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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
