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Subject: Re: [OMPI users] openmpi credits for eager messages
From: Patrick Geoffray (patrick_at_[hidden])
Date: 2008-02-04 14:25:09


Brightwell, Ronald wrote:
> Not to muddy the point, but if there's enough ambiguity in the Standard
> for people to ignore the progress rule, then I think (hope) there's enough
> ambiguity for people to ignore the sender throttling issue too ;)

I understand your position, and I used to agree until I was forced to
change my mind by naive users :-)

Poorly written MPI codes won't likely segfault or deadlock because the
progress rule was ignored. However, users will proudly tell you that you
have a memory leak if you don't limit the size of the unexpected queue
and their codes with no flow control blow up.

You don't have to make it very efficient (per-sender credits
definitively does not scale), but you need to have a way to stall/slow
the sender when the unexpected queue gets too big. That's quite easy to
do without affecting the common case.