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Subject: [OMPI users] IO performance
From: Tom Rosmond (rosmond_at_[hidden])
Date: 2012-02-03 13:46:21

Recently the organization I work for bought a modest sized Linux cluster
for running large atmospheric data assimilation systems. In my
experience a glaring problem with systems of this kind is poor IO
performance. Typically they have 2 types of network: 1) A high speed,
low latency, e.g. Infiniband, network dedicated to MPI communications,
and, 2) A lower speed network, e.g 1Gb or 10Gb ethernet, for IO. On
clusters this second network is usually the basis for a global parallel
file system (GPFS), through which nearly all IO traffic must pass. So
the IO performance of applications such as ours is completely dependent
on the speed of the GPFS, and therefore on the network hardware it uses.

We have seen that a cluster with a GPFS based on a 1Gb network is
painfully slow for our applications, and of course with a 10Gb network
is much better. Therefore we are making the case to the IT staff that
all our systems should have GPFS running on 10Gb networks. Some of them
have a hard time accepting this, since they don't really understand the
requirements of our applications.

With all of this, here is my MPI related question. I recently added an
option to use MPI-IO to do the heavy IO lifting in our applications. I
would like to know what the relative importance of the dedicated MPI
network vis-a-vis the GPFS network for typical MPIIO collective reads
and writes. I assume there must be some hand-off of data between the
networks during the process, but how is it done, and are there any rules
to help understand it. Any insights would be welcome.

T. Rosmond

P.S. I am running with Open-mpi 1.4.2.