0 Remote Memory Access working group > >> >> >> > >> >> >> 09/16/2009 01:09 PM > >> >> >> > >> >> >> Sent by: > >> >> >> > >> >> >> mpi3-rma-bounces(a)lists.mpi-forum.org > >> >> >> > >> >> >> Please respond to "MPI 3.0 Remote Memory Access working group" > >> >> >> > >> >> >> But, going back to Bill's point: performance across a range > of > >> >> >> platforms is key. While you can't have a function for every > >> usage > >> >> >> (well, you can, but it would get cumbersome at some point), it > >> may > >> >> >> be important to have a few levels of specialization in the > API. > >> >> >> E.g. you could have two variants: > >> >> >> > >> >> >> MPI_Fast_RMA_xfer(): no data types, no communicators, etc. > >> >> >> MPI_Slow_RMA_xfer(): include the kitchen sink. > >> >> >> > >> >> >> Yes, the naming is a little tongue in cheek ;-) > >> >> >> > >> >> >> Keith > >> >> >> > >> >> >> <snip> > >> >> > > >> >> > _______________________________________________ > >> >> > mpi3-rma mailing list > >> >> > mpi3-rma(a)lists.mpi-forum.org > >> >> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > >> >> > > >> >> > > >> >> > >> >> > >> >> > >> >> -- > >> >> Jeff Hammond > >> >> Argonne Leadership Computing Facility > >> >> jhammond(a)mcs.anl.gov / (630) 252-5381 > >> >> http://www.linkedin.com/in/jeffhammond > >> >> http://home.uchicago.edu/~jhammond/ > >> >> > >> >> _______________________________________________ > >> >> mpi3-rma mailing list > >> >> mpi3-rma(a)lists.mpi-forum.org > >> >> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > >> > > >> > _______________________________________________ > >> > mpi3-rma mailing list > >> > mpi3-rma(a)lists.mpi-forum.org > >> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > >> > > >> > > >> > >> > >> > >> -- > >> Jeff Hammond > >> Argonne Leadership Computing Facility > >> jhammond(a)mcs.anl.gov / (630) 252-5381 > >> http://www.linkedin.com/in/jeffhammond > >> http://home.uchicago.edu/~jhammond/ > >> > >> _______________________________________________ > >> mpi3-rma mailing list > >> mpi3-rma(a)lists.mpi-forum.org > >> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > > > > _______________________________________________ > > mpi3-rma mailing list > > mpi3-rma(a)lists.mpi-forum.org > > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > > > > > > -- > Jeff Hammond > Argonne Leadership Computing Facility > jhammond(a)mcs.anl.gov / (630) 252-5381 > http://www.linkedin.com/in/jeffhammond > http://home.uchicago.edu/~jhammond/ > > _______________________________________________ > mpi3-rma mailing list > mpi3-rma(a)lists.mpi-forum.org > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma

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Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?)
by William Gropp 16 Sep '09

16 Sep '09

I agree that we need to be forward looking - but its also important to remember that some parallel systems use their own, integrated interconnect (I don't want to say network) which supports some set of remote atomic operations even now. MPI should support these systems as well. Bill Underwood, Keith D wrote: > > But we have to be very careful here. We don’t want to overly > constrain what can be thought of as “fast”. For example, I think it > is perfectly reasonable to implement accumulate on a NIC. Just > because it doesn’t exist today doesn’t mean that it shouldn’t be part > of the “fast” MPI call. > > > > Now, datatype conversion… it is nominally possible that a NIC could do > datatype conversion – just like it is nominally possible to for a NIC > to be hooked to a Rube Goldberg device to implement MPI_Make_Breakfast ;-) > > > > Anyway, the point is that we need to be forward looking in defining > “fast” and “slow”, not backward looking. > > > > Keith > > > > *From:* mpi3-rma-bounces(a)lists.mpi-forum.org > [mailto:[email protected]] *On Behalf Of *Richard > Treumann > *Sent:* Wednesday, September 16, 2009 2:06 PM > *To:* MPI 3.0 Remote Memory Access working group > *Subject:* Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > pack/unpack (?) > > > > BINGO Jeff > > We might also remove the datatype argument and twin count arguments > from MPI_RMA_Raw_xfer just to eliminate the expectation that basic > put/get do datatype conversions when origin and target are on > heterogeneous nodes. There would be a single "count" argument and it > represents the number of contiguous bytes to be transferred. > > The assertion would be that there is no use of complex RMA. It would > give the implementation the option to leave its software agent > dormant. Note that having this assertion as an option for > MPI_Init_asserted does not allow an MPI implementation to avoid having > an agent available. An application that does not use the assertion can > count on the agent being ready for any call to "full baked" RMA. > > Dick > > Dick Treumann - MPI Team > IBM Systems & Technology Group > Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 > Tele (845) 433-7846 Fax (845) 433-8363 > > > mpi3-rma-bounces(a)lists.mpi-forum.org wrote on 09/16/2009 03:43:15 PM: > > > [image removed] > > > > Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?) > > > > Jeff Hammond > > > > to: > > > > MPI 3.0 Remote Memory Access working group > > > > 09/16/2009 03:44 PM > > > > Sent by: > > > > mpi3-rma-bounces(a)lists.mpi-forum.org > > > > Please respond to "MPI 3.0 Remote Memory Access working group" > > > > I think that there is a need for two interfaces; one which is a > > portable interface to the low-level truly one-sided bulk transfer > > operation and another which is completely general and is permitted to > > do operations which require remote agency. > > > > For example, I am aware of no NIC which can do accumulate on its own, > > hence RMA_ACC_SUM and related operations require remote agency, and > > thus this category of RMA operations are not truly one-sided. > > > > Thus the standard might support two xfer calls: > > > > MPI_RMA_Raw_xfer(origin_addr, origin_count, origin_datatype, > > target_mem, target_disp, target_count , target_rank, request) > > > > which is exclusively for transferring contiguous bytes from one place > > to another, i.e. does raw put/get only, and the second, which has been > > described already, which handles the general case, including > > accumulation, non-contiguous and other complex operations. > > > > The distinction over remote agency is extremely important from a > > implementation perspective since contiguous put/get operations can be > > performed in a fully asynchronous non-interrupting way with a variety > > of interconnects, and thus exposing this procedure in the MPI standard > > will allow for very efficient implementations on some systems. It > > should also encourage MPI users to think about their RMA needs and how > > they might restructure their code to take advantage of the faster > > flavor of xfer when doing so requires little modification. > > > > Jeff > > > > On Wed, Sep 16, 2009 at 1:49 PM, Vinod tipparaju > > <tipparajuv(a)hotmail.com> wrote: > > >>My argument is that any RMA depends on a call at the origin being > able to > > >> trigger activity at the target. Modern RMA hardware has the > hooksto do the > > >> remote side of MPI_Fast_RMA_xfer() efficiently based on a call at the > > >> origin. Because these hooks are in the hardware they are simply > there. They > > >> do not use the CPU or hurt performance of things that do use the CPU. > > > > > > I read this as an argument that says two interfaces are not necessary. > > > Having application author promise (during init) it will not do > anything that > > > needs an agent is certainly useful. Particularly when, as you > state, "having > > > this agent standing by hurts general performance". > > > The things that potentially cannot be done without an agent > (technically, > > > everything but atomics could be done with out need for any > agents)are users > > > choice through explicit usage. Users choses these attributes being > aware of > > > their cost hence they can indicate that they will not use them > ahead of time > > > when they don't use them. > > > I have repeatedly considered dropping the atomicity attribute, I am > unable > > > to because it makes programming (and thinking) so much easier for many > > > applications. > > > Vinod. > > > > > > > > > ________________________________ > > > To: mpi3-rma(a)lists.mpi-forum.org > > > From: treumann(a)us.ibm.com > > > Date: Wed, 16 Sep 2009 14:18:15 -0400 > > > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > > > pack/unpack (?) > > > > > > The assertion could then be: MPI_NO_SLOW_RMA (also a bit tongue in > cheek) > > > > > > My argument is that any RMA depends on a call at the origin being > able to > > > trigger activity at the target. Modern RMA hardware has the hooks > to do the > > > remote side of MPI_Fast_RMA_xfer() efficiently based on a call at the > > > origin. Because these hooks are in the hardware they are simply > there. They > > > do not use the CPU or hurt performance of things that do use the CPU. > > > > > > RMA hardware may not have the hooks to do the target side of any > arbitrary > > > MPI_Slow_RMA_xfer(). As a result, support for the more complex > RMA_xfer may > > > require a wake-able software agent (thread maybe) to be standing by > at all > > > tasks just because they may become target of a Slow_RMA_xfer. > > > > > > If having this agent standing by hurts general performance of MPI > > > applications that will never make a call to Slow_RMA_xfer, why not > let the > > > applications author promise up front "I have no need of this agent." > > > > > > An MPI implementation that can support Slow_RMA_xfer with no extra > costs > > > (send/recv latency, memory, packet interrupts, CPU contention) will > simply > > > ignore the assertion. > > > > > > BTW - I just took a look at the broad proposal and it may contain > several > > > things that cannot be done without a wake-able remote software > agent. That > > > argues for Keith's idea of an RMA operation which closely matches > what RMA > > > hardware does and a second one that brings along all the bells > andwhistles. > > > Maybe the assertion for an application that only uses the basic RMA > call or > > > uses no RMA at all could be MPI_NO_KITCHEN_SINK (even more tongue > in cheek). > > > > > > Dick > > > > > > > > > Dick Treumann - MPI Team > > > IBM Systems & Technology Group > > > Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 > > > Tele (845) 433-7846 Fax (845) 433-8363 > > > > > > > > > mpi3-rma-bounces(a)lists.mpi-forum.org wrote on 09/16/2009 01:08:51 PM: > > > > > >> [image removed] > > >> > > >> Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > pack/unpack (?) > > >> > > >> Underwood, Keith D > > >> > > >> to: > > >> > > >> MPI 3.0 Remote Memory Access working group > > >> > > >> 09/16/2009 01:09 PM > > >> > > >> Sent by: > > >> > > >> mpi3-rma-bounces(a)lists.mpi-forum.org > > >> > > >> Please respond to "MPI 3.0 Remote Memory Access working group" > > >> > > >> But, going back to Bill’s point: performance across a range of > > >> platforms is key. While you can’t have a function for every usage > > >> (well, you can, but it would get cumbersome at some point), it may > > >> be important to have a few levels of specialization in the API. > > >> E.g. you could have two variants: > > >> > > >> MPI_Fast_RMA_xfer(): no data types, no communicators, etc. > > >> MPI_Slow_RMA_xfer(): include the kitchen sink. > > >> > > >> Yes, the naming is a little tongue in cheek ;-) > > >> > > >> Keith > > >> > > >> <snip> > > > > > > _______________________________________________ > > > mpi3-rma mailing list > > > mpi3-rma(a)lists.mpi-forum.org > > > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > > > > > > > > > > > > > > -- > > Jeff Hammond > > Argonne Leadership Computing Facility > > jhammond(a)mcs.anl.gov / (630) 252-5381 > > http://www.linkedin.com/in/jeffhammond > > http://home.uchicago.edu/~jhammond/ > <http://home.uchicago.edu/%7Ejhammond/> > > > > _______________________________________________ > > mpi3-rma mailing list > > mpi3-rma(a)lists.mpi-forum.org > > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma >

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Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?)
by Jeff Hammond 16 Sep '09

16 Sep '09

Keith, I don't really think in terms of the fast/slow dichotomy, rather the raw/complex one. This is really meant to align with the philosophical dichotomy of MPI users - one group is using less than 10 functions and are perfectly happy with the results, while another group's needs push the limits of the standard, or at least require a non-trivial subset of the MPI standard's calls. Raw_xfer should be simple and fast when that is all that is needed, but should not be the only available option. I contend, as someone who has not implemented nor will most likely not implement the MPI standard, but only as a rather heavy user of GA/ARMCI, that excluding Raw_xfer from the standard will prevent optimal performance on some systems, particularly the ones at the bleeding-edge. I would love it if the standard branch-predicted future hardware and defined RMA accordingly, but I think that the raw/complex dichotomy is appropriate for what we know to be true now and that it in no way handicaps the standard from responding to future hardware as new capability is realized. The only way I would modify the Raw_xfer based upon my optimistic vision of the future of hardware would be to add a stride argument such that if someone develops hardware support for strided transfer over primitive types, then they can seamlessly become part of Raw_xfer. Dick, If you believe that the MPI implementations can do all the datatype decoding required in the general purpose with ~very~ little overhead, then Raw_xfer is not needed. However, I'm pessimistic that this will be possible. Local completion of contiguous transfers on BlueGene/P occurs in less than 10000 clock cycles. If the datatype cache is large and is not prefetched to L1 cache, its a challenge to decode the general case on that time-scale. I echo Keith's most recent post along these lines. I appreciate Vinod's comments that BlueGene/P is a special case, but few vendors are talking about clock speeds more than 5 times what PPC450 runs at (0.85 GHz) and neither have I heard that we're going to get to exascale with slower latency interconnects, so I hope 2-3 microsecond latency becomes available on more machines than just BlueGene/X varieties. Given that intra-node memory pipelines aren't getting any faster, any MPI function call that has to go to a database, i.e. for cached datatypes, is going to take a performance hit now and into the future. Best, Jeff On Wed, Sep 16, 2009 at 3:43 PM, Underwood, Keith D <keith.d.underwood(a)intel.com> wrote: > It is a very fine line to walk. ï¿½If you have "fast" and "slow" functions that map to what is possible today, then you have a standard which is nominally fast out of the box; however, you are then stuck having to implement EVERYTHING in the "slow" function before you can move that into the "fast" category for users. > > In contrast, if you branch predict (a little) what networks will be able to do in a couple of years and push on the vendors a bit in the definition of the "fast" interface, then the day the standard ships, it may not be of optimal speed, but you have hope of getting a slightly broader set of functionality in the fast category. > > Since I don't really expect that the new RMA ops will be available the same day as the first printed standard, I tend to favor the latter category. ï¿½Let's define a subset that is a good subset, rather than simply the subset that exists in DCMF and Verbs. > > Keith > >> -----Original Message----- >> From: mpi3-rma-bounces(a)lists.mpi-forum.org [mailto:mpi3-rma- >> bounces(a)lists.mpi-forum.org] On Behalf Of Jeff Hammond >> Sent: Wednesday, September 16, 2009 2:37 PM >> To: MPI 3.0 Remote Memory Access working group >> Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided >> pack/unpack (?) >> >> I was having the same thoughts during my hideous commute today. >> Clearly, future systems may have very smart NICs or OSs which >> seamlessly manage communication co-processors to support, among other >> things, on-the-fly compression of non-contiguous buffers. ï¿½However, >> until some vendor starts taking seriously about this, I don't think it >> is good idea to ignore the present hardware support of RMA and create >> a standard which can perform well when the first MPI-3 libraries >> become available. >> >> I'm not sure I understand all this talk about assertions upon >> initialization, since I was hoping that Raw_xfer and general-purpose >> xfer (Gen_xfer) would both always be available but that the former >> would be much faster in certain contexts, since implementing the Raw >> version close to hardware doesn't require much work and would not >> interfere with how Gen_xfer operates. ï¿½Am I missing something? >> >> When the day finally comes that NICs can do Gen_xfer for a complex >> non-contiguous datatype as fast as Raw_xfer, would not the standard >> still be sufficient, since implementers would just produce a faster >> Gen_xfer and Raw_xfer would fade into the background? >> >> Raw_xfer is a pragmatic response to the present and near-future state >> of HPC systems rather than some sort of marquee feature to tape to the >> metaphorical refrigerator door. >> >> Best, >> >> Jeff >> >> On Wed, Sep 16, 2009 at 3:14 PM, Underwood, Keith D >> <keith.d.underwood(a)intel.com> wrote: >> > But we have to be very careful here.ï¿½ We don't want to overly >> constrain what >> > can be thought of as "fast".ï¿½ For example, I think it is perfectly >> > reasonable to implement accumulate on a NIC.ï¿½ Just because it doesn't >> exist >> > today doesn't mean that it shouldn't be part of the "fast" MPI call. >> > >> > >> > >> > Now, datatype conversion. it is nominally possible that a NIC could >> do >> > datatype conversion - just like it is nominally possible to for a NIC >> to be >> > hooked to a Rube Goldberg device to implement MPI_Make_Breakfast ;-) >> > >> > >> > >> > Anyway, the point is that we need to be forward looking in defining >> "fast" >> > and "slow", not backward looking. >> > >> > >> > >> > Keith >> > >> > >> > >> > From: mpi3-rma-bounces(a)lists.mpi-forum.org >> > [mailto:[email protected]] On Behalf Of Richard >> Treumann >> > Sent: Wednesday, September 16, 2009 2:06 PM >> > To: MPI 3.0 Remote Memory Access working group >> > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided >> > pack/unpack (?) >> > >> > >> > >> > BINGO Jeff >> > >> > We might also remove the datatype argument and twin count arguments >> from >> > MPI_RMA_Raw_xfer just to eliminate the expectation that basic put/get >> do >> > datatype conversions when origin and target are on heterogeneous >> nodes. >> > There would be a single "count" argument and it represents the number >> of >> > contiguous bytes to be transferred. >> > >> > The assertion would be that there is no use of complex RMA. It would >> give >> > the implementation the option to leave its software agent dormant. >> Note that >> > having this assertion as an option for MPI_Init_asserted does not >> allow an >> > MPI implementation to avoid having an agent available. An application >> that >> > does not use the assertion can count on the agent being ready for any >> call >> > to "full baked" RMA. >> > >> > Dick >> > >> > Dick Treumann - MPI Team >> > IBM Systems & Technology Group >> > Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 >> > Tele (845) 433-7846 Fax (845) 433-8363 >> > >> > >> > mpi3-rma-bounces(a)lists.mpi-forum.org wrote on 09/16/2009 03:43:15 PM: >> > >> >> [image removed] >> >> >> >> Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack >> (?) >> >> >> >> Jeff Hammond >> >> >> >> to: >> >> >> >> MPI 3.0 Remote Memory Access working group >> >> >> >> 09/16/2009 03:44 PM >> >> >> >> Sent by: >> >> >> >> mpi3-rma-bounces(a)lists.mpi-forum.org >> >> >> >> Please respond to "MPI 3.0 Remote Memory Access working group" >> >> >> >> I think that there is a need for two interfaces; one which is a >> >> portable interface to the low-level truly one-sided bulk transfer >> >> operation and another which is completely general and is permitted >> to >> >> do operations which require remote agency. >> >> >> >> For example, I am aware of no NIC which can do accumulate on its >> own, >> >> hence RMA_ACC_SUM and related operations require remote agency, and >> >> thus this category of RMA operations are not truly one-sided. >> >> >> >> Thus the standard might support two xfer calls: >> >> >> >> MPI_RMA_Raw_xfer(origin_addr, origin_count, origin_datatype, >> >> target_mem, target_disp, target_count , target_rank, request) >> >> >> >> which is exclusively for transferring contiguous bytes from one >> place >> >> to another, i.e. does raw put/get only, and the second, which has >> been >> >> described already, which handles the general case, including >> >> accumulation, non-contiguous and other complex operations. >> >> >> >> The distinction over remote agency is extremely important from a >> >> implementation perspective since contiguous put/get operations can >> be >> >> performed in a fully asynchronous non-interrupting way with a >> variety >> >> of interconnects, and thus exposing this procedure in the MPI >> standard >> >> will allow for very efficient implementations on some systems. ï¿½It >> >> should also encourage MPI users to think about their RMA needs and >> how >> >> they might restructure their code to take advantage of the faster >> >> flavor of xfer when doing so requires little modification. >> >> >> >> Jeff >> >> >> >> On Wed, Sep 16, 2009 at 1:49 PM, Vinod tipparaju >> >> <tipparajuv(a)hotmail.com> wrote: >> >> >>My argument is that any RMA depends on a call at the origin being >> able >> >> >> to >> >> >> trigger activity at the target. Modern RMA hardware has the >> hooksto do >> >> >> the >> >> >> remote side ofï¿½MPI_Fast_RMA_xfer()ï¿½efficiently based on a call at >> the >> >> >> origin. Because these hooks are in the hardware they are simply >> there. >> >> >> They >> >> >> do not use the CPU or hurt performance of things that do use the >> CPU. >> >> > >> >> > I read this as an argument that says two interfaces are not >> necessary. >> >> > Having application author promise (during init) it will not do >> anything >> >> > that >> >> > needs an agent is certainly useful. Particularly when, as you >> state, >> >> > "having >> >> > this agent standing by hurts general performance". >> >> > The things that potentially cannot be done without an agent >> >> > (technically, >> >> > everything but atomics could be done with out need for any >> agents)are >> >> > users >> >> > choice through explicit usage. Users choses these attributes being >> aware >> >> > of >> >> > their cost hence they can indicate that they will not use them >> ahead of >> >> > time >> >> > when they don't use them. >> >> > I have repeatedly considered dropping the atomicity attribute, I >> am >> >> > unable >> >> > to because it makes programming (and thinking) so much easier for >> many >> >> > applications. >> >> > Vinod. >> >> > >> >> > >> >> > ________________________________ >> >> > To: mpi3-rma(a)lists.mpi-forum.org >> >> > From: treumann(a)us.ibm.com >> >> > Date: Wed, 16 Sep 2009 14:18:15 -0400 >> >> > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided >> >> > pack/unpack (?) >> >> > >> >> > The assertion could then be: MPI_NO_SLOW_RMA (also a bit tongue in >> >> > cheek) >> >> > >> >> > My argument is that any RMA depends on a call at the origin being >> able >> >> > to >> >> > trigger activity at the target. Modern RMA hardware has the hooks >> to do >> >> > the >> >> > remote side ofï¿½MPI_Fast_RMA_xfer() efficiently based on a call at >> the >> >> > origin. Because these hooks are in the hardware they are simply >> there. >> >> > They >> >> > do not use the CPU or hurt performance of things that do use the >> CPU. >> >> > >> >> > RMA hardware may not have the hooks to do the target side of any >> >> > arbitrary >> >> > MPI_Slow_RMA_xfer(). ï¿½As a result, support for the more complex >> RMA_xfer >> >> > may >> >> > require a wake-able software agent (thread maybe) to be standing >> by at >> >> > all >> >> > tasks just because they may become target of a Slow_RMA_xfer. >> >> > >> >> > If having this agent standing by hurts general performance of MPI >> >> > applications that will never make a call to Slow_RMA_xfer, why not >> let >> >> > the >> >> > applications author promise up front "I have no need of this >> agent." >> >> > >> >> > An MPI implementation that can support Slow_RMA_xfer with no extra >> costs >> >> > (send/recv latency, memory, packet interrupts, CPU contention) >> will >> >> > simply >> >> > ignore the assertion. >> >> > >> >> > BTW - I just took a look at the broad proposal and it may contain >> >> > several >> >> > things that cannot be done without a wake-able remote software >> agent. >> >> > That >> >> > argues for Keith's idea of an RMA operation which closely matches >> what >> >> > RMA >> >> > hardware does and a second one that brings along all the bells >> >> > andwhistles. >> >> > Maybe the assertion for an application that only uses the basic >> RMA call >> >> > or >> >> > uses no RMA at all could be MPI_NO_KITCHEN_SINK (even more tongue >> in >> >> > cheek). >> >> > >> >> > ï¿½ ï¿½ ï¿½ ï¿½ ï¿½ ï¿½Dick >> >> > >> >> > >> >> > Dick Treumann - MPI Team >> >> > IBM Systems & Technology Group >> >> > Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 >> >> > Tele (845) 433-7846 Fax (845) 433-8363 >> >> > >> >> > >> >> > mpi3-rma-bounces(a)lists.mpi-forum.org wrote on 09/16/2009 01:08:51 >> PM: >> >> > >> >> >> [image removed] >> >> >> >> >> >> Re: [Mpi3-rma] non-contiguous support in RMA & one-sided >> pack/unpack >> >> >> (?) >> >> >> >> >> >> Underwood, Keith D >> >> >> >> >> >> to: >> >> >> >> >> >> MPI 3.0 Remote Memory Access working group >> >> >> >> >> >> 09/16/2009 01:09 PM >> >> >> >> >> >> Sent by: >> >> >> >> >> >> mpi3-rma-bounces(a)lists.mpi-forum.org >> >> >> >> >> >> Please respond to "MPI 3.0 Remote Memory Access working group" >> >> >> >> >> >> But, going back to Bill's point: ï¿½performance across a range of >> >> >> platforms is key. ï¿½While you can't have a function for every >> usage >> >> >> (well, you can, but it would get cumbersome at some point), it >> may >> >> >> be important to have a few levels of specialization in the API. >> >> >> E.g. you could have two variants: >> >> >> >> >> >> MPI_Fast_RMA_xfer(): ï¿½no data types, no communicators, etc. >> >> >> MPI_Slow_RMA_xfer(): include the kitchen sink. >> >> >> >> >> >> Yes, the naming is a little tongue in cheek ;-) >> >> >> >> >> >> Keith >> >> >> >> >> >> <snip> >> >> > >> >> > _______________________________________________ >> >> > mpi3-rma mailing list >> >> > mpi3-rma(a)lists.mpi-forum.org >> >> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma >> >> > >> >> > >> >> >> >> >> >> >> >> -- >> >> Jeff Hammond >> >> Argonne Leadership Computing Facility >> >> jhammond(a)mcs.anl.gov / (630) 252-5381 >> >> http://www.linkedin.com/in/jeffhammond >> >> http://home.uchicago.edu/~jhammond/ >> >> >> >> _______________________________________________ >> >> mpi3-rma mailing list >> >> mpi3-rma(a)lists.mpi-forum.org >> >> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma >> > >> > _______________________________________________ >> > mpi3-rma mailing list >> > mpi3-rma(a)lists.mpi-forum.org >> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma >> > >> > >> >> >> >> -- >> Jeff Hammond >> Argonne Leadership Computing Facility >> jhammond(a)mcs.anl.gov / (630) 252-5381 >> http://www.linkedin.com/in/jeffhammond >> http://home.uchicago.edu/~jhammond/ >> >> _______________________________________________ >> mpi3-rma mailing list >> mpi3-rma(a)lists.mpi-forum.org >> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > > _______________________________________________ > mpi3-rma mailing list > mpi3-rma(a)lists.mpi-forum.org > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > -- Jeff Hammond Argonne Leadership Computing Facility jhammond(a)mcs.anl.gov / (630) 252-5381 http://www.linkedin.com/in/jeffhammond http://home.uchicago.edu/~jhammond/

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Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?)
by Underwood, Keith D 16 Sep '09

16 Sep '09

Do you really think it is just a couple of ifs? If this is not MPI_COMM_WORLD... if this is a derived datatype but not contiguous locally... if this is a derived datatype but not contiguous remotely... if these are not the same datatypes... if this is not heterogeneous... You also get into the trade-offs (that we obviously have to make even with two routines as well)... so, should contiguous derived datatypes be fast or slow? Well, obviously the NIC based agent can handle them if you do the math in the MPI library, but that math takes instructions. One of the things that was put on the table in Chicago was "should we have specialized functions for each native datatype?" Ultimately, it is all about what your performance targets are in terms of latency and issue rate. On one end of the spectrum, you have MPI_Isend(). On the other you have the speed of a native processor store. Which end of the spectrum do we want to be on? If we want broader penetration for RMA, we need to move toward the latter. So, yeah, "a handful of ifs" may be an issue... Particularly on any architecture where a given thread isn't all that speedy... Keith From: mpi3-rma-bounces(a)lists.mpi-forum.org [mailto:[email protected]] On Behalf Of Richard Treumann Sent: Wednesday, September 16, 2009 3:05 PM To: MPI 3.0 Remote Memory Access working group Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?) Hi Jeff I do not understand the need for two distinct origin side operations if you will assume the agency infrastructure must always be standing by ready to be activated. The MPI implementation will be able to determine at the origin side if the parameters passed into a complex RMA fit the capabilities of its NIC. It can decide whether to use the intrinsic NIC mode or the software agent mode. If it has a NIC that does integer accumulate but not floating point accumulate, the implementation can pick between NIC mode and agent mode. When a new NIC that does floating point accumulate comes along, the implementation can exploit it with no need for application rewrite. On the other hand, the RAW RMA can be used by by applications that want to exploit NIC capabilities that are widely available today and do not want to drag in overheads for an agent waiting on the sidelines. An agent that they may know will never be needed. If someday in the future the typical NIC has certain additional capabilities we can add a richer RAW operation to the MPI standard. Can you explain to me what you expect to have always available as a software agent that will be there at no significant cost in memory or harm to performance of non-RMA operations? Are you thinking we need two distinct routines just to avoid a couple "if" tests to decide inside libmpi whether to use NIC based or agent based RMA? Dick Dick Treumann - MPI Team IBM Systems & Technology Group Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 Tele (845) 433-7846 Fax (845) 433-8363 [cid:[email protected]]Jeff Hammond ---09/16/2009 04:37:33 PM---I was having the same thoughts during my hideous commute today. Clearly, future systems may have ver From: Jeff Hammond <jeff.science(a)gmail.com> To: "MPI 3.0 Remote Memory Access working group" <mpi3-rma(a)lists.mpi-forum.org> Date: 09/16/2009 04:37 PM Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?) Sent by: mpi3-rma-bounces(a)lists.mpi-forum.org ________________________________ <snip> I'm not sure I understand all this talk about assertions upon initialization, since I was hoping that Raw_xfer and general-purpose xfer (Gen_xfer) would both always be available but that the former would be much faster in certain contexts, since implementing the Raw version close to hardware doesn't require much work and would not interfere with how Gen_xfer operates. Am I missing something? <snip>

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Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?)
by Richard Treumann 16 Sep '09

16 Sep '09

Hi Jeff I do not understand the need for two distinct origin side operations if you will assume the agency infrastructure must always be standing by ready to be activated. The MPI implementation will be able to determine at the origin side if the parameters passed into a complex RMA fit the capabilities of its NIC. It can decide whether to use the intrinsic NIC mode or the software agent mode. If it has a NIC that does integer accumulate but not floating point accumulate, the implementation can pick between NIC mode and agent mode. When a new NIC that does floating point accumulate comes along, the implementation can exploit it with no need for application rewrite. On the other hand, the RAW RMA can be used by by applications that want to exploit NIC capabilities that are widely available today and do not want to drag in overheads for an agent waiting on the sidelines. An agent that they may know will never be needed. If someday in the future the typical NIC has certain additional capabilities we can add a richer RAW operation to the MPI standard. Can you explain to me what you expect to have always available as a software agent that will be there at no significant cost in memory or harm to performance of non-RMA operations? Are you thinking we need two distinct routines just to avoid a couple "if" tests to decide inside libmpi whether to use NIC based or agent based RMA? Dick Dick Treumann - MPI Team IBM Systems & Technology Group Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 Tele (845) 433-7846 Fax (845) 433-8363 From: Jeff Hammond <jeff.science(a)gmail.com> To: "MPI 3.0 Remote Memory Access working group" <mpi3-rma(a)lists.mpi-forum.org> Date: 09/16/2009 04:37 PM Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?) Sent by: mpi3-rma-bounces(a)lists.mpi-forum.org <snip> I'm not sure I understand all this talk about assertions upon initialization, since I was hoping that Raw_xfer and general-purpose xfer (Gen_xfer) would both always be available but that the former would be much faster in certain contexts, since implementing the Raw version close to hardware doesn't require much work and would not interfere with how Gen_xfer operates. Am I missing something? <snip>

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Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?)
by Underwood, Keith D 16 Sep '09

16 Sep '09

It is a very fine line to walk. If you have "fast" and "slow" functions that map to what is possible today, then you have a standard which is nominally fast out of the box; however, you are then stuck having to implement EVERYTHING in the "slow" function before you can move that into the "fast" category for users. In contrast, if you branch predict (a little) what networks will be able to do in a couple of years and push on the vendors a bit in the definition of the "fast" interface, then the day the standard ships, it may not be of optimal speed, but you have hope of getting a slightly broader set of functionality in the fast category. Since I don't really expect that the new RMA ops will be available the same day as the first printed standard, I tend to favor the latter category. Let's define a subset that is a good subset, rather than simply the subset that exists in DCMF and Verbs. Keith > -----Original Message----- > From: mpi3-rma-bounces(a)lists.mpi-forum.org [mailto:mpi3-rma- > bounces(a)lists.mpi-forum.org] On Behalf Of Jeff Hammond > Sent: Wednesday, September 16, 2009 2:37 PM > To: MPI 3.0 Remote Memory Access working group > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > pack/unpack (?) > > I was having the same thoughts during my hideous commute today. > Clearly, future systems may have very smart NICs or OSs which > seamlessly manage communication co-processors to support, among other > things, on-the-fly compression of non-contiguous buffers. However, > until some vendor starts taking seriously about this, I don't think it > is good idea to ignore the present hardware support of RMA and create > a standard which can perform well when the first MPI-3 libraries > become available. > > I'm not sure I understand all this talk about assertions upon > initialization, since I was hoping that Raw_xfer and general-purpose > xfer (Gen_xfer) would both always be available but that the former > would be much faster in certain contexts, since implementing the Raw > version close to hardware doesn't require much work and would not > interfere with how Gen_xfer operates. Am I missing something? > > When the day finally comes that NICs can do Gen_xfer for a complex > non-contiguous datatype as fast as Raw_xfer, would not the standard > still be sufficient, since implementers would just produce a faster > Gen_xfer and Raw_xfer would fade into the background? > > Raw_xfer is a pragmatic response to the present and near-future state > of HPC systems rather than some sort of marquee feature to tape to the > metaphorical refrigerator door. > > Best, > > Jeff > > On Wed, Sep 16, 2009 at 3:14 PM, Underwood, Keith D > <keith.d.underwood(a)intel.com> wrote: > > But we have to be very careful here.ï¿½ We don't want to overly > constrain what > > can be thought of as "fast".ï¿½ For example, I think it is perfectly > > reasonable to implement accumulate on a NIC.ï¿½ Just because it doesn't > exist > > today doesn't mean that it shouldn't be part of the "fast" MPI call. > > > > > > > > Now, datatype conversion. it is nominally possible that a NIC could > do > > datatype conversion - just like it is nominally possible to for a NIC > to be > > hooked to a Rube Goldberg device to implement MPI_Make_Breakfast ;-) > > > > > > > > Anyway, the point is that we need to be forward looking in defining > "fast" > > and "slow", not backward looking. > > > > > > > > Keith > > > > > > > > From: mpi3-rma-bounces(a)lists.mpi-forum.org > > [mailto:[email protected]] On Behalf Of Richard > Treumann > > Sent: Wednesday, September 16, 2009 2:06 PM > > To: MPI 3.0 Remote Memory Access working group > > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > > pack/unpack (?) > > > > > > > > BINGO Jeff > > > > We might also remove the datatype argument and twin count arguments > from > > MPI_RMA_Raw_xfer just to eliminate the expectation that basic put/get > do > > datatype conversions when origin and target are on heterogeneous > nodes. > > There would be a single "count" argument and it represents the number > of > > contiguous bytes to be transferred. > > > > The assertion would be that there is no use of complex RMA. It would > give > > the implementation the option to leave its software agent dormant. > Note that > > having this assertion as an option for MPI_Init_asserted does not > allow an > > MPI implementation to avoid having an agent available. An application > that > > does not use the assertion can count on the agent being ready for any > call > > to "full baked" RMA. > > > > Dick > > > > Dick Treumann - MPI Team > > IBM Systems & Technology Group > > Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 > > Tele (845) 433-7846 Fax (845) 433-8363 > > > > > > mpi3-rma-bounces(a)lists.mpi-forum.org wrote on 09/16/2009 03:43:15 PM: > > > >> [image removed] > >> > >> Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack > (?) > >> > >> Jeff Hammond > >> > >> to: > >> > >> MPI 3.0 Remote Memory Access working group > >> > >> 09/16/2009 03:44 PM > >> > >> Sent by: > >> > >> mpi3-rma-bounces(a)lists.mpi-forum.org > >> > >> Please respond to "MPI 3.0 Remote Memory Access working group" > >> > >> I think that there is a need for two interfaces; one which is a > >> portable interface to the low-level truly one-sided bulk transfer > >> operation and another which is completely general and is permitted > to > >> do operations which require remote agency. > >> > >> For example, I am aware of no NIC which can do accumulate on its > own, > >> hence RMA_ACC_SUM and related operations require remote agency, and > >> thus this category of RMA operations are not truly one-sided. > >> > >> Thus the standard might support two xfer calls: > >> > >> MPI_RMA_Raw_xfer(origin_addr, origin_count, origin_datatype, > >> target_mem, target_disp, target_count , target_rank, request) > >> > >> which is exclusively for transferring contiguous bytes from one > place > >> to another, i.e. does raw put/get only, and the second, which has > been > >> described already, which handles the general case, including > >> accumulation, non-contiguous and other complex operations. > >> > >> The distinction over remote agency is extremely important from a > >> implementation perspective since contiguous put/get operations can > be > >> performed in a fully asynchronous non-interrupting way with a > variety > >> of interconnects, and thus exposing this procedure in the MPI > standard > >> will allow for very efficient implementations on some systems. ï¿½It > >> should also encourage MPI users to think about their RMA needs and > how > >> they might restructure their code to take advantage of the faster > >> flavor of xfer when doing so requires little modification. > >> > >> Jeff > >> > >> On Wed, Sep 16, 2009 at 1:49 PM, Vinod tipparaju > >> <tipparajuv(a)hotmail.com> wrote: > >> >>My argument is that any RMA depends on a call at the origin being > able > >> >> to > >> >> trigger activity at the target. Modern RMA hardware has the > hooksto do > >> >> the > >> >> remote side ofï¿½MPI_Fast_RMA_xfer()ï¿½efficiently based on a call at > the > >> >> origin. Because these hooks are in the hardware they are simply > there. > >> >> They > >> >> do not use the CPU or hurt performance of things that do use the > CPU. > >> > > >> > I read this as an argument that says two interfaces are not > necessary. > >> > Having application author promise (during init) it will not do > anything > >> > that > >> > needs an agent is certainly useful. Particularly when, as you > state, > >> > "having > >> > this agent standing by hurts general performance". > >> > The things that potentially cannot be done without an agent > >> > (technically, > >> > everything but atomics could be done with out need for any > agents)are > >> > users > >> > choice through explicit usage. Users choses these attributes being > aware > >> > of > >> > their cost hence they can indicate that they will not use them > ahead of > >> > time > >> > when they don't use them. > >> > I have repeatedly considered dropping the atomicity attribute, I > am > >> > unable > >> > to because it makes programming (and thinking) so much easier for > many > >> > applications. > >> > Vinod. > >> > > >> > > >> > ________________________________ > >> > To: mpi3-rma(a)lists.mpi-forum.org > >> > From: treumann(a)us.ibm.com > >> > Date: Wed, 16 Sep 2009 14:18:15 -0400 > >> > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > >> > pack/unpack (?) > >> > > >> > The assertion could then be: MPI_NO_SLOW_RMA (also a bit tongue in > >> > cheek) > >> > > >> > My argument is that any RMA depends on a call at the origin being > able > >> > to > >> > trigger activity at the target. Modern RMA hardware has the hooks > to do > >> > the > >> > remote side ofï¿½MPI_Fast_RMA_xfer() efficiently based on a call at > the > >> > origin. Because these hooks are in the hardware they are simply > there. > >> > They > >> > do not use the CPU or hurt performance of things that do use the > CPU. > >> > > >> > RMA hardware may not have the hooks to do the target side of any > >> > arbitrary > >> > MPI_Slow_RMA_xfer(). ï¿½As a result, support for the more complex > RMA_xfer > >> > may > >> > require a wake-able software agent (thread maybe) to be standing > by at > >> > all > >> > tasks just because they may become target of a Slow_RMA_xfer. > >> > > >> > If having this agent standing by hurts general performance of MPI > >> > applications that will never make a call to Slow_RMA_xfer, why not > let > >> > the > >> > applications author promise up front "I have no need of this > agent." > >> > > >> > An MPI implementation that can support Slow_RMA_xfer with no extra > costs > >> > (send/recv latency, memory, packet interrupts, CPU contention) > will > >> > simply > >> > ignore the assertion. > >> > > >> > BTW - I just took a look at the broad proposal and it may contain > >> > several > >> > things that cannot be done without a wake-able remote software > agent. > >> > That > >> > argues for Keith's idea of an RMA operation which closely matches > what > >> > RMA > >> > hardware does and a second one that brings along all the bells > >> > andwhistles. > >> > Maybe the assertion for an application that only uses the basic > RMA call > >> > or > >> > uses no RMA at all could be MPI_NO_KITCHEN_SINK (even more tongue > in > >> > cheek). > >> > > >> > ï¿½ ï¿½ ï¿½ ï¿½ ï¿½ ï¿½Dick > >> > > >> > > >> > Dick Treumann - MPI Team > >> > IBM Systems & Technology Group > >> > Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 > >> > Tele (845) 433-7846 Fax (845) 433-8363 > >> > > >> > > >> > mpi3-rma-bounces(a)lists.mpi-forum.org wrote on 09/16/2009 01:08:51 > PM: > >> > > >> >> [image removed] > >> >> > >> >> Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > pack/unpack > >> >> (?) > >> >> > >> >> Underwood, Keith D > >> >> > >> >> to: > >> >> > >> >> MPI 3.0 Remote Memory Access working group > >> >> > >> >> 09/16/2009 01:09 PM > >> >> > >> >> Sent by: > >> >> > >> >> mpi3-rma-bounces(a)lists.mpi-forum.org > >> >> > >> >> Please respond to "MPI 3.0 Remote Memory Access working group" > >> >> > >> >> But, going back to Bill's point: ï¿½performance across a range of > >> >> platforms is key. ï¿½While you can't have a function for every > usage > >> >> (well, you can, but it would get cumbersome at some point), it > may > >> >> be important to have a few levels of specialization in the API. > >> >> E.g. you could have two variants: > >> >> > >> >> MPI_Fast_RMA_xfer(): ï¿½no data types, no communicators, etc. > >> >> MPI_Slow_RMA_xfer(): include the kitchen sink. > >> >> > >> >> Yes, the naming is a little tongue in cheek ;-) > >> >> > >> >> Keith > >> >> > >> >> <snip> > >> > > >> > _______________________________________________ > >> > mpi3-rma mailing list > >> > mpi3-rma(a)lists.mpi-forum.org > >> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > >> > > >> > > >> > >> > >> > >> -- > >> Jeff Hammond > >> Argonne Leadership Computing Facility > >> jhammond(a)mcs.anl.gov / (630) 252-5381 > >> http://www.linkedin.com/in/jeffhammond > >> http://home.uchicago.edu/~jhammond/ > >> > >> _______________________________________________ > >> mpi3-rma mailing list > >> mpi3-rma(a)lists.mpi-forum.org > >> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > > > > _______________________________________________ > > mpi3-rma mailing list > > mpi3-rma(a)lists.mpi-forum.org > > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > > > > > > > > -- > Jeff Hammond > Argonne Leadership Computing Facility > jhammond(a)mcs.anl.gov / (630) 252-5381 > http://www.linkedin.com/in/jeffhammond > http://home.uchicago.edu/~jhammond/ > > _______________________________________________ > mpi3-rma mailing list > mpi3-rma(a)lists.mpi-forum.org > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma

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Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?)
by Jeff Hammond 16 Sep '09

16 Sep '09

I was having the same thoughts during my hideous commute today. Clearly, future systems may have very smart NICs or OSs which seamlessly manage communication co-processors to support, among other things, on-the-fly compression of non-contiguous buffers. However, until some vendor starts taking seriously about this, I don't think it is good idea to ignore the present hardware support of RMA and create a standard which can perform well when the first MPI-3 libraries become available. I'm not sure I understand all this talk about assertions upon initialization, since I was hoping that Raw_xfer and general-purpose xfer (Gen_xfer) would both always be available but that the former would be much faster in certain contexts, since implementing the Raw version close to hardware doesn't require much work and would not interfere with how Gen_xfer operates. Am I missing something? When the day finally comes that NICs can do Gen_xfer for a complex non-contiguous datatype as fast as Raw_xfer, would not the standard still be sufficient, since implementers would just produce a faster Gen_xfer and Raw_xfer would fade into the background? Raw_xfer is a pragmatic response to the present and near-future state of HPC systems rather than some sort of marquee feature to tape to the metaphorical refrigerator door. Best, Jeff On Wed, Sep 16, 2009 at 3:14 PM, Underwood, Keith D <keith.d.underwood(a)intel.com> wrote: > But we have to be very careful here.ï¿½ We donï¿½t want to overly constrain what > can be thought of as ï¿½fastï¿½.ï¿½ For example, I think it is perfectly > reasonable to implement accumulate on a NIC.ï¿½ Just because it doesnï¿½t exist > today doesnï¿½t mean that it shouldnï¿½t be part of the ï¿½fastï¿½ MPI call. > > > > Now, datatype conversionï¿½ it is nominally possible that a NIC could do > datatype conversion ï¿½ just like it is nominally possible to for a NIC to be > hooked to a Rube Goldberg device to implement MPI_Make_Breakfast ;-) > > > > Anyway, the point is that we need to be forward looking in defining ï¿½fastï¿½ > and ï¿½slowï¿½, not backward looking. > > > > Keith > > > > From: mpi3-rma-bounces(a)lists.mpi-forum.org > [mailto:[email protected]] On Behalf Of Richard Treumann > Sent: Wednesday, September 16, 2009 2:06 PM > To: MPI 3.0 Remote Memory Access working group > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided > pack/unpack (?) > > > > BINGO Jeff > > We might also remove the datatype argument and twin count arguments from > MPI_RMA_Raw_xfer just to eliminate the expectation that basic put/get do > datatype conversions when origin and target are on heterogeneous nodes. > There would be a single "count" argument and it represents the number of > contiguous bytes to be transferred. > > The assertion would be that there is no use of complex RMA. It would give > the implementation the option to leave its software agent dormant. Note that > having this assertion as an option for MPI_Init_asserted does not allow an > MPI implementation to avoid having an agent available. An application that > does not use the assertion can count on the agent being ready for any call > to "full baked" RMA. > > Dick > > Dick Treumann - MPI Team > IBM Systems & Technology Group > Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 > Tele (845) 433-7846 Fax (845) 433-8363 > > > mpi3-rma-bounces(a)lists.mpi-forum.org wrote on 09/16/2009 03:43:15 PM: > >> [image removed] >> >> Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?) >> >> Jeff Hammond >> >> to: >> >> MPI 3.0 Remote Memory Access working group >> >> 09/16/2009 03:44 PM >> >> Sent by: >> >> mpi3-rma-bounces(a)lists.mpi-forum.org >> >> Please respond to "MPI 3.0 Remote Memory Access working group" >> >> I think that there is a need for two interfaces; one which is a >> portable interface to the low-level truly one-sided bulk transfer >> operation and another which is completely general and is permitted to >> do operations which require remote agency. >> >> For example, I am aware of no NIC which can do accumulate on its own, >> hence RMA_ACC_SUM and related operations require remote agency, and >> thus this category of RMA operations are not truly one-sided. >> >> Thus the standard might support two xfer calls: >> >> MPI_RMA_Raw_xfer(origin_addr, origin_count, origin_datatype, >> target_mem, target_disp, target_count , target_rank, request) >> >> which is exclusively for transferring contiguous bytes from one place >> to another, i.e. does raw put/get only, and the second, which has been >> described already, which handles the general case, including >> accumulation, non-contiguous and other complex operations. >> >> The distinction over remote agency is extremely important from a >> implementation perspective since contiguous put/get operations can be >> performed in a fully asynchronous non-interrupting way with a variety >> of interconnects, and thus exposing this procedure in the MPI standard >> will allow for very efficient implementations on some systems. ï¿½It >> should also encourage MPI users to think about their RMA needs and how >> they might restructure their code to take advantage of the faster >> flavor of xfer when doing so requires little modification. >> >> Jeff >> >> On Wed, Sep 16, 2009 at 1:49 PM, Vinod tipparaju >> <tipparajuv(a)hotmail.com> wrote: >> >>My argument is that any RMA depends on a call at the origin being able >> >> to >> >> trigger activity at the target. Modern RMA hardware has the hooksto do >> >> the >> >> remote side ofï¿½MPI_Fast_RMA_xfer()ï¿½efficiently based on a call at the >> >> origin. Because these hooks are in the hardware they are simply there. >> >> They >> >> do not use the CPU or hurt performance of things that do use the CPU. >> > >> > I read this as an argument that says two interfaces are not necessary. >> > Having application author promise (during init) it will not do anything >> > that >> > needs an agent is certainly useful. Particularly when, as you state, >> > "having >> > this agent standing by hurts general performance". >> > The things that potentially cannot be done without an agent >> > (technically, >> > everything but atomics could be done with out need for any agents)are >> > users >> > choice through explicit usage. Users choses these attributes being aware >> > of >> > their cost hence they can indicate that they will not use them ahead of >> > time >> > when they don't use them. >> > I have repeatedly considered dropping the atomicity attribute, I am >> > unable >> > to because it makes programming (and thinking) so much easier for many >> > applications. >> > Vinod. >> > >> > >> > ________________________________ >> > To: mpi3-rma(a)lists.mpi-forum.org >> > From: treumann(a)us.ibm.com >> > Date: Wed, 16 Sep 2009 14:18:15 -0400 >> > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided >> > pack/unpack (?) >> > >> > The assertion could then be: MPI_NO_SLOW_RMA (also a bit tongue in >> > cheek) >> > >> > My argument is that any RMA depends on a call at the origin being able >> > to >> > trigger activity at the target. Modern RMA hardware has the hooks to do >> > the >> > remote side ofï¿½MPI_Fast_RMA_xfer() efficiently based on a call at the >> > origin. Because these hooks are in the hardware they are simply there. >> > They >> > do not use the CPU or hurt performance of things that do use the CPU. >> > >> > RMA hardware may not have the hooks to do the target side of any >> > arbitrary >> > MPI_Slow_RMA_xfer(). ï¿½As a result, support for the more complex RMA_xfer >> > may >> > require a wake-able software agent (thread maybe) to be standing by at >> > all >> > tasks just because they may become target of a Slow_RMA_xfer. >> > >> > If having this agent standing by hurts general performance of MPI >> > applications that will never make a call to Slow_RMA_xfer, why not let >> > the >> > applications author promise up front "I have no need of this agent." >> > >> > An MPI implementation that can support Slow_RMA_xfer with no extra costs >> > (send/recv latency, memory, packet interrupts, CPU contention) will >> > simply >> > ignore the assertion. >> > >> > BTW - I just took a look at the broad proposal and it may contain >> > several >> > things that cannot be done without a wake-able remote software agent. >> > That >> > argues for Keith's idea of an RMA operation which closely matches what >> > RMA >> > hardware does and a second one that brings along all the bells >> > andwhistles. >> > Maybe the assertion for an application that only uses the basic RMA call >> > or >> > uses no RMA at all could be MPI_NO_KITCHEN_SINK (even more tongue in >> > cheek). >> > >> > ï¿½ ï¿½ ï¿½ ï¿½ ï¿½ ï¿½Dick >> > >> > >> > Dick Treumann - MPI Team >> > IBM Systems & Technology Group >> > Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601 >> > Tele (845) 433-7846 Fax (845) 433-8363 >> > >> > >> > mpi3-rma-bounces(a)lists.mpi-forum.org wrote on 09/16/2009 01:08:51 PM: >> > >> >> [image removed] >> >> >> >> Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack >> >> (?) >> >> >> >> Underwood, Keith D >> >> >> >> to: >> >> >> >> MPI 3.0 Remote Memory Access working group >> >> >> >> 09/16/2009 01:09 PM >> >> >> >> Sent by: >> >> >> >> mpi3-rma-bounces(a)lists.mpi-forum.org >> >> >> >> Please respond to "MPI 3.0 Remote Memory Access working group" >> >> >> >> But, going back to Billï¿½s point: ï¿½performance across a range of >> >> platforms is key. ï¿½While you canï¿½t have a function for every usage >> >> (well, you can, but it would get cumbersome at some point), it may >> >> be important to have a few levels of specialization in the API. >> >> E.g. you could have two variants: >> >> >> >> MPI_Fast_RMA_xfer(): ï¿½no data types, no communicators, etc. >> >> MPI_Slow_RMA_xfer(): include the kitchen sink. >> >> >> >> Yes, the naming is a little tongue in cheek ;-) >> >> >> >> Keith >> >> >> >> <snip> >> > >> > _______________________________________________ >> > mpi3-rma mailing list >> > mpi3-rma(a)lists.mpi-forum.org >> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma >> > >> > >> >> >> >> -- >> Jeff Hammond >> Argonne Leadership Computing Facility >> jhammond(a)mcs.anl.gov / (630) 252-5381 >> http://www.linkedin.com/in/jeffhammond >> http://home.uchicago.edu/~jhammond/ >> >> _______________________________________________ >> mpi3-rma mailing list >> mpi3-rma(a)lists.mpi-forum.org >> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > > _______________________________________________ > mpi3-rma mailing list > mpi3-rma(a)lists.mpi-forum.org > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma > > -- Jeff Hammond Argonne Leadership Computing Facility jhammond(a)mcs.anl.gov / (630) 252-5381 http://www.linkedin.com/in/jeffhammond http://home.uchicago.edu/~jhammond/

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