Re: [Xen-users] Automatic CPU Pinning

[Dario Faggioli <raistlin@xxxxxxxx>]

On Fri, 2012-03-23 at 14:30 -0700, Matthew Hook wrote: 
> Hi there,
> 
Hi,

> in the past you used to be able to pin CPU's to virtual CPU's in the
> config files when using xm.
> However, I notice that the new xl command is ignoring the cpus option
> in the config file and this link detailing the xl config file syntax
> does not present this feature anymore.
>
Support for "cpus=" is there in xl in the xen-unstable tree, thus,
pinning via config file will be part supported in 4.2 release.

> http://xenbits.xen.org/docs/unstable/man/xl.cfg.5.html
> 
Mmm... It's more than possible that we failed to updated the docs wrt
that (where "we", basically means "me" here :-D). I'll go check and fix
this if it is the case. Thanks for pointing it out!

> Is there a newer better way to pin cpu's to vcpu's in config files?
> I see something called cpu-pools.  How can I do this correctly.
> 
Yep, all the variants of the old xm syntax should be working (with
xen-unstable, as I said), e.g., 'cpus="1,3-7,^4". Just be careful that
using the following syntax 'cpus=[1, 3, 5]' would just end up in being
equivalent to the following command `xl vcpu-pin <domid> all "1,3,5"'
and _NOT_ to the old xend/xm syntax which was quite counterintuitive and
hard to deal with properly.

Also, as you were mentioning, if you have cpupools configured, you can
specify a poolid for your to-be-created domain. That's something
different then vcpu pinning in quite a bit of respects, but might
achieve similar effects, depending on what your requirements and
expectations are. :-)

> Regarding that, for cache friendliness, in the old xm way a VCPU could
> float over a range of physical CPU's.
>
What do you mean by "a range"? Moreover, do you mean with or without
vcpu-pinning? And do you refer to cases where you pin the vcpus to pcpus
at creation time (via config file) or post-creation?

> Wouldn't this could have a negative impact on applications with
> regards to the CPU caches?
> 
Well, it depends. For sure, the most you can retain locality, the better
for memory accessing as a whole. However, actual benefits or
disadvantages are related to many other factors as well, and they tend
to be very workload specific...

Thanks and Regards,
Dario

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

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