Re: [Xen-users] Xen 4.6 and Intel MKL Linpack behaviour

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Mon, 2016-05-09 at 15:37 +0200, Marko Đukić wrote:
> On 9 May 2016 at 14:57, Dario Faggioli <dario.faggioli@xxxxxxxxxx>
> As you can see, in HVM guest shows 4 cpus and each entry shows "cpu
> cores: 4" and a different core id for each entry.
> In PVH guest shows "cpu cores: 1" and "core_id: 0" for each entry.
>
Ok, so you do get 4 vcpus, but they are all listed as if the were part
of the same core.

This happens because of the difference between CPUID (and topology
handling in general) implementation between PV(H) and HVM that Roger
was mentioning.

There is work ongoing for improving this (making things more
configurable and reliable), both on the Xen and Linux kernel sides, but
patches are not upstream yet.

If LINPACK relies on this 'core id' (or something equivalent/based on
the same low level topology info), there's few that we can do, apart
from changing LINPACK itself.

Maybe (before embarking in changing LINPAK), fiddling with the cpuid
config option is worth a try.

For example, in this thread:
 http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg04423.html

it's suggested to set it to:
 cpuid=['0x1:ebx=xxxxxxxx00000001xxxxxxxxxxxxxxxx']

This have an effect, and if it is not exactly what you want, you can
change things a bit by playing with the value.

This is a workaroud, and I don't know if it can be considered good
enough for what you're tring to achieve.

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
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Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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