Almost :-) seqBegin() and seqEnd() takes a communicator as a parameter. So attribute caching is good for performance (only) if more than one sequence *per communicator* is used in the application.
Cheers, Gilles On Sun, Dec 16, 2018 at 11:38 PM 邹海峰 <haifengzou1...@gmail.com> wrote: > > Thank you very much for the reply. > > According to your explanation and the content from the website, if there is > only one sequential execution in the program, then it doesn't matter whether > using the attribute. But if there are multiple sequential execution, each > process only needs to use MPI_Comm_dup() once with the help of attribute, > instead of duplicating the MPI_COMM_WORLD whenever there is a sequential > execution. Am I right? > > Best wishes! > > Gilles Gouaillardet <gilles.gouaillar...@gmail.com> 于2018年12月16日周日 上午8:14写道： >> >> Hi, >> >> Your understanding is incorrect : >> "Attributes are local to the process and specific to the communicator >> to which they are attached." >> (per >> https://www.mcs.anl.gov/research/projects/mpi/mpi-standard/mpi-report-1.1/node119.htm) >> >> think an attribute is often a pointer, and really bad things can >> happen if rank 0 uses a pointer that is valid on rank 1, >> so if attributes were global, they would be virtually unusable. >> >> note the comment before the barrier is incorrect. this is a simple >> barrier and all MPI tasks will block until all of them invoke seqEnd() >> >> The main goal of using attributes in this example is to invoke >> MPI_Comm_dup() once per communicator (instead of once per sequence, >> since this is an expensive operation). >> >> >> Cheers, >> >> Gilles >> On Sun, Dec 16, 2018 at 1:04 AM 邹海峰 <haifengzou1...@gmail.com> wrote: >> > >> > Hi there, >> > >> > At first, I think attribute is just like global variable that attached to >> > a specific communicator. I can define and set the value on one process, >> > then get and modify the value on another process as long as those >> > processes belonging to the same communicator. But when I was reading >> > chapter 6 of the book using mpi: portable parallel programming with the >> > message-passing interface. I was confused by the usage of caching >> > attribute. >> > >> > The purpose the code is to make the execution sequential. the main >> > function is >> > >> > seqBegin( MPI_COMM_WORLD ); >> > printf( "My rank is %d\n", wrank ); >> > fflush( stdout ); >> > seqEnd( MPI_COMM_WORLD ); >> > >> > which is simple to understand. The program will print the rank in order. >> > The defination of the function "seqBegin()" is >> > >> > static int seqKeyval = MPI_KEYVAL_INVALID; >> > >> > void seqBegin( MPI_Comm comm ) >> > { >> > MPI_Comm lcomm; >> > int flag, mysize, myrank; >> > seqInfo *info; >> > if (seqKeyval == MPI_KEYVAL_INVALID) { >> > MPI_Comm_create_keyval( MPI_NULL_COPY_FN, seqDelFn, &seqKeyval, NULL ); >> > } >> > MPI_Comm_get_attr( comm, seqKeyval, &info, &flag ); >> > if (!flag) { >> > info = (seqInfo *)malloc( sizeof(seqInfo) ); >> > MPI_Comm_dup( comm, &info->lcomm ); >> > MPI_Comm_rank( info->lcomm, &myrank ); >> > MPI_Comm_size( info->lcomm, &mysize ); >> > info->prevRank = myrank - 1; >> > if (info->prevRank < 0) info->prevRank = MPI_PROC_NULL; >> > info->nextRank = myrank + 1; >> > if (info->nextRank >= mysize) info->nextRank = MPI_PROC_NULL; >> > if (verbose) { >> > printf( "seqbegin: prev = %d, next = %d\n", >> > info->prevRank, info->nextRank ); >> > } >> > MPI_Comm_set_attr( comm, seqKeyval, info ); >> > } >> > MPI_Recv( NULL, 0, MPI_INT, info->prevRank, 0, info->lcomm, >> > MPI_STATUS_IGNORE ); >> > } >> > >> > and the defination of function "seqEnd()" is >> > >> > void seqEnd( MPI_Comm comm ) >> > { >> > seqInfo *info; >> > int flag; >> > >> > /* Sanity check */ >> > if (seqKeyval == MPI_KEYVAL_INVALID) >> > MPI_Abort( MPI_COMM_WORLD, 1 ); >> > MPI_Comm_get_attr( comm, seqKeyval, &info, &flag ); >> > if (!info || !flag) >> > MPI_Abort( MPI_COMM_WORLD, 1 ); >> > if (verbose) { >> > printf( "seqend: prev = %d, next = %d\n", >> > info->prevRank, info->nextRank ); >> > } >> > MPI_Send( NULL, 0, MPI_INT, info->nextRank, 0, info->lcomm ); >> > >> > /* Make everyone wait until all have completed their send */ >> > MPI_Barrier( info->lcomm ); >> > } >> > >> > Other details are omitted. In fact, all the codes can be found in >> > https://www.mcs.anl.gov/research/projects/mpi/usingmpi/examples-usingmpi/libraries/index.html >> > which is provided by the author of the book. >> > >> > The program uses send and recv to block the execution. Only if the process >> > receive the message from last process, the process can continue to >> > execute, otherwise it is blocked, which resulting in the sequential >> > execution. The part I don't understand is in function "seqBegin()". If my >> > undertstanding about attribute is right, only one process will enter the >> > if condition and set the value of attribute, other processes just get the >> > value . Here comes the question: since other processes don't set the >> > value, how can they get the prevRank and nextRank of their own. >> > >> > The code can be executed as expected. But I still can't get the rational >> > behind this and there is little reference about attribute caching, so I >> > come here for help. Thank you very much ! >> > >> > Best Wishes ! >> > _______________________________________________ >> > 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 > > _______________________________________________ > 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
