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 !
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> users mailing list
> users@lists.open-mpi.org
> https://lists.open-mpi.org/mailman/listinfo/users
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