- mpiwg-rma - mailman3.cels.anl.gov

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

16 Sep '09

It might make sense to have several assertables: MPI_RMA_NO_HETERO MPI_RMA_NO_ACCUM, MPI_RMA_NO_BREAKFAST, etc. Of course what these would be may be just as difficult, e.g., accumulate may be easy to do on a NIC for integers but not floating point. -d On 09/16/2009 03:14 PM, 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/ >> >> _______________________________________________ >> 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

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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

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

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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

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

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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 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 hooks to 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 and whistles. > 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/

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

16 Sep '09

>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. 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 and whistles. 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>

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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

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 and whistles. 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). That particular option naming might be misinterpreted to apply to a bit more of the standard than just RMA ;-) Keith

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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

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 and whistles. 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>

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

16 Sep '09

I am OK with having two interfaces, one which is WRT comm_world and only supports MPI_BYTE and the other a more general one. Thanks,Vinod. From: keith.d.underwood(a)intel.com To: mpi3-rma(a)lists.mpi-forum.org Date: Wed, 16 Sep 2009 11:08:51 -0600 Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?) 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 From: mpi3-rma-bounces(a)lists.mpi-forum.org [mailto:[email protected]] On Behalf Of Vinod tipparaju Sent: Wednesday, September 16, 2009 10:58 AM To: MPI 3.0 Remote Memory Access working group Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?) "else" condition is non-trivial when > one considers the latency of the BlueGene/P interconnect, the memory > latency, the clock rate and the absence of dynamic communication > threads. Forgive me for being specific to one machine, but it is the > benchmark data I have obtained that inspires me to comment on these > issues You have a valid point, but, as you already note, BG is an exception. It is a "slowed down" exception. In most cases, latency associated with branching will have to continue to be insignificant (by 2 orders of magnitude) WRT to network latency. Hence what needs to be done for non-contiguous data WRT and in the network is more important than a mere branch. What the network does or doesn't do to support non-contiguous data transfers is critical. The input/guidance forum gives hence is more important when taken during the network design -- many things may be fixed in an implementations but one cannot fix what a network lack here. Basically, you believe having two calls instead of one will help because you believe you can help the implementation by giving hints on what you need from this particular RMA transfer. I added attributes for a similar reason. Usages are many, we cannot have an interface for each usage. We can however use attributes to give hints/orders that may help implementations utilize networks better or know when to give up. We can also support features such as Binding (discussed in the last meeting) to help reduce latency. Thanks, Vinod. > Date: Wed, 16 Sep 2009 09:28:31 -0500 > From: jeff.science(a)gmail.com > To: mpi3-rma(a)lists.mpi-forum.org > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?) > > > 1) MPI is implemented as a library. There is no connection between the > > analysis the compiler can do and the MPI implementation so what can be > > "known" by the compiler cannot be communicated to the implementation. > > Yes, I am well aware of this. What I mean is that I, the programmer, > can choose to use in my source code the function or function arguments > which invoke the fastest correct call, thus preventing MPI from > proceeding through all the logic necessary to decipher the most > general case. > > For example, if "xfer" contained a flag just for > contiguous/non-contiguous, I could do the following in my code: > > ====================================== > /* get row */ > ( invoke xfer_get with contig flag set and using the simple datatype > for a contiguous vector of floats) > > /* get column */ > ( invoke xfer_get with noncontig flag set and using derived datatype > for the column vector of a matrix of floats) > ====================================== > > knowing that MPI can implement "xfer" like this: > > ====================================== > xfer_get ( ... ){ > if (contig){ > low_level_comm_api_get(...) > } else { > process_datatype(...) /* figure out how many contiguous segments > are in this */ > #ifdef (FAST_INJECTION_RATE) > (for all contig segments in datatype){ > low_level_comm_api_get(...,datatype,...) > } > #elseif (SLOW_INJECTION_RATE) > active_message(invoke_remote_pack(datatype,packed_buffer)) > low_level_comm_api_get(...,packed_buffer,...) > local_unpack(packed_buffer,datatype) > #endif > } > ====================================== > > If you look at ARMCI or the MPI-2 one-sided implementation over DCMF, > the overhead associated with the "else" condition is non-trivial when > one considers the latency of the BlueGene/P interconnect, the memory > latency, the clock rate and the absence of dynamic communication > threads. Forgive me for being specific to one machine, but it is the > benchmark data I have obtained that inspires me to comment on these > issues. > > On machines with capped message injection rates, implementing the > "else" condition might be best done in a completely different manner > with one-sided packing, which is why I brought that up in the first > place. Of course, the overhead of packing is large and thus should be > reserved exclusively for the case where hardware prefers it, and then > only for "xfer" calls where it is absolutely necessary, otherwise it > should be ignored completely. > > > 2) There is no requirement in MPI that all tasks of an application run the > > same executable so even if one piece of source code that will be used in an > > application is free of complex datatypes, only the human being who knows if > > other pieces of code will be used by some tasks can make the judgement that > > there will be no calls with complex datatypes by any participating task. > > While this may be true for MPI ("no requirement that all tasks of an > application run the same executable"), it may not be true for all > systems on which MPI is to be run. Hence, I do not think this is a > useful solution. > > It is exactly my point that the human knows which calls require > complex datatypes and should be able to make calls to "xfer" in light > of this knowledge. Currently the "xfer" API does not support > bypassing complex datatypes and it is my contention that, in some > cases, doing so will improve performance markedly. > > Best, > > Jeff > > -- > 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 Underwood, Keith D 16 Sep '09

16 Sep '09

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 From: mpi3-rma-bounces(a)lists.mpi-forum.org [mailto:[email protected]] On Behalf Of Vinod tipparaju Sent: Wednesday, September 16, 2009 10:58 AM To: MPI 3.0 Remote Memory Access working group Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?) "else" condition is non-trivial when > one considers the latency of the BlueGene/P interconnect, the memory > latency, the clock rate and the absence of dynamic communication > threads. Forgive me for being specific to one machine, but it is the > benchmark data I have obtained that inspires me to comment on these > issues You have a valid point, but, as you already note, BG is an exception. It is a "slowed down" exception. In most cases, latency associated with branching will have to continue to be insignificant (by 2 orders of magnitude) WRT to network latency. Hence what needs to be done for non-contiguous data WRT and in the network is more important than a mere branch. What the network does or doesn't do to support non-contiguous data transfers is critical. The input/guidance forum gives hence is more important when taken during the network design -- many things may be fixed in an implementations but one cannot fix what a network lack here. Basically, you believe having two calls instead of one will help because you believe you can help the implementation by giving hints on what you need from this particular RMA transfer. I added attributes for a similar reason. Usages are many, we cannot have an interface for each usage. We can however use attributes to give hints/orders that may help implementations utilize networks better or know when to give up. We can also support features such as Binding (discussed in the last meeting) to help reduce latency. Thanks, Vinod. > Date: Wed, 16 Sep 2009 09:28:31 -0500 > From: jeff.science(a)gmail.com > To: mpi3-rma(a)lists.mpi-forum.org > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?) > > > 1) MPI is implemented as a library. There is no connection between the > > analysis the compiler can do and the MPI implementation so what can be > > "known" by the compiler cannot be communicated to the implementation. > > Yes, I am well aware of this. What I mean is that I, the programmer, > can choose to use in my source code the function or function arguments > which invoke the fastest correct call, thus preventing MPI from > proceeding through all the logic necessary to decipher the most > general case. > > For example, if "xfer" contained a flag just for > contiguous/non-contiguous, I could do the following in my code: > > ====================================== > /* get row */ > ( invoke xfer_get with contig flag set and using the simple datatype > for a contiguous vector of floats) > > /* get column */ > ( invoke xfer_get with noncontig flag set and using derived datatype > for the column vector of a matrix of floats) > ====================================== > > knowing that MPI can implement "xfer" like this: > > ====================================== > xfer_get ( ... ){ > if (contig){ > low_level_comm_api_get(...) > } else { > process_datatype(...) /* figure out how many contiguous segments > are in this */ > #ifdef (FAST_INJECTION_RATE) > (for all contig segments in datatype){ > low_level_comm_api_get(...,datatype,...) > } > #elseif (SLOW_INJECTION_RATE) > active_message(invoke_remote_pack(datatype,packed_buffer)) > low_level_comm_api_get(...,packed_buffer,...) > local_unpack(packed_buffer,datatype) > #endif > } > ====================================== > > If you look at ARMCI or the MPI-2 one-sided implementation over DCMF, > the overhead associated with the "else" condition is non-trivial when > one considers the latency of the BlueGene/P interconnect, the memory > latency, the clock rate and the absence of dynamic communication > threads. Forgive me for being specific to one machine, but it is the > benchmark data I have obtained that inspires me to comment on these > issues. > > On machines with capped message injection rates, implementing the > "else" condition might be best done in a completely different manner > with one-sided packing, which is why I brought that up in the first > place. Of course, the overhead of packing is large and thus should be > reserved exclusively for the case where hardware prefers it, and then > only for "xfer" calls where it is absolutely necessary, otherwise it > should be ignored completely. > > > 2) There is no requirement in MPI that all tasks of an application run the > > same executable so even if one piece of source code that will be used in an > > application is free of complex datatypes, only the human being who knows if > > other pieces of code will be used by some tasks can make the judgement that > > there will be no calls with complex datatypes by any participating task. > > While this may be true for MPI ("no requirement that all tasks of an > application run the same executable"), it may not be true for all > systems on which MPI is to be run. Hence, I do not think this is a > useful solution. > > It is exactly my point that the human knows which calls require > complex datatypes and should be able to make calls to "xfer" in light > of this knowledge. Currently the "xfer" API does not support > bypassing complex datatypes and it is my contention that, in some > cases, doing so will improve performance markedly. > > Best, > > Jeff > > -- > 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 Vinod tipparaju 16 Sep '09

16 Sep '09

"else" condition is non-trivial when > one considers the latency of the BlueGene/P interconnect, the memory > latency, the clock rate and the absence of dynamic communication > threads. Forgive me for being specific to one machine, but it is the > benchmark data I have obtained that inspires me to comment on these > issues You have a valid point, but, as you already note, BG is an exception. It is a "slowed down" exception. In most cases, latency associated with branching will have to continue to be insignificant (by 2 orders of magnitude) WRT to network latency. Hence what needs to be done for non-contiguous data WRT and in the network is more important than a mere branch. What the network does or doesn't do to support non-contiguous data transfers is critical. The input/guidance forum gives hence is more important when taken during the network design -- many things may be fixed in an implementations but one cannot fix what a network lack here. Basically, you believe having two calls instead of one will help because you believe you can help the implementation by giving hints on what you need from this particular RMA transfer. I added attributes for a similar reason. Usages are many, we cannot have an interface for each usage. We can however use attributes to give hints/orders that may help implementations utilize networks better or know when to give up. We can also support features such as Binding (discussed in the last meeting) to help reduce latency. Thanks,Vinod. > Date: Wed, 16 Sep 2009 09:28:31 -0500 > From: jeff.science(a)gmail.com > To: mpi3-rma(a)lists.mpi-forum.org > Subject: Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?) > > > 1) MPI is implemented as a library. There is no connection between the > > analysis the compiler can do and the MPI implementation so what can be > > "known" by the compiler cannot be communicated to the implementation. > > Yes, I am well aware of this. What I mean is that I, the programmer, > can choose to use in my source code the function or function arguments > which invoke the fastest correct call, thus preventing MPI from > proceeding through all the logic necessary to decipher the most > general case. > > For example, if "xfer" contained a flag just for > contiguous/non-contiguous, I could do the following in my code: > > ====================================== > /* get row */ > ( invoke xfer_get with contig flag set and using the simple datatype > for a contiguous vector of floats) > > /* get column */ > ( invoke xfer_get with noncontig flag set and using derived datatype > for the column vector of a matrix of floats) > ====================================== > > knowing that MPI can implement "xfer" like this: > > ====================================== > xfer_get ( ... ){ > if (contig){ > low_level_comm_api_get(...) > } else { > process_datatype(...) /* figure out how many contiguous segments > are in this */ > #ifdef (FAST_INJECTION_RATE) > (for all contig segments in datatype){ > low_level_comm_api_get(...,datatype,...) > } > #elseif (SLOW_INJECTION_RATE) > active_message(invoke_remote_pack(datatype,packed_buffer)) > low_level_comm_api_get(...,packed_buffer,...) > local_unpack(packed_buffer,datatype) > #endif > } > ====================================== > > If you look at ARMCI or the MPI-2 one-sided implementation over DCMF, > the overhead associated with the "else" condition is non-trivial when > one considers the latency of the BlueGene/P interconnect, the memory > latency, the clock rate and the absence of dynamic communication > threads. Forgive me for being specific to one machine, but it is the > benchmark data I have obtained that inspires me to comment on these > issues. > > On machines with capped message injection rates, implementing the > "else" condition might be best done in a completely different manner > with one-sided packing, which is why I brought that up in the first > place. Of course, the overhead of packing is large and thus should be > reserved exclusively for the case where hardware prefers it, and then > only for "xfer" calls where it is absolutely necessary, otherwise it > should be ignored completely. > > > 2) There is no requirement in MPI that all tasks of an application run the > > same executable so even if one piece of source code that will be used in an > > application is free of complex datatypes, only the human being who knows if > > other pieces of code will be used by some tasks can make the judgement that > > there will be no calls with complex datatypes by any participating task. > > While this may be true for MPI ("no requirement that all tasks of an > application run the same executable"), it may not be true for all > systems on which MPI is to be run. Hence, I do not think this is a > useful solution. > > It is exactly my point that the human knows which calls require > complex datatypes and should be able to make calls to "xfer" in light > of this knowledge. Currently the "xfer" API does not support > bypassing complex datatypes and it is my contention that, in some > cases, doing so will improve performance markedly. > > Best, > > Jeff > > -- > 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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