linux-kernel - Re: [PATCH 1/5] Add a global synchronization point for pvclock

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Message-ID: <20100420015733.GA28249@amt.cnet>
Date:	Mon, 19 Apr 2010 22:57:33 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, avi@...hat.com,
	Zachary Amsden <zamsden@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

On Mon, Apr 19, 2010 at 03:25:43PM -0300, Glauber Costa wrote:
> On Mon, Apr 19, 2010 at 09:19:38AM -0700, Jeremy Fitzhardinge wrote:
> > On 04/19/2010 07:26 AM, Glauber Costa wrote:
> > >> Is the problem that the tscs are starting out of sync, or that they're
> > >> drifting relative to each other over time?  Do the problems become worse
> > >> the longer the uptime?  How large are the offsets we're talking about here?
> > >>     
> > > The offsets usually seem pretty small, under a microsecond. So I don't think
> > > it has anything to do with tscs starting out of sync. Specially because the
> > > delta-based calculation has the exact purpose of handling that case.
> > >   
> > 
> > So you think they're drifting out of sync from an initially synced
> > state?  If so, what would bound the drift?
> I think delta calculation introduces errors.

Yes.

> Marcelo can probably confirm it, but he has a nehalem with an appearently
> very good tsc source. Even this machine warps.
> 
> It stops warping if we only write pvclock data structure once and forget it,
> (which only updated tsc_timestamp once), according to him.

Yes. So its not as if the guest visible TSCs go out of sync (they don't
on this machine Glauber mentioned, or even on a multi-core Core 2 Duo),
but the delta calculation is very hard (if not impossible) to get right.

The timewarps i've seen were in the 0-200ns range, and very rare (once
every 10 minutes or so).

> Obviously, we can't do that everywhere.
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