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Section: 3.9 Good programming practice with SMC
Scali MPI Connect Release 4.4 Users Guide 32
3.9 Good programming practice with SMC
3.9.1 Matching MPI_Recv() with MPI_Probe()
During development and testing of SMC, Scali has come across several application programs
with the following code sequence:
while (...) {
MPI_Probe(MPI_ANY_SOURCE, MPI_ANY_TAG, comm, sts);
if (sts->MPI_TAG == SOME_VALUE) {
MPI_Recv(buf, cnt, dtype, MPI_ANY_SOURCE, MPI_ANY_TAG, comm, sts);
doStuff();
}
doOtherStuff();
}
For MPI implementations that have one, and only one, receive-queue for all senders, the
program’s code sequence works as desired. However, the code will not work as expected with
SMC. SMC uses one receive-queue per sender (inside each MPI-process). Thus, a message
from one sender can bypass the message from another sender. In the time-gap between the
completion of MPI_Probe() and before MPI_Recv() matches a message, another new message
from a different MPI-process could arrive, i.e. it is not certain that the message found by
MPI_Probe() is identical to one that MPI_Recv() matches.
To make the program work as expected, the code sequence should be corrected to:
while (...) {
MPI_Probe(MPI_ANY_SOURCE, MPI_ANY_TAG, comm, sts);
if (sts->MPI_TAG == SOME_VALUE) {
MPI_Recv(buf, cnt, dtype, sts->MPI_SOURCE, sts->MPI_TAG, comm, sts);
doStuff();
}
doOtherStuff();
}
3.9.2 Using MPI_Isend(), MPI_Irecv()
If communication and calculations do not overlap, using immediate calls, e.g., MPI_Isend()
and MPI_Irecv(), is usually performance ineffective.
3.9.3 Using MPI_Bsend()
Using buffered send, e.g., MPI_Bsend(), usually degrade performance significantly in
comparison with their unbuffered relatives.
3.9.4 Avoid starving MPI-processes - fairness
MPI programs may, if not special care is taken, be unfair and may starve MPI-processes, e.g.,
by using MPI_Waitany(), as illustrated for a client-server application in examples 3.15 and
3.16 in the MPI 1.1 standard [1]. Fairness can also be enforced, e.g. through the use of several
tags or separate communicators.