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

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Message-ID: <20100419182542.GI14158@mothafucka.localdomain>
Date:	Mon, 19 Apr 2010 15:25:43 -0300
From:	Glauber Costa <glommer@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	Marcelo Tosatti <mtosatti@...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 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.

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.

Obviously, we can't do that everywhere.
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