On Fri, Jan 21, 2011 at 09:48:29AM -0500, Rik van Riel wrote: > >>Why? If a VCPU can't make progress because its waiting for some > >>resource, then why not schedule something else instead? > > > >In the process, "something else" can get more share of cpu resource than its > >entitled to and that's where I was bit concerned. I guess one could > >employ hard-limits to cap "something else's" bandwidth where it is of real > >concern (like clouds). > > I'd like to think I fixed those things in my yield_task_fair + > yield_to + kvm_vcpu_on_spin patch series from yesterday. Speaking of the spinlock-in-virtualized-environment problem as whole, IMHO I don't think that kvm_vcpu_on_spin + yield changes will provide the best results, especially where ticketlocks are involved and they are paravirtualized in a manner being discussed in this thread. An important focus of pv-ticketlocks is to reduce the lock _acquisition_ time by ensuring that the next-in-line vcpu gets to run asap when a ticket lock is released. With the way kvm_vcpu_on_spin+yield_to is implemented, I don't see how we can provide the best lock acquisition times for threads. It would be nice though to compare the two approaches (kvm_vcpu_on_spin optimization and the pv-ticketlock scheme) to get some real-world numbers. I unfortunately don't have access to a PLE capable hardware which is required to test your kvm_vcpu_on_spin changes? Also it may be possible for the pv-ticketlocks to track owning vcpu and make use of a yield-to interface as further optimization to avoid the "others-get-more-time" problem, but Peterz rightly pointed that PI would be a better solution there than yield-to. So overall IMO kvm_vcpu_on_spin+yield_to could be the best solution for unmodified guests, while paravirtualized ticketlocks + some sort of PI would be a better solution where we have the luxury of modifying guest sources! - 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
