Re: [PATCH] [RESEND] KVM:VMX: Add support for Pause-Loop Exiting

[Joerg Roedel <joro@8bytes.org>]

On Sun, Sep 27, 2009 at 03:47:55PM +0200, Avi Kivity wrote:
> On 09/27/2009 03:46 PM, Joerg Roedel wrote:
>>
>>> We can't find exactly which vcpu, but we can:
>>>
>>> - rule out threads that are not vcpus for this guest
>>> - rule out threads that are already running
>>>
>>> A major problem with sleep() is that it effectively reduces the vm
>>> priority relative to guests that don't have spinlock contention.  By
>>> selecting a random nonrunnable vcpu belonging to this guest, we at least
>>> preserve the guest's timeslice.
>>>      
>> Ok, that makes sense. But before trying that we should probably try to
>> call just yield() instead of schedule()? I remember someone from our
>> team here at AMD did this for Xen a while ago and already had pretty
>> good results with that. Xen has a completly other scheduler but maybe
>> its worth trying?
>>    
>
> yield() is a no-op in CFS.

Hmm, true. At least when kernel.sched_compat_yield == 0, which it is on my
distro.
If the scheduler would give us something like a real_yield() function
which asumes kernel.sched_compat_yield = 1 might help. At least its
better than sleeping for some random amount of time.

	Joerg