Re: [PATCH RFC 2/4] Add yield hypercall for KVM guests

[Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>]

On Tue, Aug 03, 2010 at 10:46:59AM +0530, Srivatsa Vaddagiri wrote:
> On Mon, Aug 02, 2010 at 11:40:23AM +0300, Avi Kivity wrote:
> > >>Can you do a directed yield?
> > >We don't have that support yet in Linux scheduler.
> > 
> > If you think it's useful, it would be good to design it into the
> > interface, and fall back to ordinary yield if the host doesn't
> > support it.
> > 
> > A big advantage of directed yield vs yield is that you conserve
> > resources within a VM; a simple yield will cause the guest to drop
> > its share of cpu to other guest.
> 
> Hmm .. I see possibility of modifying yield to reclaim its "lost" timeslice when
> its scheduled next as well. Basically remember what timeslice we have given
> up and add that as its "bonus" when it runs next. That would keep the dynamics
> of yield donation/reclaim local to the (physical) cpu and IMHO is less complex
> than dealing with directed yield between tasks located across different physical
> cpus. That would also address the fairness issue with yield you are pointing at?

Basically with directed yield, we need to deal with these issues:

- Timeslice inflation of target (lock-holder) vcpu affecting fair-time of other
  guests vcpus.
- Intra-VM fairness - different vcpus could get different fair-time, depending
  on how much of a lock-holder/spinner a vcpu is

By simply educating yield to reclaim its lost share, I feel we can avoid these
complexities and get most of the benefit of yield-on-contention.

CCing other shceduler experts for their opinion of directed yield.

- vatsa