Not saying we won't change the behavior. Just saying the user can't expect a particular alignment as there is no guarantee in the standard. In Open MPI we just don't bother to align the pointer so right now so it naturally aligns as 64-bit. It isn't about wasting memory.
Also remember that by default the shared memory regions are contiguous across local ranks so each rank will get a buffer alignment dictated by the sizes of the allocations specified by the prior ranks in addition to the zero rank buffer alignment. -Nathan > On May 3, 2018, at 9:43 PM, Jeff Hammond <jeff.scie...@gmail.com> wrote: > > Given that this seems to break user experience on a relatively frequent > basis, I’d like to know the compelling reason why MPI implementers aren’t > willing to do something utterly trivial to fix it. > > And don’t tell me that 16B alignment wastes memory versus 8B alignment. > Open-MPI “wastes” 4B relative to MPICH for every handle on I32LP64 systems. > The internal state associated with MPI allocations - particularly windows - > is bigger than 8B. I recall ptmalloc uses something like 32B per heap > allocation. > > Jeff > >> On Thu, May 3, 2018 at 5:20 PM Nathan Hjelm <hje...@me.com> wrote: >> That is probably it. When there are 4 ranks there are 4 int64’s just before >> the user data (for PSCW). With 1 rank we don’t even bother, its just malloc >> (16-byte aligned). With any other odd number of ranks the user data is after >> an odd number of int64’s and is 8-byte aligned. There is no requirement in >> MPI to provide 16-byte alignment (which is required for _Atomic __int128 >> because of the alignment requirement of cmpxchg16b) so you have to align it >> yourself. >> >> -Nathan >> >> > On May 3, 2018, at 2:16 PM, Joseph Schuchart <schuch...@hlrs.de> wrote: >> > >> > Martin, >> > >> > You say that you allocate shared memory, do you mean shared memory >> > windows? If so, this could be the reason: >> > https://github.com/open-mpi/ompi/issues/4952 >> > >> > The alignment of memory allocated for MPI windows is not suitable for >> > 128bit values in Open MPI (only 8-byte alignment is guaranteed atm). I >> > have seen the alignment change depending on the number of processes. Could >> > you check the alignment of the memory you are trying to access? >> > >> > Cheers, >> > Joseph >> > >> > On 05/03/2018 08:48 PM, Martin Böhm wrote: >> >> Dear all, >> >> I have a problem with a segfault on a user-built OpenMPI 3.0.1 running on >> >> Ubuntu 16.04 >> >> in local mode (only processes on a single computer). >> >> The problem manifests itself as a segfault when allocating shared memory >> >> for >> >> (at least) one 128-bit atomic variable (say std::atomic<__int128>) and >> >> then >> >> storing into this variable. This error only occurs when there are at >> >> least two >> >> processes, even though only the process of shared rank 0 is the one doing >> >> both >> >> the allocation and the writing. The compile flags -march=core2 or >> >> -march=native >> >> or -latomic were tried, none of which helped. >> >> An example of the code that triggers it on my computers is this: >> >> https://github.com/bohm/binstretch/blob/parallel-classic/algorithm/bug.cpp >> >> The code works fine with mpirun -np 1 and segfaults with mpirun -np 2, 3 >> >> and 4; >> >> if line 41 is commented out (the 128-bit atomic write), everything works >> >> fine >> >> with -np 2 or more. >> >> As for Ubuntu's stock package containing OpenMPI 1.10.2, the code >> >> segfaults with >> >> "-np 2" and "-np 3" but not "-np 1" or "-np 4". >> >> Thank you for any assistance concerning this problem. I would suspect my >> >> own >> >> code to be the most likely culprit, since it triggers on both the stock >> >> package >> >> and custom-built OpenMPI. >> >> I attach the config.log.bz2 and ompi_info.log. Below I list some runs of >> >> the program >> >> and what errors are produced. >> >> Thank you for any assistance. I have tried googling and searching the >> >> mailing list >> >> for this problem; if I missed something, I apologize. >> >> Martin Böhm >> >> ----- Ubuntu 16.04 stock mpirun and mpic++ ----- >> >> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/bin/mpirun -np 1 >> >> ../tests/bug >> >> ex1 success. >> >> ex2 success. >> >> Inserted into ex1. >> >> Inserted into ex2. >> >> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/bin/mpirun -np 4 >> >> ../tests/bug >> >> Thread 2: ex1 success. >> >> Thread 3: ex1 success. >> >> ex1 success. >> >> Thread 1: ex1 success. >> >> ex2 success. >> >> Thread 2: ex2 success. >> >> Thread 1: ex2 success. >> >> Thread 3: ex2 success. >> >> Inserted into ex1. >> >> Inserted into ex2. >> >> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/bin/mpirun -np 2 >> >> ../tests/bug >> >> Thread 1: ex1 success. >> >> ex1 success. >> >> Thread 1: ex2 success. >> >> ex2 success. >> >> Inserted into ex1. >> >> [kamenice:13662] *** Process received signal *** >> >> [kamenice:13662] Signal: Segmentation fault (11) >> >> [kamenice:13662] Signal code: (128) >> >> [kamenice:13662] Failing at address: (nil) >> >> [kamenice:13662] [ 0] >> >> /lib/x86_64-linux-gnu/libc.so.6(+0x354b0)[0x7f31773844b0] >> >> [kamenice:13662] [ 1] ../tests/bug[0x40d8ac] >> >> [kamenice:13662] [ 2] ../tests/bug[0x408997] >> >> [kamenice:13662] [ 3] ../tests/bug[0x408bf0] >> >> [kamenice:13662] [ 4] >> >> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7f317736f830] >> >> [kamenice:13662] [ 5] ../tests/bug[0x4086e9] >> >> [kamenice:13662] *** End of error message *** >> >> ----- Ubuntu 16.04 custom-compiled OpenMPI 3.0.1, installed to /usr/local >> >> (the stock packages were uninstalled) ----- >> >> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/local/bin/mpirun -np 1 >> >> ../tests/bug >> >> ex1 success. >> >> ex2 success. >> >> Inserted into ex1. >> >> Inserted into ex2. >> >> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/local/bin/mpirun -np 2 >> >> ../tests/bug >> >> ex1 success. >> >> Thread 1: ex1 success. >> >> Inserted into ex1. >> >> [kamenice:22794] *** Process received signal *** >> >> ex2 success. >> >> Thread 1: ex2 success. >> >> [kamenice:22794] Signal: Segmentation fault (11) >> >> [kamenice:22794] Signal code: (128) >> >> [kamenice:22794] Failing at address: (nil) >> >> [kamenice:22794] [ 0] >> >> /lib/x86_64-linux-gnu/libc.so.6(+0x354b0)[0x7ff8bad084b0] >> >> [kamenice:22794] [ 1] ../tests/bug[0x401010] >> >> [kamenice:22794] [ 2] ../tests/bug[0x400d27] >> >> [kamenice:22794] [ 3] ../tests/bug[0x400f80] >> >> [kamenice:22794] [ 4] >> >> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7ff8bacf3830] >> >> [kamenice:22794] [ 5] ../tests/bug[0x400a79] >> >> [kamenice:22794] *** End of error message *** >> >> ------------------------------------------------------- >> >> Primary job terminated normally, but 1 process returned >> >> a non-zero exit code. Per user-direction, the job has been aborted. >> >> ------------------------------------------------------- >> >> -------------------------------------------------------------------------- >> >> mpirun noticed that process rank 0 with PID 0 on node kamenice exited on >> >> signal 11 (Segmentation fault). >> >> -------------------------------------------------------------------------- >> >> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/local/bin/mpirun -np 4 >> >> --oversubscribe ../tests/bug >> >> ex1 success. >> >> Thread 1: ex1 success. >> >> Thread 2: ex1 success. >> >> Thread 3: ex1 success. >> >> ex2 success. >> >> Thread 1: ex2 success. >> >> Thread 2: ex2 success. >> >> Thread 3: ex2 success. >> >> Inserted into ex1. >> >> [kamenice:22728] *** Process received signal *** >> >> [kamenice:22728] Signal: Segmentation fault (11) >> >> [kamenice:22728] Signal code: (128) >> >> [kamenice:22728] Failing at address: (nil) >> >> [kamenice:22728] [ 0] >> >> /lib/x86_64-linux-gnu/libc.so.6(+0x354b0)[0x7f826a6294b0] >> >> [kamenice:22728] [ 1] ../tests/bug[0x401010] >> >> [kamenice:22728] [ 2] ../tests/bug[0x400d27] >> >> [kamenice:22728] [ 3] ../tests/bug[0x400f80] >> >> [kamenice:22728] [ 4] >> >> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7f826a614830] >> >> [kamenice:22728] [ 5] ../tests/bug[0x400a79] >> >> [kamenice:22728] *** End of error message *** >> >> ------------------------------------------------------- >> >> Primary job terminated normally, but 1 process returned >> >> a non-zero exit code. Per user-direction, the job has been aborted. >> >> ------------------------------------------------------- >> >> -------------------------------------------------------------------------- >> >> mpirun noticed that process rank 0 with PID 0 on node kamenice exited on >> >> signal 11 (Segmentation fault). >> >> -------------------------------------------------------------------------- >> >> bohm@kamenice:~/cl/w/b/classic/algorithm$ /usr/local/bin/mpirun -np 2 >> >> valgrind ../tests/bug >> >> ==22814== Memcheck, a memory error detector >> >> ==22814== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al. >> >> ==22814== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright >> >> info >> >> ==22814== Command: ../tests/bug >> >> ==22814== >> >> ==22815== Memcheck, a memory error detector >> >> ==22815== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al. >> >> ==22815== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright >> >> info >> >> ==22815== Command: ../tests/bug >> >> ==22815== >> >> ex1 success. >> >> Thread 1: ex1 success. >> >> Thread 1: ex2 success. >> >> ex2 success. >> >> Inserted into ex1. >> >> [kamenice:22814] *** Process received signal *** >> >> [kamenice:22814] Signal: Segmentation fault (11) >> >> [kamenice:22814] Signal code: (128) >> >> [kamenice:22814] Failing at address: (nil) >> >> [kamenice:22814] [ 0] /lib/x86_64-linux-gnu/libc.so.6(+0x354b0)[0x51704b0] >> >> [kamenice:22814] [ 1] ../tests/bug[0x401010] >> >> [kamenice:22814] [ 2] ../tests/bug[0x400d27] >> >> [kamenice:22814] [ 3] ../tests/bug[0x400f80] >> >> [kamenice:22814] [ 4] >> >> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x515b830] >> >> [kamenice:22814] [ 5] ../tests/bug[0x400a79] >> >> [kamenice:22814] *** End of error message *** >> >> ==22814== >> >> ==22814== Process terminating with default action of signal 11 (SIGSEGV) >> >> ==22814== at 0x5170428: raise (raise.c:54) >> >> ==22814== by 0x5827E4D: show_stackframe (in >> >> /usr/local/lib/libopen-pal.so.40.1.0) >> >> ==22814== by 0x51704AF: ??? (in /lib/x86_64-linux-gnu/libc-2.23.so) >> >> ==22814== by 0x40100F: std::atomic<__int128>::store(__int128, >> >> std::memory_order) (atomic:225) >> >> ==22814== by 0x400D26: shared_memory_init(int, int) (bug.cpp:41) >> >> ==22814== by 0x400F7F: main (bug.cpp:80) >> >> ==22814== >> >> ==22814== HEAP SUMMARY: >> >> ==22814== in use at exit: 2,759,989 bytes in 9,014 blocks >> >> ==22814== total heap usage: 20,168 allocs, 11,154 frees, 3,820,707 >> >> bytes allocated >> >> ==22814== >> >> ==22814== LEAK SUMMARY: >> >> ==22814== definitely lost: 12 bytes in 1 blocks >> >> ==22814== indirectly lost: 0 bytes in 0 blocks >> >> ==22814== possibly lost: 608 bytes in 2 blocks >> >> ==22814== still reachable: 2,759,369 bytes in 9,011 blocks >> >> ==22814== suppressed: 0 bytes in 0 blocks >> >> ==22814== Rerun with --leak-check=full to see details of leaked memory >> >> ==22814== >> >> ==22814== For counts of detected and suppressed errors, rerun with: -v >> >> ==22814== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0) >> >> ------------------------------------------------------- >> >> Primary job terminated normally, but 1 process returned >> >> a non-zero exit code. Per user-direction, the job has been aborted. >> >> ------------------------------------------------------- >> >> ==22815== >> >> ==22815== Process terminating with default action of signal 15 (SIGTERM) >> >> ==22815== at 0x523674D: ??? (syscall-template.S:84) >> >> ==22815== by 0x583B4A7: poll (poll2.h:46) >> >> ==22815== by 0x583B4A7: poll_dispatch (poll.c:165) >> >> ==22815== by 0x5831BDE: opal_libevent2022_event_base_loop >> >> (event.c:1630) >> >> ==22815== by 0x57F210D: progress_engine (in >> >> /usr/local/lib/libopen-pal.so.40.1.0) >> >> ==22815== by 0x5FD76B9: start_thread (pthread_create.c:333) >> >> ==22815== by 0x524241C: clone (clone.S:109) >> >> ==22815== >> >> ==22815== HEAP SUMMARY: >> >> ==22815== in use at exit: 2,766,405 bytes in 9,017 blocks >> >> ==22815== total heap usage: 20,167 allocs, 11,150 frees, 3,823,751 >> >> bytes allocated >> >> ==22815== >> >> ==22815== LEAK SUMMARY: >> >> ==22815== definitely lost: 12 bytes in 1 blocks >> >> ==22815== indirectly lost: 0 bytes in 0 blocks >> >> ==22815== possibly lost: 608 bytes in 2 blocks >> >> ==22815== still reachable: 2,765,785 bytes in 9,014 blocks >> >> ==22815== suppressed: 0 bytes in 0 blocks >> >> ==22815== Rerun with --leak-check=full to see details of leaked memory >> >> ==22815== >> >> ==22815== For counts of detected and suppressed errors, rerun with: -v >> >> ==22815== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0) >> >> -------------------------------------------------------------------------- >> >> mpirun noticed that process rank 0 with PID 0 on node kamenice exited on >> >> signal 9 (Killed). >> >> -------------------------------------------------------------------------- >> >> _______________________________________________ >> >> users mailing list >> >> users@lists.open-mpi.org >> >> https://lists.open-mpi.org/mailman/listinfo/users >> > >> > >> > -- >> > Dipl.-Inf. Joseph Schuchart >> > High Performance Computing Center Stuttgart (HLRS) >> > Nobelstr. 19 >> > D-70569 Stuttgart >> > >> > Tel.: +49(0)711-68565890 >> > Fax: +49(0)711-6856832 >> > E-Mail: schuch...@hlrs.de >> > _______________________________________________ >> > users mailing list >> > users@lists.open-mpi.org >> > https://lists.open-mpi.org/mailman/listinfo/users >> >> _______________________________________________ >> users mailing list >> users@lists.open-mpi.org >> https://lists.open-mpi.org/mailman/listinfo/users > -- > Jeff Hammond > jeff.scie...@gmail.com > http://jeffhammond.github.io/ > _______________________________________________ > users mailing list > users@lists.open-mpi.org > https://lists.open-mpi.org/mailman/listinfo/users
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