On 5/6/2010 10:30 PM, John Hearns wrote:
On 7 May 2010 03:17, Jeff Squyres<jsquy...@cisco.com> wrote:
Indeed. I have seen some people have HT enabled in the bios just so that they
can have the software option of turning them off via linux -- then you can run
with HT and without it and see what it does to your specific codes.
I may have missed this on the thread, but how do you do that?
The Nehalem systems I have came delivered with HT enabled in the BIOS
- I know it is not a real pain to reboot and configure, but it would
be a lot easir to leave it on and switch off in software - also if you
wanted to do back-to-back testing of performance with/without HT.
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I don't know what Jeff meant by that, but we haven't seen a feasible way
of disabling HT without rebooting and using the BIOS options. It is
feasible to place 1 MPI process or thread per core. With careful
affinity, performance when using 1 logical per core normally is
practically the same as with HT disabled.
--
Tim Prince
