On 5/6/2010 10:30 PM, John Hearns wrote:
> On 7 May 2010 03:17, Jeff Squyres<jsquyres_at_[hidden]> 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.
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.