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Subject: Re: [OMPI users] Ensuring use of real cores
From: Ralph Castain (rhc_at_[hidden])
Date: 2012-09-12 08:05:00

On Sep 12, 2012, at 4:57 AM, "John R. Cary" <cary_at_[hidden]> wrote:

> I do want in fact to bind first to one HT of each core
> before binding to two HTs of one core. So that will
> be possible in 1.7?

Yes - you can get a copy of the 1.7 nightly tarball and experiment with it in advance, if you like. You'll want

mpirun --map-by core --bind-to hwthread ....

Add --report-bindings to see what happens, but I believe that will do what you want. You'll map one process to each core, and bind it to only the first hwthread on that core.

Let me know either way - if it doesn't, we have time to adjust it.

> Thx....John
> On 9/11/12 11:19 PM, Ralph Castain wrote:
>> Not entirely sure I know what you mean. If you are talking about running without specifying binding, then it makes no difference - we'll run wherever the OS puts us, so you would need to tell the OS not to use the virtual cores (i.e., disable HT).
>> If you are talking about binding, then pre-1.7 releases all bind to core at the lowest level. On a hyperthread-enabled machine, that binds you to both HT's of a core. Starting with the upcoming 1.7 release, you can bind to the separate HTs, but that doesn't sound like something you want to do.
>> HTH
>> Ralph
>> On Sep 11, 2012, at 6:34 PM, John R. Cary <cary_at_[hidden]> wrote:
>>> Our code gets little benefit from using virtual cores (hyperthreading),
>>> so when we run with mpiexec on an 8 real plus 8 virtual machine, we
>>> would like to be certain that it uses only the 8 real cores.
>>> Is there a way to do this with openmpi?
>>> Thx....John
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