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From: Daniel Rozenbaum (drozenbaum_at_[hidden])
Date: 2007-10-03 13:41:55

Hi again,

I'm trying to debug the problem I posted on several times recently; I thought I'd try asking a more focused question:

I have the following sequence in the client code:
MPI_Status stat;
ret = MPI_Probe(0, MPI_ANY_TAG, MPI_COMM_WORLD, &stat);
assert(ret == MPI_SUCCESS);
ret = MPI_Get_elements(&stat, MPI_BYTE, &count);
assert(ret == MPI_SUCCESS);
char *buffer = malloc(count);
assert(buffer != NULL);
ret = MPI_Recv((void *)buffer, count, MPI_BYTE, 0, stat.MPI_TAG, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
assert(ret == MPI_SUCCESS);
fprintf(stderr, "MPI_Recv done\n");
<proceed to taking action on the received buffer, send response to server>
Each MPI_ call in the lines above is surrounded by debug prints that print out the client's rank, current time, the action about to be taken with all its parameters' values, and the action's result. After the first cycle (receive message from server -- process it -- send response -- wait for next message) works out as expected, the next cycle get stuck in MPI_Recv. What I get in my debug prints is more or less the following:
MPI_Probe(source= 0, tag= MPI_ANY_TAG, comm= MPI_COMM_WORKD, status= <address1>)
MPI_Probe done, source= 0, tag= 2, error= 0
MPI_Get_elements(status= <address1>, dtype= MPI_BYTE, count= <address2>)
MPI_Get_elements done, count= 2731776
MPI_Recv(buf= <address3>, count= 2731776, dtype= MPI_BYTE, src= 0, tag= 2, comm= MPI_COMM_WORLD, stat= MPI_STATUS_IGNORE)
<nothing beyond this point. Some time afterwards there're "readv failed" errors in server's stderr>
My question then is this - what would cause MPI_Recv to not return, after the immediately preceding MPI_Probe and MPI_Get_elements return properly?