From: Anthony Liguori <anthony@codemonkey.ws>
To: Jan Kiszka <jan.kiszka@web.de>
Cc: Anthony Liguori <anthony@codemonkey.ws>,
qemu-devel <qemu-devel@nongnu.org>,
Glauber Costa <glommer@redhat.com>,
Ulrich Obergfell <uobergfe@redhat.com>, kvm <kvm@vger.kernel.org>,
Avi Kivity <avi@redhat.com>
Subject: Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift
Date: Thu, 03 Feb 2011 20:06:27 -0600 [thread overview]
Message-ID: <4D4B5F23.7040801@codemonkey.ws> (raw)
In-Reply-To: <4D4B1CF8.8040800@web.de>
On 02/03/2011 03:24 PM, Jan Kiszka wrote:
> On 2011-02-03 21:07, Anthony Liguori wrote:
>
>> On 02/03/2011 09:28 AM, Jan Kiszka wrote:
>>
>>> On 2011-02-03 14:43, Ulrich Obergfell wrote:
>>>
>>>
>>>> Hi,
>>>>
>>>> I am observing severe backward time drift in a MS Windows Vista(tm)
>>>> guest running on a Fedora 14 KVM host. I can reproduce the problem
>>>> with the following steps:
>>>>
>>>> 1. Use 'vncviewer' to connect to the guest's desktop.
>>>> 2. Click on the menu title bar of a window on the guest's desktop.
>>>> 3. Move that window around on the guest's desktop.
>>>>
>>>> While I keep on moving the window around for one minute, the guest
>>>> time falls up to 15 seconds behind host time.
>>>>
>>>> The problem is caused by delayed callbacks of hpet_timer(). A timer
>>>> interrupt is injected into the guest during each callback. However,
>>>> interrupts are lost if delays are greater than a comparator period.
>>>>
>>>>
>>>>
>>> Yes, that's a well known limitation of qemu, in fact. We are lacking a
>>> generic irq coalescing infrastructure. That, once designed and
>>> available, would also allow to fix the HPET.
>>>
>>>
>> I don't think it requires anything that sophisticated.
>>
>> It's just the period calculation of the HPET that's wrong and doesn't
>> account for loss.
>>
> Blind (/wrt the guest state) reinjection from the iothread will
> compensate for lost time of *that* thread but not of the target vcpu(s).
>
Is the case your concern about where you try to set an interrupt from
the I/O thread and the VCPU is not scheduled such that it doesn't
actually get injected into KVM until after the period is over?
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.
> So, depending on your workload, you may reduce the drift more or less,
> but you won't fix it this way.
>
There is no such thing as "fix" it. Time drift can happen on bare metal
too. Interrupts can be coalesced due to crappy SMM code. It's
something we see quite a lot in practice.
My point is that there's really low hanging fruit and while for some
curious reason I don't actually see this patch, I believe that a patch
like this probably can help us quite a lot in the short term.
The think the two biggest problems we have right now are bad period
calculations due to sloppy unit conversion (PIT/RTC) and lack of
accounting for missed periods.
It's worth noting again that if you don't use a gradual catchup policy,
interrupt notifiers are extremely important because you're not going to
inject before the end of the interrupt window. However, with a gradual
policy, it shouldn't be a huge issue.
Regards,
Anthony Liguori
> Jan
>
>
next prev parent reply other threads:[~2011-02-04 2:06 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <480481933.225059.1296734409954.JavaMail.root@zmail07.collab.prod.int.phx2.redhat.com>
2011-02-03 13:43 ` [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift Ulrich Obergfell
2011-02-03 15:28 ` Jan Kiszka
2011-02-03 20:07 ` [Qemu-devel] " Anthony Liguori
2011-02-03 21:24 ` Jan Kiszka
2011-02-04 2:06 ` Anthony Liguori [this message]
2011-02-04 8:56 ` Jan Kiszka
2011-02-07 12:34 ` Avi Kivity
2011-02-07 13:11 ` Anthony Liguori
2011-02-07 13:14 ` Avi Kivity
2011-02-07 13:23 ` Anthony Liguori
2011-02-07 13:34 ` Avi Kivity
2011-02-07 13:41 ` Gleb Natapov
2011-02-07 13:46 ` Avi Kivity
2011-02-07 13:48 ` Gleb Natapov
2011-02-07 13:51 ` Avi Kivity
2011-02-07 13:54 ` Gleb Natapov
2011-02-07 14:10 ` Jan Kiszka
2011-02-07 14:28 ` Anthony Liguori
2011-02-07 14:43 ` Jan Kiszka
2011-02-07 14:54 ` Anthony Liguori
2011-02-07 14:57 ` Jan Kiszka
2011-02-07 15:01 ` Anthony Liguori
2011-02-07 15:08 ` Jan Kiszka
2011-02-07 15:13 ` Avi Kivity
2011-02-07 15:17 ` Jan Kiszka
2011-02-07 15:29 ` Avi Kivity
2011-02-07 19:30 ` Anthony Liguori
2011-02-08 9:11 ` Avi Kivity
2011-02-07 15:20 ` Jan Kiszka
2011-02-07 15:30 ` Avi Kivity
2011-02-07 19:28 ` Anthony Liguori
2011-02-07 14:58 ` Avi Kivity
2011-02-07 15:01 ` Jan Kiszka
2011-02-07 15:08 ` Avi Kivity
2011-02-07 15:14 ` Jan Kiszka
2011-02-07 15:16 ` Avi Kivity
2011-02-07 15:22 ` Anthony Liguori
2011-02-07 15:18 ` Anthony Liguori
2011-02-04 9:52 ` Ulrich Obergfell
2011-02-07 10:44 ` Ulrich Obergfell
2011-02-07 13:24 ` Anthony Liguori
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