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

From: Greg Holmberg <g...@holmberg.to>
To: FreeBSD PR kern/153620 <bug-follo...@freebsd.org>
Cc:  
Subject: Re: kern/153620: Xen guest system clock drifts in AWS EC2 (FreeBSD
 9.0-CURRENT i386 T1-micro)
Date: Tue, 4 Jan 2011 02:10:29 +0100

 > On Sun, Jan 02, 2011 at 03:16:52AM -0800, Colin Percival wrote:
 >
 > 3. Did the clock _drift_, or _jump_?
 >
 
 I started a new VM (ami-5b82b72f) using the latest available 2011-01-01 code.
 
 The problem -- the system clock losing 2200 seconds -- is still present.
 
 Last night, the clock in the new AMI seems to have lost 2200 seconds twice in 
a 14 hour idle period.
 
 The system clock seemed to be stopped before login. It started incrementing 
smoothly again when I logged in to check on the system in the morning. The 
system clock itself only managed to advance 14 minutes (837 seconds) overnight.
 
 After subtracting the accumulated drift from the previous evening's work, we 
see that the offset with a nearby NTP server at the moment I logged in was 
4398.107077. This value is very close to  2 * (2^41) / 1e9.
 
    system clock            NTP source
 
    ...
    20110103-141513UTC      0.437444     # drift is gradual, linear
    20110103-141613UTC      0.438124     # clock is running slow
    20110103-141713UTC      0.438786
    20110103-141813UTC      0.439584
    20110103-141914UTC      0.440247
    20110103-142014UTC      0.440957
    20110103-142114UTC      0.441663
    20110103-142214UTC      0.442313
    20110103-142314UTC      0.443064
    20110103-143711UTC      4398.550141  <--- 4398.107077 second difference
    20110103-143811UTC      4398.550755       between slow clock and NTP src
    ...
 
    
 I have noticed that it only seems to happen when there are no active login 
sessions. I have seen a large jump in system time happen with a monitor script 
backgrounded from a single login shell, sometime after logout.  I have seen it 
happen when the script runs in the foreground of a window in GNU screen.
 
 I have not been able to provoke it during an interactive login session. Maybe 
I just need to be more creative.
 
 Best regards,
 
 Greg Holmberg
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