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Subject: Re: [OMPI devel] kernel 2.6.23 vs 2.6.24 - communication/wait times
From: Peter Kjellstrom (cap_at_[hidden])
Date: 2010-04-19 14:14:53

On Monday 19 April 2010, Oliver Geisler wrote:
> > Ah, that could do it. Open MPI's shared memory files are under /tmp. So
> > if /tmp is NFS, you could get extremely high latencies because of dirty
> > page writes out through NFS.
> >
> > You don't necessarily have to make /tmp disk-full -- if you just make
> > OMPI's session directories go into a ramdisk instead of to NFS, that
> > should also be sufficient.
> I just browsed FAQ and "ompi_info --param all all", but didn't find the
> answer:
> How do I set the OMPI session directory to point it to a ramdisk?
> And another question:
> What would be a good size for the ram disk? One general value was
> supposed by the FAQ with 128MB, but what is your experience?
> (maybe a large topic by itself, so I have to try out, I guess)

I just wanted to add that space not used on a tmpfs (mount -t tmpfs ...) is
not wasted. You can have an 8G tmpfs mounted but if you only use 100M that's
how much RAM it uses.