Here's my advice: Don't trust anyones advice. Benchmark it yourself and
see.
The problems vary so wildly that only you can tell if your problem will
benefit from oversubscription. It really depends on too many factors to
accurately predict: schedulers, memory usage, network/interconnect
hardware, disk seek times, and probably a hundred other things.
I've even seen mixed results from oversubscribing within a single
algorithm. (Granted this is mostly with the older generation
hyperthreading, so I'm not sure how things fare with nehalem). The most
notable effect I've observed is related to cache use. If the problem
fits in cache it is much faster. With cores sharing cache it can even
be advantageous to *undersubscribe* the problem. i.e. schedule 2
processes on a quad core so each can have the full cache.
-- Mark Borgerding
Klymak Jody wrote:
Hi Robert,
I ran some very crude tests and found that things slowed down once you
got over 8 cores at a time. However, they didn't slow down by 50% if
you went to 16 processes. Sadly, the tests were so crude, I did not
keep good notes (it appears).
I'm running a gcm, so my benchmarks may not be very useful to most
folks. If there was an easy-to-compile benhmark that I could run on
my cluster, I'd be curious what the results are too.
Thanks, Jody
On 11-Jul-09, at 2:16 PM, Robert Kubrick wrote:
The Open MPI FAQ recommends not to oversubscribe the available cores
for best performances, but is this still true? The new Nehalem
processors are built to run 2 threads on each core. On a 8 sockets
systems, that sums up to 128 threads that Intel claims can be run
without significant performance degradation. I guess the last word is
to those who have tried to run some benchmarks and applications on
the new Intel processors. Any experience to share?
http://www.open-mpi.org/faq/?category=running#oversubscribing
http://en.wikipedia.org/wiki/Simultaneous_multithreading
http://communities.intel.com/community/openportit/server/blog/2009/06/11/nehalem-ex-brings-new-economics-to-scalable-systems
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Mark Borgerding
3dB Labs, Inc
Innovate. Develop. Deliver.
