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Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy lo
From: |
Gleb Natapov |
Subject: |
Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load. |
Date: |
Thu, 6 Nov 2008 16:51:42 +0200 |
On Thu, Nov 06, 2008 at 08:40:09AM -0600, Anthony Liguori wrote:
> Paul Brook wrote:
>>> You see time drift with 100hz timers in the guest?? That makes very
>>> little sense as even without hr timers, the host should have no problem
>>> delivering a 10ms timer.
>>>
>>
>> If your host has HZ=100 and no HR timers than a 10ms interval is fairly
>> borderline, and it's not that hard to end up missing an interrupt,
>> especially under heavy load.
>>
>
> If the host is otherwise idle, then the guest should have ample time to
> run. With /dev/rtc, you should be able to get a reliably 1ms timer.
And only one guest.
BTW I am saying that guest timer is 100HZ since I assume that windows2003
behaves like XP in this regard, but may be default timer frequency of 2003
is different. I'll check this.
>
> Gleb: are you perhaps using a qcow2 file in conjunction with -snapshot?
I am using qcow2, but without -snapshot.
> If that's the case, and you're doing an IO intensive workload, then you
> would be expanding the qcow2 file and updating the metadata is
> synchronous. This could cause lost time for the guest.
>
I think that's what happens when I see a huge drift, yes.
> If you are using qcow2, can you still reproduce with a raw file and not
> using -snapshot?
I'll try next week to run with a raw file.
>
>> If part of qemu gets swapped out then all bets are off, and you can
>> easily stall for significant fractions of a second. No amount of host
>> high resolution time support will help you there.
>>
>
> Running a steady workload, you aren't going to be partially swapped.
>
We want to oversubscribe host as much as possible, and workload will
vary during a lifetime of the VMs.
--
Gleb.
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., (continued)
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Anthony Liguori, 2008/11/05
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Jamie Lokier, 2008/11/05
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Gleb Natapov, 2008/11/06
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Anthony Liguori, 2008/11/06
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Paul Brook, 2008/11/06
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Anthony Liguori, 2008/11/06
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load.,
Gleb Natapov <=
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Anthony Liguori, 2008/11/06
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Gleb Natapov, 2008/11/08
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Dor Laor, 2008/11/08
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Gleb Natapov, 2008/11/09
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Anthony Liguori, 2008/11/09
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Avi Kivity, 2008/11/09
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Anthony Liguori, 2008/11/09
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Gleb Natapov, 2008/11/10
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Anthony Liguori, 2008/11/10
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Gleb Natapov, 2008/11/10