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Re: [mpich-discuss] Fortran 2008 MPI Interface Bug
by Zhou, Hui 21 Nov '19

21 Nov '19

Hi Thomas, Just to follow up. I will try to isolate a reproducing example at some point (unless someone beats me). — Hui Zhou On Nov 15, 2019, at 12:48 PM, Thomas König via discuss <discuss(a)mpich.org<mailto:[email protected]>> wrote: Hi Junchao, Could you check if it is a gcc bug in handling type(*), dimension(..) Unfortunately, I don't know the code that well to be able to do it. If you could put together a simple, self-contained test case showing i there is a problem, you could submit that to gcc's bugzilla (or I could do that for you). I could also check a simplified test case against current gfortran trunk, which will not compile mpich 3.3.1 because it now flags argument mismatches as errors. (And yes, argument mismatches are real errors, which cause wrong code; see for example https://bugs.debian.org/944127 .) _______________________________________________ discuss mailing list discuss(a)mpich.org<mailto:[email protected]> To manage subscription options or unsubscribe: https://lists.mpich.org/mailman/listinfo/discuss

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Re: [mpich-discuss] Fortran 2008 MPI Interface Bug
by Zhou, Hui 21 Nov '19

21 Nov '19

Hi Pedro Ricardo, I have added some follow up for the reported issue at https://github.com/pmodels/mpich/issues/4170. gfortran 9.2 confirms your observation. Intel compiler, ifort, works correctly. I will try further narrow down the issue when I find some time; but it appears this is an issue in gfortran implementation. The `use_f08` binding can be implemented with a thin Fortran layer, essentially wrapping around the `F77` binding, or it can be implemented nearly entirely with Fortran iso-c-binding, wrapping around a native mpi c library. MPICH adopts the latter approach. If you are interested, you may refer to this paper: https://www.mcs.anl.gov/~balaji/pubs/2014/eurompi/eurompi14.f08binding.pdf — Hui Zhou On Nov 13, 2019, at 4:45 PM, Zhou, Hui via discuss <discuss(a)mpich.org<mailto:[email protected]>> wrote: Hi Pedro Ricardo, Thanks for reporting the issue. I have opened a GitHub issue, https://github.com/pmodels/mpich/issues/4170, for better tracking. `mpi_f08` tries to use the iso-c-binding as much as it can and it has not been receiving as much test as `use mpi`. One of the reason is the compiler support has been sporadic. May I ask which compiler (include version) were you using? Anyway, we’ll look into it and hopefully get it fixed shortly. — Hui Zhou Principal Software Development Specialist MCS | ANL On Nov 13, 2019, at 1:59 PM, Pedro Ricardo Souza via discuss <discuss(a)mpich.org<mailto:[email protected]>> wrote: Hi there, My name is Pedro Ricardo and I'm currently a researcher at a Fluid Dynamics Laboratory in Brazil. We have a CFD software with 12 continuous years of development mostly in Fortran, and we've been using Mpich and Openmpi since the very beginning. Recently we started to refactor the code and write it using Fortran2008 standard and this includes the usage of the the newest MPI module for Fortran "mpi_f08". So far no errors were present in Openmpi implementations and everything worked like a charm, but when we started compiling with Mpich lots of errors happened during execution. I was able so isolate and reproduce the error in a simpler code. Whenever "use mpi_f08" is present, the arrays passed to a MPI subroutine return with C-like indexes. If I allocate an standard array in Fortran (that starts with index 1), after the call of some MPI routine this array come back starting with index 0. And you can imagine that all access to this array will be wrong after that. It is a funny behavior because if you put "use mpi" instead of "use mpi_f08", the indexes do not change. The test is simple, allocate and array, print it's bound indexes, calls a MPI_Reduce routine and print the array bounds again. I've also tested other routines like MPI_ISend and the error also happens. program check_bounds !Check Works ! use mpi !Check Fails use mpi_f08 implicit none integer:: mpi_ierr, proc_id, n_proc real, allocatable, dimension(:):: t1, t1_0 call MPI_Init(mpi_ierr) call MPI_Comm_rank(MPI_COMM_WORLD, proc_id, mpi_ierr) call MPI_Comm_size(MPI_COMM_WORLD, n_proc, mpi_ierr) allocate(t1(5), t1_0(5)) t1 = 1.0; t1_0 = 1.0 if (proc_id==0) then write(*,*)'Array Configuration Before:' write(*,'(a6,2(1x,i2))')'Low:',lbound(t1),lbound(t1_0) write(*,'(a6,2(1x,i2))')'Upper:',ubound(t1),ubound(t1_0) end if CALL MPI_REDUCE(t1, t1_0, 5, MPI_REAL, MPI_SUM, 0, MPI_COMM_WORLD, mpi_ierr) if (proc_id==0) then write(*,*)'Array Configuration After:' write(*,'(a6,2(1x,i2))')'Low:',lbound(t1),lbound(t1_0) write(*,'(a6,2(1x,i2))')'Upper:',ubound(t1),ubound(t1_0) end if deallocate(t1, t1_0) call MPI_Finalize(mpi_ierr) end program check_bounds Thanks for the attention, _______________________________________________ discuss mailing list discuss(a)mpich.org<mailto:[email protected]> To manage subscription options or unsubscribe: https://lists.mpich.org/mailman/listinfo/discuss

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Re: [mpich-discuss] Overlapping non-blocking collectives leads to deadlock
by Raffenetti, Kenneth J. 18 Nov '19

18 Nov '19

Correct. The ordering requirement is per-communicator, so creating additional communicators is one way to ensure safety. Ken On 11/18/19 11:52 AM, Joachim Protze via discuss wrote: > Hi Mark, > > I agree with Guiseppe. This is an issue in your code. You can use > different communicators (e.g., duplicates of comm-world) if you want to > concurrently execute collective communication by different threads. > > - Joachim > > On 11/18/19 5:25 PM, Mark Davis via discuss wrote: >> Hi Giuseppe, thank you for the fast response -- that clarifies things >> for me. >> >> On Mon, Nov 18, 2019 at 10:39 AM Congiu, Giuseppe via discuss >> <discuss(a)mpich.org> wrote: >>> >>> Hello Mark, >>> >>> I don’t think that is a bug in MPICH, it’s a bug in your code. The >>> MPI standard requires that collectives (non-blocking ones are no >>> exception) must be invoked in the same order in all processes. If T0 >>> in process 0 runs first and T1 in process 1 runs first you have a >>> mismatch and a resulting deadlock. >>> >>> —Giuseppe >>> >>>> On Nov 18, 2019, at 8:27 AM, Mark Davis via discuss >>>> <discuss(a)mpich.org> wrote: >>>> >>>> I realized something else relevant: I mentioned above that this >>>> deadlock occurs sometimes but not all of the time; I think I've >>>> narrowed down when it happens. Here's the above example with thread >>>> IDs annotated in: >>>> >>>> >>>> PROCESS 0 (root for ireduce and ibcast): >>>> // T0 is always the thread that calls MPI functions >>>> T0: MPI_Ireduce(..., &req) >>>> T0: MPI_Wait(&req); <-- blocking here >>>> ... >>>> T0: MPI_Ibcast(..., &req2); >>>> T0: MPI_Wait(&req2); >>>> >>>> PROCESS 1 (non-root for ireduce and ibcast): >>>> // T0 is the root for the reduce >>>> T0: MPI_Ireduce(..., &req) >>>> T0:MPI_Wait(&req); >>>> ... >>>> // T1 is the root for the bcast >>>> T1: MPI_Ibcast(..., &req2); >>>> T1: MPI_Wait(&req2); <-- blocking here >>>> >>>> Note that the non-root process has two different threads, T0 and T1, >>>> and T0 does the Ireduce and T1 does the bcast. I believe the T0 call >>>> to MPI_Ireduce is concurrent with the T1 call to MPI_Ibcast (both as >>>> non-roots). >>>> >>>> So, I believe the question is: is it legal in MPI to have two threads >>>> in a given MPI process call different non-blocking collectives (e.g., >>>> reduce and bcast) concurrently with MPI_THREAD_MULTIPLE enabled? >>>> >>>> Thank you >>>> >>>> On Mon, Nov 18, 2019 at 10:05 AM Mark Davis >>>> <markdavisinboston(a)gmail.com> wrote: >>>>> >>>>> Hello, I'm experimenting with non-blocking collectives using MPICH in >>>>> a multithreaded C++ program (with MPI_THREAD_MULTIPLE initialization). >>>>> >>>>> I'm currently doing a non-blocking reduce followed by a non-blocking >>>>> broadcast (I realize I can just use an allreduce but for my >>>>> experiment, I need to serialize these operations). I was able to >>>>> produce this bug with only two MPI processes. I see in gdb that the >>>>> root process is stuck trying to execute the MPI_Ireduce in cases where >>>>> the non-root process does the MPI_Ireduce and gets to the MPI_Ibcast >>>>> quickly. That is, process 0 (root) isn't able to complete the >>>>> MPI_Ireduce wait while process 1 is stuck in the MPI_Ibcast wait. >>>>> >>>>> PROCESS 0 (root for ireduce and ibcast): >>>>> MPI_Ireduce(..., &req) >>>>> MPI_Wait(&req); <-- blocking here >>>>> ... >>>>> MPI_Ibcast(..., &req2); >>>>> MPI_Wait(&req2); >>>>> >>>>> PROCESS 1 (non-root for ireduce and ibcast): >>>>> MPI_Ireduce(..., &req) >>>>> MPI_Wait(&req); >>>>> ... >>>>> MPI_Ibcast(..., &req2); >>>>> MPI_Wait(&req2); <-- blocking here >>>>> >>>>> Much of the time, the program deadlocks as shown above; sometimes this >>>>> works fine, though, perhaps due to subtle timing differences. I >>>>> mentioned above that this is a multithreaded program. I'm able to >>>>> produce the issue with two threads with two MPI procs. The other >>>>> threads are not calling MPI functions -- they are helping with other >>>>> computation. I've verified that I don't have any TSAN or ASAN errors >>>>> in this program. However, when I only have one thread per process, I >>>>> don't have this issue. I think there's a decent chance, though, that >>>>> this has to do with timing differences as opposed to changing anything >>>>> with the MPI calls. I have verified that only one thread per process >>>>> is calling the MPI routines in the multithreaded case. >>>>> >>>>> When I change the MPI_Ireduce to a blocking MPI_Reduce and I keep the >>>>> MPI_Ibcast non-blocking, the program runs fine. Only when BOTH >>>>> MPI_Ireduce and MPI_Ibcast happen serially do I see this deadlock >>>>> (again, some of the time). >>>>> >>>>> Unfortunately, this program is part of a very large system and it's >>>>> not straightforward to give a fully working example. So, I'm just >>>>> looking for any ideas anyone has for what sort of thing may be >>>>> happening, any information that may be helpful about how two >>>>> coincident non-blocking requests could interact with each other, etc. >>>>> >>>>> Also, if anyone has tips on how to debug this sort of thing in gdb >>>>> that would be helpful. For example, are there ways to introspect the >>>>> MPI_Request object, etc.? >>>>> >>>>> Thanks >>>> _______________________________________________ >>>> discuss mailing list discuss(a)mpich.org >>>> To manage subscription options or unsubscribe: >>>> https://lists.mpich.org/mailman/listinfo/discuss >>> >>> _______________________________________________ >>> discuss mailing list discuss(a)mpich.org >>> To manage subscription options or unsubscribe: >>> https://lists.mpich.org/mailman/listinfo/discuss >> _______________________________________________ >> discuss mailing list discuss(a)mpich.org >> To manage subscription options or unsubscribe: >> https://lists.mpich.org/mailman/listinfo/discuss >> > > > > _______________________________________________ > discuss mailing list discuss(a)mpich.org > To manage subscription options or unsubscribe: > https://lists.mpich.org/mailman/listinfo/discuss >

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Re: [mpich-discuss] Overlapping non-blocking collectives leads to deadlock
by Joachim Protze 18 Nov '19

18 Nov '19

Hi Mark, I agree with Guiseppe. This is an issue in your code. You can use different communicators (e.g., duplicates of comm-world) if you want to concurrently execute collective communication by different threads. - Joachim On 11/18/19 5:25 PM, Mark Davis via discuss wrote: > Hi Giuseppe, thank you for the fast response -- that clarifies things for me. > > On Mon, Nov 18, 2019 at 10:39 AM Congiu, Giuseppe via discuss > <discuss(a)mpich.org> wrote: >> >> Hello Mark, >> >> I don’t think that is a bug in MPICH, it’s a bug in your code. The MPI standard requires that collectives (non-blocking ones are no exception) must be invoked in the same order in all processes. If T0 in process 0 runs first and T1 in process 1 runs first you have a mismatch and a resulting deadlock. >> >> —Giuseppe >> >>> On Nov 18, 2019, at 8:27 AM, Mark Davis via discuss <discuss(a)mpich.org> wrote: >>> >>> I realized something else relevant: I mentioned above that this >>> deadlock occurs sometimes but not all of the time; I think I've >>> narrowed down when it happens. Here's the above example with thread >>> IDs annotated in: >>> >>> >>> PROCESS 0 (root for ireduce and ibcast): >>> // T0 is always the thread that calls MPI functions >>> T0: MPI_Ireduce(..., &req) >>> T0: MPI_Wait(&req); <-- blocking here >>> ... >>> T0: MPI_Ibcast(..., &req2); >>> T0: MPI_Wait(&req2); >>> >>> PROCESS 1 (non-root for ireduce and ibcast): >>> // T0 is the root for the reduce >>> T0: MPI_Ireduce(..., &req) >>> T0:MPI_Wait(&req); >>> ... >>> // T1 is the root for the bcast >>> T1: MPI_Ibcast(..., &req2); >>> T1: MPI_Wait(&req2); <-- blocking here >>> >>> Note that the non-root process has two different threads, T0 and T1, >>> and T0 does the Ireduce and T1 does the bcast. I believe the T0 call >>> to MPI_Ireduce is concurrent with the T1 call to MPI_Ibcast (both as >>> non-roots). >>> >>> So, I believe the question is: is it legal in MPI to have two threads >>> in a given MPI process call different non-blocking collectives (e.g., >>> reduce and bcast) concurrently with MPI_THREAD_MULTIPLE enabled? >>> >>> Thank you >>> >>> On Mon, Nov 18, 2019 at 10:05 AM Mark Davis <markdavisinboston(a)gmail.com> wrote: >>>> >>>> Hello, I'm experimenting with non-blocking collectives using MPICH in >>>> a multithreaded C++ program (with MPI_THREAD_MULTIPLE initialization). >>>> >>>> I'm currently doing a non-blocking reduce followed by a non-blocking >>>> broadcast (I realize I can just use an allreduce but for my >>>> experiment, I need to serialize these operations). I was able to >>>> produce this bug with only two MPI processes. I see in gdb that the >>>> root process is stuck trying to execute the MPI_Ireduce in cases where >>>> the non-root process does the MPI_Ireduce and gets to the MPI_Ibcast >>>> quickly. That is, process 0 (root) isn't able to complete the >>>> MPI_Ireduce wait while process 1 is stuck in the MPI_Ibcast wait. >>>> >>>> PROCESS 0 (root for ireduce and ibcast): >>>> MPI_Ireduce(..., &req) >>>> MPI_Wait(&req); <-- blocking here >>>> ... >>>> MPI_Ibcast(..., &req2); >>>> MPI_Wait(&req2); >>>> >>>> PROCESS 1 (non-root for ireduce and ibcast): >>>> MPI_Ireduce(..., &req) >>>> MPI_Wait(&req); >>>> ... >>>> MPI_Ibcast(..., &req2); >>>> MPI_Wait(&req2); <-- blocking here >>>> >>>> Much of the time, the program deadlocks as shown above; sometimes this >>>> works fine, though, perhaps due to subtle timing differences. I >>>> mentioned above that this is a multithreaded program. I'm able to >>>> produce the issue with two threads with two MPI procs. The other >>>> threads are not calling MPI functions -- they are helping with other >>>> computation. I've verified that I don't have any TSAN or ASAN errors >>>> in this program. However, when I only have one thread per process, I >>>> don't have this issue. I think there's a decent chance, though, that >>>> this has to do with timing differences as opposed to changing anything >>>> with the MPI calls. I have verified that only one thread per process >>>> is calling the MPI routines in the multithreaded case. >>>> >>>> When I change the MPI_Ireduce to a blocking MPI_Reduce and I keep the >>>> MPI_Ibcast non-blocking, the program runs fine. Only when BOTH >>>> MPI_Ireduce and MPI_Ibcast happen serially do I see this deadlock >>>> (again, some of the time). >>>> >>>> Unfortunately, this program is part of a very large system and it's >>>> not straightforward to give a fully working example. So, I'm just >>>> looking for any ideas anyone has for what sort of thing may be >>>> happening, any information that may be helpful about how two >>>> coincident non-blocking requests could interact with each other, etc. >>>> >>>> Also, if anyone has tips on how to debug this sort of thing in gdb >>>> that would be helpful. For example, are there ways to introspect the >>>> MPI_Request object, etc.? >>>> >>>> Thanks >>> _______________________________________________ >>> discuss mailing list discuss(a)mpich.org >>> To manage subscription options or unsubscribe: >>> https://lists.mpich.org/mailman/listinfo/discuss >> >> _______________________________________________ >> discuss mailing list discuss(a)mpich.org >> To manage subscription options or unsubscribe: >> https://lists.mpich.org/mailman/listinfo/discuss > _______________________________________________ > discuss mailing list discuss(a)mpich.org > To manage subscription options or unsubscribe: > https://lists.mpich.org/mailman/listinfo/discuss > -- Dipl.-Inf. Joachim Protze IT Center Group: High Performance Computing Division: Computational Science and Engineering RWTH Aachen University Seffenter Weg 23 D 52074 Aachen (Germany) Tel: +49 241 80- 24765 Fax: +49 241 80-624765 protze(a)itc.rwth-aachen.de www.itc.rwth-aachen.de

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Re: [mpich-discuss] Overlapping non-blocking collectives leads to deadlock
by Mark Davis 18 Nov '19

18 Nov '19

Hi Giuseppe, thank you for the fast response -- that clarifies things for me. On Mon, Nov 18, 2019 at 10:39 AM Congiu, Giuseppe via discuss <discuss(a)mpich.org> wrote: > > Hello Mark, > > I don’t think that is a bug in MPICH, it’s a bug in your code. The MPI standard requires that collectives (non-blocking ones are no exception) must be invoked in the same order in all processes. If T0 in process 0 runs first and T1 in process 1 runs first you have a mismatch and a resulting deadlock. > > —Giuseppe > > > On Nov 18, 2019, at 8:27 AM, Mark Davis via discuss <discuss(a)mpich.org> wrote: > > > > I realized something else relevant: I mentioned above that this > > deadlock occurs sometimes but not all of the time; I think I've > > narrowed down when it happens. Here's the above example with thread > > IDs annotated in: > > > > > > PROCESS 0 (root for ireduce and ibcast): > > // T0 is always the thread that calls MPI functions > > T0: MPI_Ireduce(..., &req) > > T0: MPI_Wait(&req); <-- blocking here > > ... > > T0: MPI_Ibcast(..., &req2); > > T0: MPI_Wait(&req2); > > > > PROCESS 1 (non-root for ireduce and ibcast): > > // T0 is the root for the reduce > > T0: MPI_Ireduce(..., &req) > > T0:MPI_Wait(&req); > > ... > > // T1 is the root for the bcast > > T1: MPI_Ibcast(..., &req2); > > T1: MPI_Wait(&req2); <-- blocking here > > > > Note that the non-root process has two different threads, T0 and T1, > > and T0 does the Ireduce and T1 does the bcast. I believe the T0 call > > to MPI_Ireduce is concurrent with the T1 call to MPI_Ibcast (both as > > non-roots). > > > > So, I believe the question is: is it legal in MPI to have two threads > > in a given MPI process call different non-blocking collectives (e.g., > > reduce and bcast) concurrently with MPI_THREAD_MULTIPLE enabled? > > > > Thank you > > > > On Mon, Nov 18, 2019 at 10:05 AM Mark Davis <markdavisinboston(a)gmail.com> wrote: > >> > >> Hello, I'm experimenting with non-blocking collectives using MPICH in > >> a multithreaded C++ program (with MPI_THREAD_MULTIPLE initialization). > >> > >> I'm currently doing a non-blocking reduce followed by a non-blocking > >> broadcast (I realize I can just use an allreduce but for my > >> experiment, I need to serialize these operations). I was able to > >> produce this bug with only two MPI processes. I see in gdb that the > >> root process is stuck trying to execute the MPI_Ireduce in cases where > >> the non-root process does the MPI_Ireduce and gets to the MPI_Ibcast > >> quickly. That is, process 0 (root) isn't able to complete the > >> MPI_Ireduce wait while process 1 is stuck in the MPI_Ibcast wait. > >> > >> PROCESS 0 (root for ireduce and ibcast): > >> MPI_Ireduce(..., &req) > >> MPI_Wait(&req); <-- blocking here > >> ... > >> MPI_Ibcast(..., &req2); > >> MPI_Wait(&req2); > >> > >> PROCESS 1 (non-root for ireduce and ibcast): > >> MPI_Ireduce(..., &req) > >> MPI_Wait(&req); > >> ... > >> MPI_Ibcast(..., &req2); > >> MPI_Wait(&req2); <-- blocking here > >> > >> Much of the time, the program deadlocks as shown above; sometimes this > >> works fine, though, perhaps due to subtle timing differences. I > >> mentioned above that this is a multithreaded program. I'm able to > >> produce the issue with two threads with two MPI procs. The other > >> threads are not calling MPI functions -- they are helping with other > >> computation. I've verified that I don't have any TSAN or ASAN errors > >> in this program. However, when I only have one thread per process, I > >> don't have this issue. I think there's a decent chance, though, that > >> this has to do with timing differences as opposed to changing anything > >> with the MPI calls. I have verified that only one thread per process > >> is calling the MPI routines in the multithreaded case. > >> > >> When I change the MPI_Ireduce to a blocking MPI_Reduce and I keep the > >> MPI_Ibcast non-blocking, the program runs fine. Only when BOTH > >> MPI_Ireduce and MPI_Ibcast happen serially do I see this deadlock > >> (again, some of the time). > >> > >> Unfortunately, this program is part of a very large system and it's > >> not straightforward to give a fully working example. So, I'm just > >> looking for any ideas anyone has for what sort of thing may be > >> happening, any information that may be helpful about how two > >> coincident non-blocking requests could interact with each other, etc. > >> > >> Also, if anyone has tips on how to debug this sort of thing in gdb > >> that would be helpful. For example, are there ways to introspect the > >> MPI_Request object, etc.? > >> > >> Thanks > > _______________________________________________ > > discuss mailing list discuss(a)mpich.org > > To manage subscription options or unsubscribe: > > https://lists.mpich.org/mailman/listinfo/discuss > > _______________________________________________ > discuss mailing list discuss(a)mpich.org > To manage subscription options or unsubscribe: > https://lists.mpich.org/mailman/listinfo/discuss

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Re: [mpich-discuss] Overlapping non-blocking collectives leads to deadlock
by Congiu, Giuseppe 18 Nov '19

18 Nov '19

Hello Mark, I don’t think that is a bug in MPICH, it’s a bug in your code. The MPI standard requires that collectives (non-blocking ones are no exception) must be invoked in the same order in all processes. If T0 in process 0 runs first and T1 in process 1 runs first you have a mismatch and a resulting deadlock. —Giuseppe > On Nov 18, 2019, at 8:27 AM, Mark Davis via discuss <discuss(a)mpich.org> wrote: > > I realized something else relevant: I mentioned above that this > deadlock occurs sometimes but not all of the time; I think I've > narrowed down when it happens. Here's the above example with thread > IDs annotated in: > > > PROCESS 0 (root for ireduce and ibcast): > // T0 is always the thread that calls MPI functions > T0: MPI_Ireduce(..., &req) > T0: MPI_Wait(&req); <-- blocking here > ... > T0: MPI_Ibcast(..., &req2); > T0: MPI_Wait(&req2); > > PROCESS 1 (non-root for ireduce and ibcast): > // T0 is the root for the reduce > T0: MPI_Ireduce(..., &req) > T0:MPI_Wait(&req); > ... > // T1 is the root for the bcast > T1: MPI_Ibcast(..., &req2); > T1: MPI_Wait(&req2); <-- blocking here > > Note that the non-root process has two different threads, T0 and T1, > and T0 does the Ireduce and T1 does the bcast. I believe the T0 call > to MPI_Ireduce is concurrent with the T1 call to MPI_Ibcast (both as > non-roots). > > So, I believe the question is: is it legal in MPI to have two threads > in a given MPI process call different non-blocking collectives (e.g., > reduce and bcast) concurrently with MPI_THREAD_MULTIPLE enabled? > > Thank you > > On Mon, Nov 18, 2019 at 10:05 AM Mark Davis <markdavisinboston(a)gmail.com> wrote: >> >> Hello, I'm experimenting with non-blocking collectives using MPICH in >> a multithreaded C++ program (with MPI_THREAD_MULTIPLE initialization). >> >> I'm currently doing a non-blocking reduce followed by a non-blocking >> broadcast (I realize I can just use an allreduce but for my >> experiment, I need to serialize these operations). I was able to >> produce this bug with only two MPI processes. I see in gdb that the >> root process is stuck trying to execute the MPI_Ireduce in cases where >> the non-root process does the MPI_Ireduce and gets to the MPI_Ibcast >> quickly. That is, process 0 (root) isn't able to complete the >> MPI_Ireduce wait while process 1 is stuck in the MPI_Ibcast wait. >> >> PROCESS 0 (root for ireduce and ibcast): >> MPI_Ireduce(..., &req) >> MPI_Wait(&req); <-- blocking here >> ... >> MPI_Ibcast(..., &req2); >> MPI_Wait(&req2); >> >> PROCESS 1 (non-root for ireduce and ibcast): >> MPI_Ireduce(..., &req) >> MPI_Wait(&req); >> ... >> MPI_Ibcast(..., &req2); >> MPI_Wait(&req2); <-- blocking here >> >> Much of the time, the program deadlocks as shown above; sometimes this >> works fine, though, perhaps due to subtle timing differences. I >> mentioned above that this is a multithreaded program. I'm able to >> produce the issue with two threads with two MPI procs. The other >> threads are not calling MPI functions -- they are helping with other >> computation. I've verified that I don't have any TSAN or ASAN errors >> in this program. However, when I only have one thread per process, I >> don't have this issue. I think there's a decent chance, though, that >> this has to do with timing differences as opposed to changing anything >> with the MPI calls. I have verified that only one thread per process >> is calling the MPI routines in the multithreaded case. >> >> When I change the MPI_Ireduce to a blocking MPI_Reduce and I keep the >> MPI_Ibcast non-blocking, the program runs fine. Only when BOTH >> MPI_Ireduce and MPI_Ibcast happen serially do I see this deadlock >> (again, some of the time). >> >> Unfortunately, this program is part of a very large system and it's >> not straightforward to give a fully working example. So, I'm just >> looking for any ideas anyone has for what sort of thing may be >> happening, any information that may be helpful about how two >> coincident non-blocking requests could interact with each other, etc. >> >> Also, if anyone has tips on how to debug this sort of thing in gdb >> that would be helpful. For example, are there ways to introspect the >> MPI_Request object, etc.? >> >> Thanks > _______________________________________________ > discuss mailing list discuss(a)mpich.org > To manage subscription options or unsubscribe: > https://lists.mpich.org/mailman/listinfo/discuss

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Re: [mpich-discuss] Overlapping non-blocking collectives leads to deadlock
by Mark Davis 18 Nov '19

18 Nov '19

I realized something else relevant: I mentioned above that this deadlock occurs sometimes but not all of the time; I think I've narrowed down when it happens. Here's the above example with thread IDs annotated in: PROCESS 0 (root for ireduce and ibcast): // T0 is always the thread that calls MPI functions T0: MPI_Ireduce(..., &req) T0: MPI_Wait(&req); <-- blocking here ... T0: MPI_Ibcast(..., &req2); T0: MPI_Wait(&req2); PROCESS 1 (non-root for ireduce and ibcast): // T0 is the root for the reduce T0: MPI_Ireduce(..., &req) T0:MPI_Wait(&req); ... // T1 is the root for the bcast T1: MPI_Ibcast(..., &req2); T1: MPI_Wait(&req2); <-- blocking here Note that the non-root process has two different threads, T0 and T1, and T0 does the Ireduce and T1 does the bcast. I believe the T0 call to MPI_Ireduce is concurrent with the T1 call to MPI_Ibcast (both as non-roots). So, I believe the question is: is it legal in MPI to have two threads in a given MPI process call different non-blocking collectives (e.g., reduce and bcast) concurrently with MPI_THREAD_MULTIPLE enabled? Thank you On Mon, Nov 18, 2019 at 10:05 AM Mark Davis <markdavisinboston(a)gmail.com> wrote: > > Hello, I'm experimenting with non-blocking collectives using MPICH in > a multithreaded C++ program (with MPI_THREAD_MULTIPLE initialization). > > I'm currently doing a non-blocking reduce followed by a non-blocking > broadcast (I realize I can just use an allreduce but for my > experiment, I need to serialize these operations). I was able to > produce this bug with only two MPI processes. I see in gdb that the > root process is stuck trying to execute the MPI_Ireduce in cases where > the non-root process does the MPI_Ireduce and gets to the MPI_Ibcast > quickly. That is, process 0 (root) isn't able to complete the > MPI_Ireduce wait while process 1 is stuck in the MPI_Ibcast wait. > > PROCESS 0 (root for ireduce and ibcast): > MPI_Ireduce(..., &req) > MPI_Wait(&req); <-- blocking here > ... > MPI_Ibcast(..., &req2); > MPI_Wait(&req2); > > PROCESS 1 (non-root for ireduce and ibcast): > MPI_Ireduce(..., &req) > MPI_Wait(&req); > ... > MPI_Ibcast(..., &req2); > MPI_Wait(&req2); <-- blocking here > > Much of the time, the program deadlocks as shown above; sometimes this > works fine, though, perhaps due to subtle timing differences. I > mentioned above that this is a multithreaded program. I'm able to > produce the issue with two threads with two MPI procs. The other > threads are not calling MPI functions -- they are helping with other > computation. I've verified that I don't have any TSAN or ASAN errors > in this program. However, when I only have one thread per process, I > don't have this issue. I think there's a decent chance, though, that > this has to do with timing differences as opposed to changing anything > with the MPI calls. I have verified that only one thread per process > is calling the MPI routines in the multithreaded case. > > When I change the MPI_Ireduce to a blocking MPI_Reduce and I keep the > MPI_Ibcast non-blocking, the program runs fine. Only when BOTH > MPI_Ireduce and MPI_Ibcast happen serially do I see this deadlock > (again, some of the time). > > Unfortunately, this program is part of a very large system and it's > not straightforward to give a fully working example. So, I'm just > looking for any ideas anyone has for what sort of thing may be > happening, any information that may be helpful about how two > coincident non-blocking requests could interact with each other, etc. > > Also, if anyone has tips on how to debug this sort of thing in gdb > that would be helpful. For example, are there ways to introspect the > MPI_Request object, etc.? > > Thanks

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Overlapping non-blocking collectives leads to deadlock
by Mark Davis 18 Nov '19

18 Nov '19

Hello, I'm experimenting with non-blocking collectives using MPICH in a multithreaded C++ program (with MPI_THREAD_MULTIPLE initialization). I'm currently doing a non-blocking reduce followed by a non-blocking broadcast (I realize I can just use an allreduce but for my experiment, I need to serialize these operations). I was able to produce this bug with only two MPI processes. I see in gdb that the root process is stuck trying to execute the MPI_Ireduce in cases where the non-root process does the MPI_Ireduce and gets to the MPI_Ibcast quickly. That is, process 0 (root) isn't able to complete the MPI_Ireduce wait while process 1 is stuck in the MPI_Ibcast wait. PROCESS 0 (root for ireduce and ibcast): MPI_Ireduce(..., &req) MPI_Wait(&req); <-- blocking here ... MPI_Ibcast(..., &req2); MPI_Wait(&req2); PROCESS 1 (non-root for ireduce and ibcast): MPI_Ireduce(..., &req) MPI_Wait(&req); ... MPI_Ibcast(..., &req2); MPI_Wait(&req2); <-- blocking here Much of the time, the program deadlocks as shown above; sometimes this works fine, though, perhaps due to subtle timing differences. I mentioned above that this is a multithreaded program. I'm able to produce the issue with two threads with two MPI procs. The other threads are not calling MPI functions -- they are helping with other computation. I've verified that I don't have any TSAN or ASAN errors in this program. However, when I only have one thread per process, I don't have this issue. I think there's a decent chance, though, that this has to do with timing differences as opposed to changing anything with the MPI calls. I have verified that only one thread per process is calling the MPI routines in the multithreaded case. When I change the MPI_Ireduce to a blocking MPI_Reduce and I keep the MPI_Ibcast non-blocking, the program runs fine. Only when BOTH MPI_Ireduce and MPI_Ibcast happen serially do I see this deadlock (again, some of the time). Unfortunately, this program is part of a very large system and it's not straightforward to give a fully working example. So, I'm just looking for any ideas anyone has for what sort of thing may be happening, any information that may be helpful about how two coincident non-blocking requests could interact with each other, etc. Also, if anyone has tips on how to debug this sort of thing in gdb that would be helpful. For example, are there ways to introspect the MPI_Request object, etc.? Thanks

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Re: [mpich-discuss] Fortran 2008 MPI Interface Bug
by Thomas König 15 Nov '19

15 Nov '19

Hi Junchao, > Could you check if it is a gcc bug in handling type(*), dimension(..) Unfortunately, I don't know the code that well to be able to do it. If you could put together a simple, self-contained test case showing i there is a problem, you could submit that to gcc's bugzilla (or I could do that for you). I could also check a simplified test case against current gfortran trunk, which will not compile mpich 3.3.1 because it now flags argument mismatches as errors. (And yes, argument mismatches are real errors, which cause wrong code; see for example https://bugs.debian.org/944127 .)

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Re: [mpich-discuss] Fortran 2008 MPI Interface Bug
by Zhang, Junchao 14 Nov '19

14 Nov '19

Hi, Thomas, I tried gcc-9-branch and I could build MPICH f08 with it. I could also reproduce the problem Pedro Ricardo Souza reported. Could you check if it is a gcc bug in handling type(*), dimension(..) --Junchao Zhang On Thu, Nov 14, 2019 at 6:27 AM Thomas König via discuss <discuss(a)mpich.org<mailto:[email protected]>> wrote: Hi, just a general remark on this. If you were using gfortran 9, that has seen a fair number of bug fixes in that field in the recent weeks, which will be included in the next release of gcc 9 and the upcoming release of gcc 10. If you can, you might give the current gcc 9 branch a spin to see if the problem goes away. Any additional bug reports would be wecome 😉 _______________________________________________ discuss mailing list discuss(a)mpich.org<mailto:[email protected]> To manage subscription options or unsubscribe: https://lists.mpich.org/mailman/listinfo/discuss

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