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Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emula
From: |
Gleb Natapov |
Subject: |
Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift |
Date: |
Mon, 7 Feb 2011 15:48:21 +0200 |
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.
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, (continued)
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Anthony Liguori, 2011/02/03
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Jan Kiszka, 2011/02/03
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Anthony Liguori, 2011/02/03
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Jan Kiszka, 2011/02/04
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Avi Kivity, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Anthony Liguori, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Avi Kivity, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Anthony Liguori, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Gleb Natapov, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Avi Kivity, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift,
Gleb Natapov <=
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Avi Kivity, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Gleb Natapov, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Jan Kiszka, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Anthony Liguori, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Jan Kiszka, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Anthony Liguori, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Jan Kiszka, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Anthony Liguori, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Jan Kiszka, 2011/02/07
- Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift, Avi Kivity, 2011/02/07