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

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

On Sun, Sep 27, 2009 at 10:31:21AM +0200, Avi Kivity wrote:
> On 09/25/2009 11:43 PM, Joerg Roedel wrote:
>> On Wed, Sep 23, 2009 at 05:09:38PM +0300, Avi Kivity wrote:
>>    
>>> We haven't sorted out what is the correct thing to do here.  I think we
>>> should go for a directed yield, but until we have it, you can use
>>> hrtimers to sleep for 100 microseconds and hope the holding vcpu will
>>> get scheduled.  Even if it doesn't, we're only wasting a few percent cpu
>>> time instead of spinning.
>>>      
>> How do you plan to find out to which vcpu thread the current thread
>> should yield?
>>    
>
> 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?

	Joerg