Andriy,
Good analysis.
However, with such a simple program as this classic Ping-Pong test
"MPI_Send" would not be a problem, AS LONG AS you change the SEND+RECV order
for one of the ranks. So one is in RECV while the other SENDs and vice versa.
This guarantees that you do not get into a deadlock, even if the
implementation does not buffer the messages.
See the new code below:
Andriy Y. Fedorov wrote:
>>I'm using MPI_Isend and MPI_Recv in a loop like in the code I attached
>>to this mail. In each iteration the code becomes slower and slower.
>>What is the reason of this behaviour? How can I
>>prevent this?
>>Using MPI_Send instead of MPI_ISend I have no problem.
>>But when I have data with a size greater than 8192 Bytes in this case
>>MPI_Send hangs.
>>
>
> . . .
>
>> for (i = 0; ; i++)
>> {
>> if (rank == 0)
>> {
>> MPI_Send (send, size, MPI_DOUBLE, 1, 1, MPI_COMM_WORLD, &status);
>> MPI_Recv (recv, size, MPI_DOUBLE, 1, 2, MPI_COMM_WORLD, &status);
>> }
>> if (rank == 1)
>> {
/* RANK 0 = SEND + RECV, RANK 1 = RECV + SEND, changed order below */
>> MPI_Recv (recv, size, MPI_DOUBLE, 0, 1, MPI_COMM_WORLD, &status);
>> MPI_Send (send, size, MPI_DOUBLE, 0, 2, MPI_COMM_WORLD, &status);
>> }
>> if ((rank == 0) && (i > 0) && (i % 1000 == 0)) {
>> printf("iteration = %i, time = %u\n", i, time(t)-oldtime);
>> oldtime = time(t);
>> }
>> }
>
>
> Yeah, I remember it took me about a day to figure that out. Wasn't that hard
> though. You have to MPI_Request_free() request objects returned by non-blocking
> MPI operations. If you don't do this, I suspect they are kept somewhere inside
> LAM, I didn't have time to dig and make sure that's really the issue.
> Performance jumps up if you do release them though. I don't know if it is
> mandatory by the MPI standard.
>
> You should NOT use MPI_Send. As MPI standards says somewhere, you should be
> able to substitute MPI_Send with MPI_Ssend to make sure your application is
> deadlock-free.
>
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