The following reply was made to PR kern/153620; it has been noted by GNATS.

From: Colin Percival <cperc...@freebsd.org>
To: bug-follo...@freebsd.org, fbsd-9.0-aws-ec2-1293964...@holmberg.to
Cc:  
Subject: Re: kern/153620: [xen] Xen guest system clock drifts in AWS EC2 
(FreeBSD
 9.0-CURRENT i386 T1-micro)
Date: Tue, 04 Jan 2011 03:32:21 -0800

 Ok, I think I see what's happening here: Under some conditions it seems that
 the clock stops running.
 
 If there are no interrupts from any source, the FreeBSD instance never gets
 scheduled; and the clock loses time in multiples of the timecounter period
 (2^41 ns) if it doesn't tick for that long.
 
 The reason this doesn't show up with an interactive login session open is that
 ssh generates enough network traffic to wake the kernel periodically; this is
 also provides a workaround for this bug: Send a ping to the instance once every
 30 minutes.
 
 -- 
 Colin Percival
 Security Officer, FreeBSD | freebsd.org | The power to serve
 Founder / author, Tarsnap | tarsnap.com | Online backups for the truly paranoid
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