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Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy lo
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load. |
Date: |
Thu, 6 Nov 2008 03:55:54 +0000 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
Anthony Liguori wrote:
> >The time drift is eliminated. If there is a spike in a load time may
> >slow down, but after that it catches up (this happens only during very
> >high loads though).
>
> How bad is time drift without it. Under workload X, we lose N seconds
> per Y hours and with this patch, under the same workload, we lose M
> seconds per Y hours and N << M.
In my experience, N seconds (for any N) is typically a problem with VM
systems in general.
> I strongly, strongly doubt that you'll be eliminating drift 100%.
I do believe that the method of "virtual clock warping" can eliminate
drift 100% for all guest OSes including guests which have their own
lost-tick compensators, provided there's enough host CPU _on average_
for the guest to tick, over an appropriate averaging window. It
should work even with guests which request a different scheme with the
Microsoft PV ops.
> If the host can awaken QEMU 1024 times a second and QEMU can deliver a
> timer interrupt each time, there is no need for time drift fixing.
>
> I would think that with high res timers on the host, you would have to
> put the host under heavy load before drift began occurring.
If two guests are running at 100% CPU on a single CPU, I suspect
you'll find each QEMU instance does _not_ get to run 1024 times per
second even with high res timers. They will behave like
non-interactive processes, running alternately with long timeslices -
so even 100 or 18 times a second won't fire. That's a reasonable
scenario, and doesn't even require any I/O or swapping.
I've personally yet to see any VM which doesn't drift
over time periods of months (i.e. servers in VMs) unless the guest is
running some kind of regular clock sync protocol - and NTP does not
always work for them, because NTP assumes a fairly low jitter clock,
which guests on a loaded host don't always manage - see above.
-- Jamie
- 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., Gleb Natapov, 2008/11/06
- Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., andrzej zaborowski, 2008/11/06
- 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., andrzej zaborowski, 2008/11/07
- 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., Paul Brook, 2008/11/06
Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., Gleb Natapov, 2008/11/05
Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load., David S. Ahern, 2008/11/06
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 <=
- 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, 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, 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