Thank you for your patience. Best wishes !
Gilles Gouaillardet <gilles.gouaillar...@gmail.com> 于2018年12月16日周日 下午11:24写道： > 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
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