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

[Avi Kivity <avi@redhat.com>]

On 09/27/2009 04:53 PM, Joerg Roedel wrote:
>
>    
>> Depends.  If it's a global yield(), yes.  If it's a local yield() that
>> doesn't rebalance the runqueues we might be left with the spinning task
>> re-running.
>>      
> Only one runable task on each cpu is unlikely in a situation of high
> vcpu overcommit (where pause filtering matters).
>
>    

I think even 2:1 overcommit can degrade performance terribly.

>> Also, if yield means "give up the reminder of our timeslice", then we
>> potentially end up sleeping a much longer random amount of time.  If we
>> yield to another vcpu in the same guest we might not care, but if we
>> yield to some other guest we're seriously penalizing ourselves.
>>      
> I agree that a directed yield with possible rebalance would be good to
> have, but this is very intrusive to the scheduler code and I think we
> should at least try if this simpler approach already gives us good
> results.
>    

No objection to trying.  I'd like to see hrtimer sleep as a baseline 
since it doesn't require any core changes, and we can play with it as we 
add more core infrastructure:

- not sleeping if all vcpus are running
- true yield() instead of sleep
- directed yield
- cross cpu directed yield

-- 
error compiling committee.c: too many arguments to function