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Subject: [OMPI users] Accessing to the send buffer
From: Alberto Canestrelli (canestrelli_at_[hidden])
Date: 2010-08-02 01:05:23


Hi,
I have a problem with a fortran code that I have parallelized with MPI.
I state in advance that I read the whole ebook "Mit Press - Mpi - The
Complete Reference, Volume 1" and I took different MPI classes, so I
have a discrete MPI knowledge. I was able to solve by myself all the
errors I bumped into but now I am not able to find the bug of my code
that provides erroneous results. Without entering in the details of my
code, I think that the cause of the problem could be reletad to the
following aspect highlighted in the above ebook (in the follow I copy
and paste from the e-book):

A nonblocking post-send call indicates that the system may start copying
data
out of the send buffer. The sender must not access any part of the send
buffer
(neither for loads nor for STORES) after a nonblocking send operation is
posted until
the complete send returns.
A nonblocking post-receive indicates that the system may start writing
data into
the receive buffer. The receiver must not access any part of the receive
buffer after
a nonblocking receive operation is posted, until the complete-receive
returns.
Rationale. We prohibit read accesses to a send buffer while it is being
used, even
though the send operation is not supposed to alter the content of this
buffer. This
may seem more stringent than necessary, but the additional restriction
causes little
loss of functionality and allows better performance on some systems-
consider
the case where data transfer is done by a DMA engine that is not
cache-coherent
with the main processor.End of rationale.

I use plenty of nonblocking post-send in my code. Is it really true that
the sender must not access any part of the send buffer not even for
STORES? Or was it a MPI 1.0 issue?
Thanks.
alberto