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

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Message-ID: <20100419142624.GE14158@mothafucka.localdomain>
Date:	Mon, 19 Apr 2010 11:26:24 -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 Fri, Apr 16, 2010 at 01:36:34PM -0700, Jeremy Fitzhardinge wrote:
> On 04/15/2010 11:37 AM, Glauber Costa wrote:
> > In recent stress tests, it was found that pvclock-based systems
> > could seriously warp in smp systems. Using ingo's time-warp-test.c,
> > I could trigger a scenario as bad as 1.5mi warps a minute in some systems.
> >   
> 
> Is that "1.5 million"?
Yes it is.

But as I said, this seem to be a very deep worst case scenario. Most of boxes
are not even close to being that bad.

> 
> > (to be fair, it wasn't that bad in most of them). Investigating further, I
> > found out that such warps were caused by the very offset-based calculation
> > pvclock is based on.
> >   
> 
> 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.


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