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Subject: [OMPI users] mpi_barrier
From: Huangwei (hz283_at_[hidden])
Date: 2013-09-28 19:19:08

Dear All,

In my code I implement mpi_send/mpi_receive for an three dimensional real
array, and process is as follows:

all other processors send the array to rank 0 and then rank 0 receives the
array and put these arrays into a complete array. Then mpi_bcast is called
to send the complete array from rank 0 to all others.

This is very basic usage of mpi_send and mpi_receive. In my fortran code I
found that if I added call mpi_barrier(...) before the mpi_send and
mpi_reive statements the wall time (60s) for this sending and receiving
will be much lower than that if mpi_barrier is not called (2s). I used
mpi_wtime to count the time.

I think mpi_send and mpi_recv are blocking subroutines and thus no
additional mpi_barrier is needed. Can anybody tell me what is the reason
for this phenomena? Thank you very much.

best regards,