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Subject: [OMPI users] Very poor performance with btl sm on twin nehalem servers with Mellanox Technologies MT26428 (ConnectX)
From: Oskar Enoksson (enok_at_[hidden])
Date: 2010-05-11 14:19:37


I have a cluster with two Intel Xeon Nehalem E5520 CPU per server
(quad-core, 2.27GHz). The interconnect is 4xQDR Infiniband (Mellanox
ConnectX).

I have compiled and installed OpenMPI 1.4.2. The kernel is 2.6.32.2 and
I have compiled the kernel myself. I use gridengine 6.2u5. Openmpi was
compiled with "--with-libnuma --with-sge.

The problem is that I get very bad performance unless I explicitly
exclude the "sm" btl and I can't figure out why. I have tried searching
the web and the OpenMPI mailing lists. I have seen reports about
non-optimal performance, but my results are far worse than any other
reports I have found.

I run the "mpi_stress" program with different packet lengths. I run on a
single server using 8 slots so that all eight cores on one server are
occupied.

When I use "-mca btl self,openib" I get pretty good results, between
450MB/s and 700MB/s depending on the packet lengths. When I use "-mca
btl self,sm" or "-mca btl self,sm,openib" I just get 25MB/s to 30MB/s
for packet length 1MB. For 10kB packets the results are around 5MB/s.
things get abour 20% faster if I set "-mca paffinity_alone 1".

What is going on? Any hints? I thought these CPU's had excellent
SM-bandwidth over quickpath. I expected several GB/s.

Hyperthreading is enabled, if that is relevant. The locked-memory limit
is 500MB and the stack limit is 64MB.

Please help!
Thanks
/Oskar