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Subject: Re: [OMPI users] How do I run OpenMPI safely on a Nehalem standalone machine?
From: Prentice Bisbal (prentice_at_[hidden])
Date: 2010-05-05 11:59:16

Douglas Guptill wrote:
> On Tue, May 04, 2010 at 05:34:40PM -0600, Ralph Castain wrote:
>> On May 4, 2010, at 4:51 PM, Gus Correa wrote:
>>> Hi Ralph
>>> Ralph Castain wrote:
>>>> One possibility is that the sm btl might not like that you have hyperthreading enabled.
>>> I remember that hyperthreading was discussed months ago,
>>> in the previous incarnation of this problem/thread/discussion on "Nehalem vs. Open MPI".
>>> (It sounds like one of those supreme court cases ... )
>>> I don't really administer that machine,
>>> or any machine with hyperthreading,
>>> so I am not much familiar to the HT nitty-gritty.
>>> How do I turn off hyperthreading?
>>> Is it a BIOS or a Linux thing?
>>> I may try that.
>> I believe it can be turned off via an admin-level cmd, but I'm not certain about it
> The challenge was too great to resist, so I yielded, and rebooted my
> Nehalem (Core i7 920 @ 2.67 GHz) to confirm my thoughts on the issue.
> Entering the BIOS setup by pressing "DEL", and "right-arrowing" over
> to "Advanced", then "down arrow" to "CPU configuration", I found a
> setting called "Intel (R) HT Technology". The help dialogue says
> "When Disabled only one thread per core is enabled".
> Mine is "Enabled", and I see 8 cpus. The Core i7, to my
> understanding, is a 4 core chip.

That's correct, HyperThreading makes it look like there are twice as
many cores as there physically are. So an n-core system will look like
it has 2n cores to the OS when HT is enabled.