February 2009 - mpiwg-ft - mailman3.cels.anl.gov

Re: [Mpi3-ft] Communicator Virtualization as a step forward
by Greg Bronevetsky 18 Feb '09

18 Feb '09

My point is that each MPI implementation will have to negotiate with each checkpointer what this call will mean on each platform, meaning that it is upto the implementation-specific call to do all that magic. It doesn't save the application any real effort to call this routine by its implementation-specific name rather than a unified name given that the other two major problems are not solved: - Providing a uniform way for checkpointers and MPI implementations to interact (THE major for having this proposal in the first place) and - Support for more than the synch-and-stop checkpointing protocol, which does scales poorly with rising failure rates. My point here is that covering one of the three major issues is not good enough to modify the MPI specification and I don't think that we'll ever be able to do more than that. We're pretty much agreed in the working group that the first point above cannot be done and I don't think that the second can be done without providing callbacks to be called for each incoming/outgoing message. Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 bronevetsky1(a)llnl.gov http://greg.bronevetsky.com At 08:40 AM 2/18/2009, Supalov, Alexander wrote: >Thanks. How will the checkpointer-specific call talk to the MPI? > >-----Original Message----- >From: mpi3-ft-bounces(a)lists.mpi-forum.org >[mailto:[email protected]] On Behalf Of Greg Bronevetsky >Sent: Wednesday, February 18, 2009 5:29 PM >To: MPI 3.0 Fault Tolerance and Dynamic Process Control working >Group; MPI 3.0 Fault Tolerance and Dynamic Process Control working Group >Subject: Re: [Mpi3-ft] Communicator Virtualization as a step forward > > > >Thanks. How will you let the MPI know the checkpoint is coming, to > >give it a fair chance to prepare to this and then recover after the > >checkpoint? This is akin to the MPI_Finalize/MPI_Init in some sense, > >midway thru the job, hence the analogy. > >Just use the checkpointer-specific call. The call is going to have >checkpointer-specific semantics, so why not give it a >checkpointer-specific name? I understand that there is some use to >allowing applications to use the same name across all checkpointers >but the bar should be higher than that for adding something to the >standard. Also, right now the whole approach inherently only supports >one checkpointing protocol: synch-and-stop. If we can work out a more >generic API that supports other protocols I think that it may have >enough value to be included in the spec. Right now it still hasn't >passed the bar. > >Greg Bronevetsky >Post-Doctoral Researcher >1028 Building 451 >Lawrence Livermore National Lab >(925) 424-5756 >bronevetsky1(a)llnl.gov >http:// greg.bronevetsky.com > >_______________________________________________ >mpi3-ft mailing list >mpi3-ft(a)lists.mpi-forum.org >http:// lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft >--------------------------------------------------------------------- >Intel GmbH >Dornacher Strasse 1 >85622 Feldkirchen/Muenchen Germany >Sitz der Gesellschaft: Feldkirchen bei Muenchen >Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer >Registergericht: Muenchen HRB 47456 Ust.-IdNr. >VAT Registration No.: DE129385895 >Citibank Frankfurt (BLZ 502 109 00) 600119052 > >This e-mail and any attachments may contain confidential material for >the sole use of the intended recipient(s). Any review or distribution >by others is strictly prohibited. If you are not the intended >recipient, please contact the sender and delete all copies. > > >_______________________________________________ >mpi3-ft mailing list >mpi3-ft(a)lists.mpi-forum.org >http:// lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft

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[Mpi3-ft] Con call
by Richard Graham 18 Feb '09

18 Feb '09

Just a reminder that we do not have a call this week we are scheduled for next week. Rich

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Re: [Mpi3-ft] Communicator Virtualization as a step forward
by Supalov, Alexander 18 Feb '09

18 Feb '09

Thanks. How will the checkpointer-specific call talk to the MPI? -----Original Message----- From: mpi3-ft-bounces(a)lists.mpi-forum.org [mailto:[email protected]] On Behalf Of Greg Bronevetsky Sent: Wednesday, February 18, 2009 5:29 PM To: MPI 3.0 Fault Tolerance and Dynamic Process Control working Group; MPI 3.0 Fault Tolerance and Dynamic Process Control working Group Subject: Re: [Mpi3-ft] Communicator Virtualization as a step forward >Thanks. How will you let the MPI know the checkpoint is coming, to >give it a fair chance to prepare to this and then recover after the >checkpoint? This is akin to the MPI_Finalize/MPI_Init in some sense, >midway thru the job, hence the analogy. Just use the checkpointer-specific call. The call is going to have checkpointer-specific semantics, so why not give it a checkpointer-specific name? I understand that there is some use to allowing applications to use the same name across all checkpointers but the bar should be higher than that for adding something to the standard. Also, right now the whole approach inherently only supports one checkpointing protocol: synch-and-stop. If we can work out a more generic API that supports other protocols I think that it may have enough value to be included in the spec. Right now it still hasn't passed the bar. Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 bronevetsky1(a)llnl.gov http://greg.bronevetsky.com _______________________________________________ mpi3-ft mailing list mpi3-ft(a)lists.mpi-forum.org http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft --------------------------------------------------------------------- Intel GmbH Dornacher Strasse 1 85622 Feldkirchen/Muenchen Germany Sitz der Gesellschaft: Feldkirchen bei Muenchen Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer Registergericht: Muenchen HRB 47456 Ust.-IdNr. VAT Registration No.: DE129385895 Citibank Frankfurt (BLZ 502 109 00) 600119052 This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies.

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Re: [Mpi3-ft] Communicator Virtualization as a step forward
by Greg Bronevetsky 18 Feb '09

18 Feb '09

>Thanks. How will you let the MPI know the checkpoint is coming, to >give it a fair chance to prepare to this and then recover after the >checkpoint? This is akin to the MPI_Finalize/MPI_Init in some sense, >midway thru the job, hence the analogy. Just use the checkpointer-specific call. The call is going to have checkpointer-specific semantics, so why not give it a checkpointer-specific name? I understand that there is some use to allowing applications to use the same name across all checkpointers but the bar should be higher than that for adding something to the standard. Also, right now the whole approach inherently only supports one checkpointing protocol: synch-and-stop. If we can work out a more generic API that supports other protocols I think that it may have enough value to be included in the spec. Right now it still hasn't passed the bar. Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 bronevetsky1(a)llnl.gov http://greg.bronevetsky.com

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Re: [Mpi3-ft] Communicator Virtualization as a step forward
by Supalov, Alexander 18 Feb '09

18 Feb '09

Thanks. How will you let the MPI know the checkpoint is coming, to give it a fair chance to prepare to this and then recover after the checkpoint? This is akin to the MPI_Finalize/MPI_Init in some sense, midway thru the job, hence the analogy. -----Original Message----- From: mpi3-ft-bounces(a)lists.mpi-forum.org [mailto:[email protected]] On Behalf Of Greg Bronevetsky Sent: Tuesday, February 17, 2009 6:21 PM To: MPI 3.0 Fault Tolerance and Dynamic Process Control working Group; MPI 3.0 Fault Tolerance and Dynamic Process Control working Group Subject: Re: [Mpi3-ft] Communicator Virtualization as a step forward >Consider interaction of the MPI_Init/MPI_Finalize with the >underlying OS and job managers. It's about as undefined as that of >the mentioned calls with the checkpointer. Nevertheless, many usable >implementations can cope with that quite nicely. By the way, this is >how the checkpoint/restarting could work as well, providing the >checkpointer at hand with the MPI ready to be checkpointed/restarted >at a well defined point of the MPI program. After all, this is how >it is done now: a configuration flag tells the MPI what checkpointer >to expect. This could be refined, turned into dynamic recognition of >the active checkpointer, etc. This is all trivial. Because this is a separate argument, I decided to answer it separately. I fully support this approach to providing checkpointing to MPI applications and I don't see any extensions to the MPI specification needed to support it. The only extensions that we've developed so far have poor semantics and thus boil down to having a single name for a bunch of semantically different operations that MPI may perform. At this point we might as well not standardize this name and let applications use multiple names for what is really a different operation on each combination of checkpointer and system. Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 bronevetsky1(a)llnl.gov http://greg.bronevetsky.com _______________________________________________ mpi3-ft mailing list mpi3-ft(a)lists.mpi-forum.org http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft --------------------------------------------------------------------- Intel GmbH Dornacher Strasse 1 85622 Feldkirchen/Muenchen Germany Sitz der Gesellschaft: Feldkirchen bei Muenchen Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer Registergericht: Muenchen HRB 47456 Ust.-IdNr. VAT Registration No.: DE129385895 Citibank Frankfurt (BLZ 502 109 00) 600119052 This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies.

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[Mpi3-ft] MPI Fault Tolerance scenarios
by Erez Haba 17 Feb '09

17 Feb '09

Hello all, In our last meeting we decided to build a set of FT scenarios/programs to help us understand the details of the interface need to support those scenarios. We also decided to start with very simple scenarios and add more complex ones as we understand the former better. I hope that starting with simple scenarios will help us build a solid foundation on which we can build the more complex solutions. When we build an FT solution we will focus on the scenario as described, without complicating the solution just because it would be needed later for a more complex one. The time will come later to modify the solution as we acquire more knowledge and built the foundations. Hence, any proposal or change that we make needs to fit exactly the scenario (and all those that we previously looked at) but no more. For example in the first scenario that we'll look at there is no need for saving communicator state or error callback; but they might be required later. Note that these scenarios focus on process FT rather than checkpoint/restart or network degradation. I assume we'll do the latter later. Scenario #1: Very Simple Master-Workers Description This is a very simple master-workers scenario. However simple, we were asked many times by customers to support FT in this scenario. In this case the MPI application running with n processes, where rank 0 is used as the master and n-1 ranks are used as workers. The master generates work (either by getting it directly from user input, or reading a file) and sends it for processing to a free worker rank. The master sends requests and receives replies using MPI point-to-point communication. The workers wait for the incoming message, upon arrival the worker computes the result and sends it back to the master. The master stores the result to a log file. Hardening: The goal is to harden the workers, the master itself is not FT, thus if it fails the entire application fails. In this case the workers are FT, and are replaced to keep computation power for this application. (a twist: if a worker cannot be recovered the master can work with a smaller set of clients up to a low watermark). Worker The worker waits on a blocking receive when a message arrives it process it. If a done message arrives the worker finalizes MPI and exit normally. Hardening: There is not special requirement for hardening here. If the worker encounters a communication problem with the master, it means that the master is down and it's okay to abort the entire job. Thus, it will use the default error handler (which aborts on errors). Note that we do not need to modify the client at all to make the application FT (except the master). Pseudo code for the hardened worker: int main() { MPI_Init() for(;;) { MPI_Recv(src=0, &query, MPI_COMM_WORLD); if(is_done_msg(query)) break; process_query(&query, &answer); MPI_Send(dst=0, &answer, MPI_COMM_WORLD); } MPI_Finalize() } Notice that for this FT code there is no requirement for the worker to rejoin the comm. As the only communicator used is MPI_COMM_WORLD. Master The master code reads queries from a stream and passes them on to the workers to process. The master goes through several phases. In the initialization phase it sends the first request to each one of the ranks; in the second one it shuts down any unnecessary ranks (if the job is too small); I the third phase it enters its progress engine where it handles replies (answers), process recovery and termination (on input end). Hardening: It is the responsibility of the master to restart any failing workers and make sure that the request (query) did not get lost if a worker fails. Hence, every time an error is detected the master will move the worker into repairing state and move its workload to other workers. The master runs with errors returned rather than aborted One thing to note about the following code: it is not optimized. I did not try to overlap computation with communication (which is possible) I tried to keep it as simple as possible for the purpose of discussion. Pseudo code for the hardened master; the code needed for repairing the failed ranks is highlighted in yellow. int main() { MPI_Init() >>> MPI_Comm_set_errhandler(MPI_COMM_WORLD, MPI_ERRORS_RETURN); MPI_Comm_size(MPI_COMM_WORLD, &n); MPI_Request r[n] = MPI_REQUEST_NULL; QueryMessage q[n]; AnswerMessage a[n]; int active_workers = 0; >>> bool repairing[n] = false; // // Phase 1: send initial requests // for(int i = 1; i < n; i++) { if(get_next_query(stream, &q[i]) == eof) break; active_workers++; MPI_Send(dest=i, &q[i], MPI_COMM_WORLD); rc = MPI_Irecv(src=i, buffer=&a[x], request=&r[x], MPI_COMM_WORLD) >>> if(rc != MPI_SUCCESS) >>> { >>> start_repair(i, repairing, q, a, r, stream); >>> } } // // Phase 2: finalize any unnecessary ranks // for(int i = active_workers + 1; i < n; i++) { MPI_Send(dest=i, &done_msg, MPI_COMM_WORLD); } // // The progress engine. Get answers; send new requests and handle // process repairs // while(active_workers != 0) { rc = MPI_Waitany(n, r, &i, MPI_STATUS_IGNORE); >>> if(!repairing[i]) >>> { >>> if(rc != MPI_SUCCESS) >>> { >>> start_repair(i, repairing, q, a, r, stream) >>> continue; >>> } process_answer(&a[i]); >>> } >>> else if(rc != MPI_SUCCESS) >>> { >>> active_workers--; >>> { if(get_next_input(stream, &q[i]) == eof) { active_workers--; MPI_Send(dest=i, &done_msg) { else { MPI_Send(dest=i, &q[i]) rc = MPI_Irecv(src=i, buffer=&a[i], request=&r[i], MPI_COMM_WORLD) >>> if(rc != MPI_SUCCESS) >>> { >>> start_repair(i, repairing, q, a, r, stream); >>> } } } MPI_Finalize() } >>> void start_repair(int i, int repairing[], Query q[], Answer q[], MPI_Request r[], Stream stream) >>> { >>> repairing[i] = true; >>> push_query_back(stream, &q[i]); >>> MPI_Comm_Irepair(MPI_COMM_WORLD, i, &r[i]); >>> } Logic description (without FT) The master code keeps track of the number of active workers through the active_workers variable. It is solely used for the purpose of shutdown. When the master is out of input, it shuts-down the workers by sending them 'done' message. It decrease the number of active workers and finalizes when this number reaches zero. The master's progress engine waits on a vector of requests (note that entry 0 is not used, as to simplify the code); one it gets an answer it processes it and sends the next query to that worker until it's out of input. Logic description (with FT) The master detects a faulty client either synchronously when it ties to initiate an async receive (no need to check the send, the assumption is that if send failed, so will the receive call), or async when the async receive completes with an error. Once an error detected (and identified as a faulty client, more about this later), the master starts an async repair of that client. If the repair succeeds, new work is sent to that client. If it does not, the number of active workers is decreased and the master has to live with less processing power. The code above assumes that if the returned code is an error, it should repair the worker; however as we discussed, there could very well be many different reasons for an error here, which not all are related to process failure; for that we might use something in lines of if(MPI_Error_event(rc) == MPI_EVENT_PROCESS_DOWN)... it would be the responsibility of the MPI implementation to encode or store the event related to the returned error code. (Note: in MPICH2 there is a mechanism that enables encoding extended error information in the error code, which then can be retrieved using MPI_Error_string) Conclusions I believe that the solution above describes what we have discussed in the last meeting. The required API's to support this FT are really minimal but already cover a good set of users. Please, send your comments. Thoughts? Thanks, .Erez P.S. I will post this on the FT wiki pages (with the feedbac). P.P.S. there is one more scenario that we discussed, and extension of the master-workers model. I will try to get it write us as-soon-as-posible.

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Re: [Mpi3-ft] Communicator Virtualization as a step forward
by Greg Bronevetsky 17 Feb '09

17 Feb '09

>Consider interaction of the MPI_Init/MPI_Finalize with the >underlying OS and job managers. It's about as undefined as that of >the mentioned calls with the checkpointer. Nevertheless, many usable >implementations can cope with that quite nicely. By the way, this is >how the checkpoint/restarting could work as well, providing the >checkpointer at hand with the MPI ready to be checkpointed/restarted >at a well defined point of the MPI program. After all, this is how >it is done now: a configuration flag tells the MPI what checkpointer >to expect. This could be refined, turned into dynamic recognition of >the active checkpointer, etc. This is all trivial. Because this is a separate argument, I decided to answer it separately. I fully support this approach to providing checkpointing to MPI applications and I don't see any extensions to the MPI specification needed to support it. The only extensions that we've developed so far have poor semantics and thus boil down to having a single name for a bunch of semantically different operations that MPI may perform. At this point we might as well not standardize this name and let applications use multiple names for what is really a different operation on each combination of checkpointer and system. Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 bronevetsky1(a)llnl.gov http://greg.bronevetsky.com

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Re: [Mpi3-ft] Communicator Virtualization as a step forward
by Greg Bronevetsky 17 Feb '09

17 Feb '09

Yes, this is definitely a topic for voice conversation but let me continue this over email because I think that I can describe my position more precisely based on your comments. The definition of whether fault notification API is working or not is Not whether the underlying MPI implementation is guaranteed to do something when some low-level event happens. It is working if the rate of unrecoverable failures is below on application-specified bound, with all the remaining failures converted into notifications. This is the same for MPI_Send/MPI_Recv, which work not if they provide a certain level of performance but whether they deliver data or not. These are both high-level specifications and we don't care about how they are implemented at the network level. Having said that, there is one large difference between the two cases, which is the social dynamics. The MPI_Send/MPI_Recv API has non-trivial support in all MPI implementations and you can get better MPI performance by either using the same MPI implementation on a better machine or by switching to a higher performing MPI implementation. This fluidity works to help encourage MPI implementations to support MPI_Send/MPI_Recv because application developers will be confident that they'll get good use out them. In contrast, with the fault notification API the odds are that some MPI implementations will provide trivial support for the fault notification API while others will provide good or configurable support. As such, if a developer wants their unrecoverable error rate to be below a certain bound, they will have less freedom to match MPI implementations to systems because if a given machine has too high an error rate, an unreliable MPI implementation will not provide sufficient reliability no matter what you do. You will need to switch to a different MPI implementation. Your claim is that this will cause application developers to not bother coding fault tolerance into their applications because they can't be sure that it will do any good on their favorite MPI implementation. This makes sense but I believe that it is incorrect. The reason is that the driver for fault tolerance is not applications or MPI implementations but the hardware systems. Today's large machines are sufficiently unreliable that today's applications have to provide their own fault tolerance solutions without any support from MPI. For large-scale applications, fault tolerance is not a luxury but a necessity, so if they need to switch MPIs in order to get it, they will (assuming that not too much performance is lost). If an existing application only runs on small, reliable systems, it will never need to be recoded to use the fault notification API. However, if it grows to a large enough scale for faults to be a problem, fault notification support will need to be coded in because there is no other choice besides just failing all the time. As such, the reduced "liquidity" of the fault notification API is more than offset by the strong driving force that system failures have on large-scale applications. Using it will not be a choice but a necessity and as such, there will be a sub-community of applications/MPI implementations that use it/provide it, and the rest who ignore it. More likely, since most MPI implementations want to support large-scale systems as well as smaller ones, these implementations will provide a way for applications to request different levels of fault notification support, which will then overcome the "liquidity" problem that you've identified. So that's my argument in favor of fault notification. Fault notification is a well-defined and useful API that is less "liquid" than much of the existing MPI specification. However, because there is a significant sub-community of applications and systems on which faults are a real problem, this weakness will be more than offset by the sheer necessity of running on systems that fail frequently. In this context the fault notification API will allow MPI implementation to bound the rate of unrecoverable failures even on unreliable hardware platforms. Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 bronevetsky1(a)llnl.gov http://greg.bronevetsky.com At 04:55 AM 2/17/2009, Supalov, Alexander wrote: >Hi, > >Thanks. I think we're coming from different corners, and the main >problem is that the two criteria of yours, namely, "self-consistent >specification and usefulness of the chosen abstraction level" are >not sufficient for me. The key here appears to be in the >"usefulness" that you seem to understand as "this can be used in >principle on my future Petascale machine running my favorite MPI" >and mine "to be used in a real application, an API should be useful >in a wide, a priori known variety of practically relevant situations >that do not depend on the MPI implementation at hand or the platform involved". > >Maybe we should have a call or something to discuss this live, >without email getting into way. The rest is just an outline of what >I'd have to say if we met. > >I say that we should define a standard set of the faults that will >be detected, and then say what implementations should provide what >level of service to be compliant with a particular level of the FT >support that the standard is to specify in clear terms. If we don't >do this, we will only prepare a good ground for MPI-4, were we will >have to fix the flaws of the MPI-3. I'm afraid that at the moment >we're driving in this very direction, and this is why. > >The ultimate test of a spec is a number of implementations that are >widely used as specified. I'm concerned that the current >notification spec is too weak to be appealing to the commercial >users and implementors. I'm concerned about this because the >semantics of an API that kicks in under unknown circumstances are >surely ill-defined, and the ROI of using it in any given application >cannot be assessed upfront. This means that the investment is >unlikely to be made - either on the commercial MPI implementation or >on the commercial MPI application side of the equation. > >In other words, why would I care to use an API if I were not aware >of when and how this would help my application run weeks rather than >days between the faults? And I don't mean here a student with a >diploma or PhD thesis to write and forget about. I mean real life >commercial developers who need to justify every bit of what they are >doing by showing positive ROI to their management in the times of >worldwide economic crisis. Justify by promising to earn more money >than they are going to spend for the development, or else. > >Now, I can't comment on the networking topology example of yours >because I cannot fully follow the logic. Let me try to give another, >hopefully more practical example. > >Consider interaction of the MPI_Init/MPI_Finalize with the >underlying OS and job managers. It's about as undefined as that of >the mentioned calls with the checkpointer. Nevertheless, many usable >implementations can cope with that quite nicely. By the way, this is >how the checkpoint/restarting could work as well, providing the >checkpointer at hand with the MPI ready to be checkpointed/restarted >at a well defined point of the MPI program. After all, this is how >it is done now: a configuration flag tells the MPI what checkpointer >to expect. This could be refined, turned into dynamic recognition of >the active checkpointer, etc. This is all trivial. > >Imagine now that we have an API instead of the MPI_Init/MPI_Finalize >that says: "Well, it sort of starts and terminates an MPI job, but >one cannot guarantee how many processes will be started nor whether >they are usable after this because one cannot explain really in what >conditions the above is true. Anyway, this is all too low level for >the MPI standard to deal with. So be it because we say this >interface is self-contained and useful." > >The provided interface is self-consistent. It sort of starts a job. >Is it practically useful? No, because one cannot know when it works >at all. What is the expected commercial user reaction to this >interface? Steer clear of it or, even worse, assume on the limited >past student experience that the MPI implementation at hand is the >only correct one, and then blame all others that they do a sloppy >job when the product should be ported elsewhere. Neither would be >good for the MPI standard. > >Cf. process spawning and 1-sided comm, the biggest failures of the >MPI standard to date in practical terms. Were they self-consistent >and useful in the sense that you advocate? Sure. Were they useful in >the sense that I advocate? No. And they duly faded into obscurity, >because they did not pass the ultimate test even though they did >meet the criteria that you claim are necessary and sufficient for >the MPI standard. I pray you consider this before we exchange >another couple of emails. I'm just trying to help. > >Best regards. > >Alexander > >-----Original Message----- >From: mpi3-ft-bounces(a)lists.mpi-forum.org >[mailto:[email protected]] On Behalf Of Greg Bronevetsky >Sent: Monday, February 16, 2009 5:58 PM >To: MPI 3.0 Fault Tolerance and Dynamic Process Control working >Group; MPI 3.0 Fault Tolerance and Dynamic Process Control working Group >Subject: Re: [Mpi3-ft] Communicator Virtualization as a step forward > >MPI never places conditions on how the MPI implementation does its >job. It never says whether MPI uses static or dynamic routing or how >much performance degrades as a result of the application using a >particular communication pattern, which depends closely on the >physical network topology. These are simply issues that are too low >level for the MPI spec to define although they very much matter to >application developers. What it does instead is define a set of >semantics for MPI_Send, MPI_Recv, etc. that apply regardless of all >those details and accepts the compromise: its an internally >self-consistent spec that doesn't fully define all the relevant facts >about the system. They key thing is that it is a useful compromise. >Thus, we have two tests for a candidate API: self-consistent >specification and usefulness of the chosen abstraction level. > >The fault notification API makes exactly the same compromise as the >overall MPI spec. It doesn't say anything about how faults are >detected since that is a very low-level network matter. However, it >presents a self-consistent high-level specification that allows >applications to react to any such errors. Furthermore, it is clearly >useful. It is a red herring to worry about which low-level events >will cause which high-level notifications. The only relevant thing is >the probability of unrecoverable errors. Applications do not want >their applications to randomly abort with a frequency higher than >once every few days or weeks. If it is higher, those unrecoverable >failures must be converted into recoverable failures by the MPI >library and given to the application via the fault notification API. >This is the entire function of the fault notification API: to allow >MPI to convert unrecoverable system failures (currently they're all >unrecoverable) into recoverable failures. This makes it possible for >customers to buy systems that fail relatively frequently while making >them usable by making their applications fault tolerant. Thus, the >fault notification API is both self-consistent and useful, passing >both tests of the MPI spec. > >In contrast, the checkpointing API is useful but not self-consistent >API. Its semantics require details (i.e. interactions with the >checkpointer) that are too low-level to be specified in the MPI spec. >As a result, it needs additional mechanisms that allow individual MPI >implementations to provide the information that cannot be detailed in >the MPI spec. > >Thus, these two APIs are not at all similar unless you wish to argue >that 1. the MPI spec is ill-defined because it doesn't specify the >network topology or that 2. the semantics of being notified of a >fault are ill-defined. If you wish to argue the latter, I would love >to see examples because they would need to be fixed before this API >is ready to go before the forum. > >Greg Bronevetsky >Post-Doctoral Researcher >1028 Building 451 >Lawrence Livermore National Lab >(925) 424-5756 >bronevetsky1(a)llnl.gov > >At 06:04 AM 2/16/2009, Supalov, Alexander wrote: > >Thanks. I think that since the notification API does not provide any > >guarantee as to what kind of faults is treated how, the whole thing > >becomes a negotiation between the MPI implementation and the > >underlying networking layers. Moreover, it becomes a negotiation of > >sorts between the application and the MPI implementation, because > >the application cannot know upfront what faults will be treated what way. > > > >This is, in my mind, is very comparable to, if not worse than the > >negotiation between the MPI_prepare_for_checkpoint & > >MPI_Restart_after_chekpoint implementation on one hand, and the > >checkpointer involved on the other hand. > > > >Frankly, I don't see any difference here, or, if any, one in favor > >of the checkpointing interface. > > > >Anyway, thanks for clarification. > > > >-----Original Message----- > >From: mpi3-ft-bounces(a)lists.mpi-forum.org > >[mailto:[email protected]] On Behalf Of Greg Bronevetsky > >Sent: Friday, February 13, 2009 7:18 PM > >To: MPI 3.0 Fault Tolerance and Dynamic Process Control working > >Group; MPI 3.0 Fault Tolerance and Dynamic Process Control working Group > >Subject: Re: [Mpi3-ft] Communicator Virtualization as a step forward > > > >At 10:10 AM 2/13/2009, Supalov, Alexander wrote: > > >Thanks. Could you please clarify to me, if possible, using some > > >practically relevant example, how fault notification for a set of > > >undefined fault types that may vary from MPI implementation to > > >implementation differs from the equally abstract > > >MPI_Checkpoint/MPI_Restart that semantically clearly prepare the MPI > > >implementation at hand for the checkpoint action done by the > > >checkpointing system involved, and then semantically clearly recover > > >the MPI part of the program after the system restore? > > > >Simple. As you've pointed out, the checkpointing API is well defined > >from the application's point of view. However, its semantics are weak > >from the checkpointer's point of view. Seen from this angle, it is > >not clear what the checkpointer can expect from the MPI library and > >the whole thing devolves into a negotiation between individual > >checkpointers and individual MPI libraries on a variety of specific > >system configurations. In contrast, the fault notification API only > >has an application view, which is in fact well-defined. The weakness > >of the fault notification API is what you've already described, that > >it provides no guarantees about the quality of the implementation in > >a way that is more significant than for other portions of MPI, such > >as network details for MPI_Send/MPI_Recv. > > > >Greg Bronevetsky > >Post-Doctoral Researcher > >1028 Building 451 > >Lawrence Livermore National Lab > >(925) 424-5756 > >bronevetsky1(a)llnl.gov > > > >_______________________________________________ > >mpi3-ft mailing list > >mpi3-ft(a)lists.mpi-forum.org > >http:// lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft > >--------------------------------------------------------------------- > >Intel GmbH > >Dornacher Strasse 1 > >85622 Feldkirchen/Muenchen Germany > >Sitz der Gesellschaft: Feldkirchen bei Muenchen > >Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer > >Registergericht: Muenchen HRB 47456 Ust.-IdNr. > >VAT Registration No.: DE129385895 > >Citibank Frankfurt (BLZ 502 109 00) 600119052 > > > >This e-mail and any attachments may contain confidential material for > >the sole use of the intended recipient(s). Any review or distribution > >by others is strictly prohibited. If you are not the intended > >recipient, please contact the sender and delete all copies. > > > > > >_______________________________________________ > >mpi3-ft mailing list > >mpi3-ft(a)lists.mpi-forum.org > >http:// lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft > >_______________________________________________ >mpi3-ft mailing list >mpi3-ft(a)lists.mpi-forum.org >http:// lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft >--------------------------------------------------------------------- >Intel GmbH >Dornacher Strasse 1 >85622 Feldkirchen/Muenchen Germany >Sitz der Gesellschaft: Feldkirchen bei Muenchen >Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer >Registergericht: Muenchen HRB 47456 Ust.-IdNr. >VAT Registration No.: DE129385895 >Citibank Frankfurt (BLZ 502 109 00) 600119052 > >This e-mail and any attachments may contain confidential material for >the sole use of the intended recipient(s). Any review or distribution >by others is strictly prohibited. If you are not the intended >recipient, please contact the sender and delete all copies. > > >_______________________________________________ >mpi3-ft mailing list >mpi3-ft(a)lists.mpi-forum.org >http:// lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft

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Re: [Mpi3-ft] Communicator Virtualization as a step forward
by Supalov, Alexander 17 Feb '09

17 Feb '09

Hi, Thanks. I think we're coming from different corners, and the main problem is that the two criteria of yours, namely, "self-consistent specification and usefulness of the chosen abstraction level" are not sufficient for me. The key here appears to be in the "usefulness" that you seem to understand as "this can be used in principle on my future Petascale machine running my favorite MPI" and mine "to be used in a real application, an API should be useful in a wide, a priori known variety of practically relevant situations that do not depend on the MPI implementation at hand or the platform involved". Maybe we should have a call or something to discuss this live, without email getting into way. The rest is just an outline of what I'd have to say if we met. I say that we should define a standard set of the faults that will be detected, and then say what implementations should provide what level of service to be compliant with a particular level of the FT support that the standard is to specify in clear terms. If we don't do this, we will only prepare a good ground for MPI-4, were we will have to fix the flaws of the MPI-3. I'm afraid that at the moment we're driving in this very direction, and this is why. The ultimate test of a spec is a number of implementations that are widely used as specified. I'm concerned that the current notification spec is too weak to be appealing to the commercial users and implementors. I'm concerned about this because the semantics of an API that kicks in under unknown circumstances are surely ill-defined, and the ROI of using it in any given application cannot be assessed upfront. This means that the investment is unlikely to be made - either on the commercial MPI implementation or on the commercial MPI application side of the equation. In other words, why would I care to use an API if I were not aware of when and how this would help my application run weeks rather than days between the faults? And I don't mean here a student with a diploma or PhD thesis to write and forget about. I mean real life commercial developers who need to justify every bit of what they are doing by showing positive ROI to their management in the times of worldwide economic crisis. Justify by promising to earn more money than they are going to spend for the development, or else. Now, I can't comment on the networking topology example of yours because I cannot fully follow the logic. Let me try to give another, hopefully more practical example. Consider interaction of the MPI_Init/MPI_Finalize with the underlying OS and job managers. It's about as undefined as that of the mentioned calls with the checkpointer. Nevertheless, many usable implementations can cope with that quite nicely. By the way, this is how the checkpoint/restarting could work as well, providing the checkpointer at hand with the MPI ready to be checkpointed/restarted at a well defined point of the MPI program. After all, this is how it is done now: a configuration flag tells the MPI what checkpointer to expect. This could be refined, turned into dynamic recognition of the active checkpointer, etc. This is all trivial. Imagine now that we have an API instead of the MPI_Init/MPI_Finalize that says: "Well, it sort of starts and terminates an MPI job, but one cannot guarantee how many processes will be started nor whether they are usable after this because one cannot explain really in what conditions the above is true. Anyway, this is all too low level for the MPI standard to deal with. So be it because we say this interface is self-contained and useful." The provided interface is self-consistent. It sort of starts a job. Is it practically useful? No, because one cannot know when it works at all. What is the expected commercial user reaction to this interface? Steer clear of it or, even worse, assume on the limited past student experience that the MPI implementation at hand is the only correct one, and then blame all others that they do a sloppy job when the product should be ported elsewhere. Neither would be good for the MPI standard. Cf. process spawning and 1-sided comm, the biggest failures of the MPI standard to date in practical terms. Were they self-consistent and useful in the sense that you advocate? Sure. Were they useful in the sense that I advocate? No. And they duly faded into obscurity, because they did not pass the ultimate test even though they did meet the criteria that you claim are necessary and sufficient for the MPI standard. I pray you consider this before we exchange another couple of emails. I'm just trying to help. Best regards. Alexander -----Original Message----- From: mpi3-ft-bounces(a)lists.mpi-forum.org [mailto:[email protected]] On Behalf Of Greg Bronevetsky Sent: Monday, February 16, 2009 5:58 PM To: MPI 3.0 Fault Tolerance and Dynamic Process Control working Group; MPI 3.0 Fault Tolerance and Dynamic Process Control working Group Subject: Re: [Mpi3-ft] Communicator Virtualization as a step forward MPI never places conditions on how the MPI implementation does its job. It never says whether MPI uses static or dynamic routing or how much performance degrades as a result of the application using a particular communication pattern, which depends closely on the physical network topology. These are simply issues that are too low level for the MPI spec to define although they very much matter to application developers. What it does instead is define a set of semantics for MPI_Send, MPI_Recv, etc. that apply regardless of all those details and accepts the compromise: its an internally self-consistent spec that doesn't fully define all the relevant facts about the system. They key thing is that it is a useful compromise. Thus, we have two tests for a candidate API: self-consistent specification and usefulness of the chosen abstraction level. The fault notification API makes exactly the same compromise as the overall MPI spec. It doesn't say anything about how faults are detected since that is a very low-level network matter. However, it presents a self-consistent high-level specification that allows applications to react to any such errors. Furthermore, it is clearly useful. It is a red herring to worry about which low-level events will cause which high-level notifications. The only relevant thing is the probability of unrecoverable errors. Applications do not want their applications to randomly abort with a frequency higher than once every few days or weeks. If it is higher, those unrecoverable failures must be converted into recoverable failures by the MPI library and given to the application via the fault notification API. This is the entire function of the fault notification API: to allow MPI to convert unrecoverable system failures (currently they're all unrecoverable) into recoverable failures. This makes it possible for customers to buy systems that fail relatively frequently while making them usable by making their applications fault tolerant. Thus, the fault notification API is both self-consistent and useful, passing both tests of the MPI spec. In contrast, the checkpointing API is useful but not self-consistent API. Its semantics require details (i.e. interactions with the checkpointer) that are too low-level to be specified in the MPI spec. As a result, it needs additional mechanisms that allow individual MPI implementations to provide the information that cannot be detailed in the MPI spec. Thus, these two APIs are not at all similar unless you wish to argue that 1. the MPI spec is ill-defined because it doesn't specify the network topology or that 2. the semantics of being notified of a fault are ill-defined. If you wish to argue the latter, I would love to see examples because they would need to be fixed before this API is ready to go before the forum. Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 bronevetsky1(a)llnl.gov At 06:04 AM 2/16/2009, Supalov, Alexander wrote: >Thanks. I think that since the notification API does not provide any >guarantee as to what kind of faults is treated how, the whole thing >becomes a negotiation between the MPI implementation and the >underlying networking layers. Moreover, it becomes a negotiation of >sorts between the application and the MPI implementation, because >the application cannot know upfront what faults will be treated what way. > >This is, in my mind, is very comparable to, if not worse than the >negotiation between the MPI_prepare_for_checkpoint & >MPI_Restart_after_chekpoint implementation on one hand, and the >checkpointer involved on the other hand. > >Frankly, I don't see any difference here, or, if any, one in favor >of the checkpointing interface. > >Anyway, thanks for clarification. > >-----Original Message----- >From: mpi3-ft-bounces(a)lists.mpi-forum.org >[mailto:[email protected]] On Behalf Of Greg Bronevetsky >Sent: Friday, February 13, 2009 7:18 PM >To: MPI 3.0 Fault Tolerance and Dynamic Process Control working >Group; MPI 3.0 Fault Tolerance and Dynamic Process Control working Group >Subject: Re: [Mpi3-ft] Communicator Virtualization as a step forward > >At 10:10 AM 2/13/2009, Supalov, Alexander wrote: > >Thanks. Could you please clarify to me, if possible, using some > >practically relevant example, how fault notification for a set of > >undefined fault types that may vary from MPI implementation to > >implementation differs from the equally abstract > >MPI_Checkpoint/MPI_Restart that semantically clearly prepare the MPI > >implementation at hand for the checkpoint action done by the > >checkpointing system involved, and then semantically clearly recover > >the MPI part of the program after the system restore? > >Simple. As you've pointed out, the checkpointing API is well defined >from the application's point of view. However, its semantics are weak >from the checkpointer's point of view. Seen from this angle, it is >not clear what the checkpointer can expect from the MPI library and >the whole thing devolves into a negotiation between individual >checkpointers and individual MPI libraries on a variety of specific >system configurations. In contrast, the fault notification API only >has an application view, which is in fact well-defined. The weakness >of the fault notification API is what you've already described, that >it provides no guarantees about the quality of the implementation in >a way that is more significant than for other portions of MPI, such >as network details for MPI_Send/MPI_Recv. > >Greg Bronevetsky >Post-Doctoral Researcher >1028 Building 451 >Lawrence Livermore National Lab >(925) 424-5756 >bronevetsky1(a)llnl.gov > >_______________________________________________ >mpi3-ft mailing list >mpi3-ft(a)lists.mpi-forum.org >http:// lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft >--------------------------------------------------------------------- >Intel GmbH >Dornacher Strasse 1 >85622 Feldkirchen/Muenchen Germany >Sitz der Gesellschaft: Feldkirchen bei Muenchen >Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer >Registergericht: Muenchen HRB 47456 Ust.-IdNr. >VAT Registration No.: DE129385895 >Citibank Frankfurt (BLZ 502 109 00) 600119052 > >This e-mail and any attachments may contain confidential material for >the sole use of the intended recipient(s). Any review or distribution >by others is strictly prohibited. If you are not the intended >recipient, please contact the sender and delete all copies. > > >_______________________________________________ >mpi3-ft mailing list >mpi3-ft(a)lists.mpi-forum.org >http:// lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft _______________________________________________ mpi3-ft mailing list mpi3-ft(a)lists.mpi-forum.org http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft --------------------------------------------------------------------- Intel GmbH Dornacher Strasse 1 85622 Feldkirchen/Muenchen Germany Sitz der Gesellschaft: Feldkirchen bei Muenchen Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer Registergericht: Muenchen HRB 47456 Ust.-IdNr. VAT Registration No.: DE129385895 Citibank Frankfurt (BLZ 502 109 00) 600119052 This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies.

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Re: [Mpi3-ft] Communicator Virtualization as a step forward
by Greg Bronevetsky 16 Feb '09

16 Feb '09

MPI never places conditions on how the MPI implementation does its job. It never says whether MPI uses static or dynamic routing or how much performance degrades as a result of the application using a particular communication pattern, which depends closely on the physical network topology. These are simply issues that are too low level for the MPI spec to define although they very much matter to application developers. What it does instead is define a set of semantics for MPI_Send, MPI_Recv, etc. that apply regardless of all those details and accepts the compromise: its an internally self-consistent spec that doesn't fully define all the relevant facts about the system. They key thing is that it is a useful compromise. Thus, we have two tests for a candidate API: self-consistent specification and usefulness of the chosen abstraction level. The fault notification API makes exactly the same compromise as the overall MPI spec. It doesn't say anything about how faults are detected since that is a very low-level network matter. However, it presents a self-consistent high-level specification that allows applications to react to any such errors. Furthermore, it is clearly useful. It is a red herring to worry about which low-level events will cause which high-level notifications. The only relevant thing is the probability of unrecoverable errors. Applications do not want their applications to randomly abort with a frequency higher than once every few days or weeks. If it is higher, those unrecoverable failures must be converted into recoverable failures by the MPI library and given to the application via the fault notification API. This is the entire function of the fault notification API: to allow MPI to convert unrecoverable system failures (currently they're all unrecoverable) into recoverable failures. This makes it possible for customers to buy systems that fail relatively frequently while making them usable by making their applications fault tolerant. Thus, the fault notification API is both self-consistent and useful, passing both tests of the MPI spec. In contrast, the checkpointing API is useful but not self-consistent API. Its semantics require details (i.e. interactions with the checkpointer) that are too low-level to be specified in the MPI spec. As a result, it needs additional mechanisms that allow individual MPI implementations to provide the information that cannot be detailed in the MPI spec. Thus, these two APIs are not at all similar unless you wish to argue that 1. the MPI spec is ill-defined because it doesn't specify the network topology or that 2. the semantics of being notified of a fault are ill-defined. If you wish to argue the latter, I would love to see examples because they would need to be fixed before this API is ready to go before the forum. Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 bronevetsky1(a)llnl.gov At 06:04 AM 2/16/2009, Supalov, Alexander wrote: >Thanks. I think that since the notification API does not provide any >guarantee as to what kind of faults is treated how, the whole thing >becomes a negotiation between the MPI implementation and the >underlying networking layers. Moreover, it becomes a negotiation of >sorts between the application and the MPI implementation, because >the application cannot know upfront what faults will be treated what way. > >This is, in my mind, is very comparable to, if not worse than the >negotiation between the MPI_prepare_for_checkpoint & >MPI_Restart_after_chekpoint implementation on one hand, and the >checkpointer involved on the other hand. > >Frankly, I don't see any difference here, or, if any, one in favor >of the checkpointing interface. > >Anyway, thanks for clarification. > >-----Original Message----- >From: mpi3-ft-bounces(a)lists.mpi-forum.org >[mailto:[email protected]] On Behalf Of Greg Bronevetsky >Sent: Friday, February 13, 2009 7:18 PM >To: MPI 3.0 Fault Tolerance and Dynamic Process Control working >Group; MPI 3.0 Fault Tolerance and Dynamic Process Control working Group >Subject: Re: [Mpi3-ft] Communicator Virtualization as a step forward > >At 10:10 AM 2/13/2009, Supalov, Alexander wrote: > >Thanks. Could you please clarify to me, if possible, using some > >practically relevant example, how fault notification for a set of > >undefined fault types that may vary from MPI implementation to > >implementation differs from the equally abstract > >MPI_Checkpoint/MPI_Restart that semantically clearly prepare the MPI > >implementation at hand for the checkpoint action done by the > >checkpointing system involved, and then semantically clearly recover > >the MPI part of the program after the system restore? > >Simple. As you've pointed out, the checkpointing API is well defined >from the application's point of view. However, its semantics are weak >from the checkpointer's point of view. Seen from this angle, it is >not clear what the checkpointer can expect from the MPI library and >the whole thing devolves into a negotiation between individual >checkpointers and individual MPI libraries on a variety of specific >system configurations. In contrast, the fault notification API only >has an application view, which is in fact well-defined. The weakness >of the fault notification API is what you've already described, that >it provides no guarantees about the quality of the implementation in >a way that is more significant than for other portions of MPI, such >as network details for MPI_Send/MPI_Recv. > >Greg Bronevetsky >Post-Doctoral Researcher >1028 Building 451 >Lawrence Livermore National Lab >(925) 424-5756 >bronevetsky1(a)llnl.gov > >_______________________________________________ >mpi3-ft mailing list >mpi3-ft(a)lists.mpi-forum.org >http:// lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft >--------------------------------------------------------------------- >Intel GmbH >Dornacher Strasse 1 >85622 Feldkirchen/Muenchen Germany >Sitz der Gesellschaft: Feldkirchen bei Muenchen >Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer >Registergericht: Muenchen HRB 47456 Ust.-IdNr. >VAT Registration No.: DE129385895 >Citibank Frankfurt (BLZ 502 109 00) 600119052 > >This e-mail and any attachments may contain confidential material for >the sole use of the intended recipient(s). Any review or distribution >by others is strictly prohibited. If you are not the intended >recipient, please contact the sender and delete all copies. > > >_______________________________________________ >mpi3-ft mailing list >mpi3-ft(a)lists.mpi-forum.org >http:// lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft

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