On Mon, Feb 07, 2011 at 03:46:54PM +0200, Avi Kivity wrote: > On 02/07/2011 03:41 PM, Gleb Natapov wrote: > >On Mon, Feb 07, 2011 at 07:23:22AM -0600, Anthony Liguori wrote: > >> On 02/07/2011 07:14 AM, Avi Kivity wrote: > >> >On 02/07/2011 03:11 PM, Anthony Liguori wrote: > >> >>On 02/07/2011 06:34 AM, Avi Kivity wrote: > >> >>>On 02/04/2011 10:56 AM, Jan Kiszka wrote: > >> >>>>> > >> >>>>> This should be a rare event. If you are missing 50% of your > >> >>>>> notifications, not amount of gradual catchup is going to > >> >>>>help you out. > >> >>>> > >> >>>>But that's the only thing this patch is after: lost ticks at > >> >>>>QEMU level. > >> >>> > >> >>>Most lost ticks will happen at the vcpu level. The iothread > >> >>>has low utilization and will therefore be scheduled promptly, > >> >>>whereas the vcpu thread may have high utilization and will > >> >>>thus be preempted. When it is preempted for longer than the > >> >>>timer tick, we will see vcpu-level coalescing. All it takes > >> >>>is 2:1 overcommit to see time go half as fast; I don't think > >> >>>you'll ever see that on bare metal. > >> >> > >> >>But that's not to say that doing something about lost ticks in > >> >>QEMU isn't still useful. > >> >> > >> > > >> >If it doesn't solve the majority of the problems it isn't very > >> >useful IMO. It's a good first step, but not sufficient for real > >> >world use with overcommit. > >> > >> Even if we have a way to detect coalescing, we still need to make > >> sure we don't lose ticks in QEMU. So regardless of whether it > >> solves the majority of problems, we need this anyway. > >> > >Actually it is very strange we lose them. Last time I checked vm_clock > >worked in such a way that if ticks were lost due to qemu not been scheduled > >for a long time timer callback was repeatedly fired to compensate for > >missed wakeups. > > > > That's quite pointless, since those interrupts will be coalesced by > the guest. > Yes, of course, and this is what I remember happening. At this point interrupt de-coalescing kicks in. -- Gleb. -- 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
