Re: [Xen-devel] cpuidle causing Dom0 soft lockups

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 21/01/2010 10:16, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:

>>> While I don't know this code well, it would seem that we're suffering
>>> from extremely long wakeup times. This suggests that there likely is
>>> a (performance) problem even for smaller numbers of vCPU-s.
>>> Hence, unless it can be fixed before 4.0 releases, I would suggest
>>> disabling C-state management by default again.
>> 
>> It's enabled in 3.4 by feault too though, already. Are there problems there
>> too?
> 
> We never shipped 3.4.x for anything that would normally be run on
> large systems. And 3.4 doesn't support more than 32 vCPU-s per
> domain, hence the problem - even if present - would be hidden.

Hm, okay. Another thing to consider is not using ticket locks when running
on Xen. Regardless of slow wakeup times (and you can't really depend on them
being fast, in general, in a virtualised environment) experiments have shown
that, as you might expect, contended ticket locks perform *abysmally* in
virtualised environments.

Turning off our major source of power-management wins by default in Xen 4.0
is not really a goer.

 -- Keir



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