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Subject: Re: [OMPI users] scaling problem with openmpi
From: Peter Kjellstrom (cap_at_[hidden])
Date: 2009-05-20 10:07:12


On Wednesday 20 May 2009, Rolf Vandevaart wrote:
...
> If I am understanding what is happening, it looks like the original
> MPI_Alltoall made use of three algorithms. (You can look in
> coll_tuned_decision_fixed.c)
>
> If message size < 200 or communicator size > 12
> bruck
> else if message size < 3000
> basic linear
> else
> pairwise
> end

And 3000 was the observed threshold for bad behaviour so it seems very likely
that "basic linear" was the culprit. My testing would suggest that "pairwise"
was a good choice for ~3000 (but maybe bruck, as configured by Pavel, is good
too).

/Peter

> With the file Pavel has provided things have changed to the following.
> (maybe someone can confirm)
>
> If message size < 8192
> bruck
> else
> pairwise
> end
>
> Rolf