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
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