From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ben Guthro <bguthro@virtualiron.com>
Subject: Re: Xen 3.1.2 domU clock goes out of sync
Date: Tue, 20 Nov 2007 08:49:44 -0500
Message-ID: <4742E5F8.40200@virtualiron.com>
References: <20071119162849.U32489@zhuryrwn.rrarg.rr>
Mime-Version: 1.0
Content-Type: multipart/mixed; boundary="===============0122836927=="
Return-path: <xen-devel-bounces@lists.xensource.com>
In-Reply-To: <20071119162849.U32489@zhuryrwn.rrarg.rr>
List-Unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>,
	<mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
List-Post: <mailto:xen-devel@lists.xensource.com>
List-Help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-Subscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>,
	<mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
Mime-version: 1.0
Sender: xen-devel-bounces@lists.xensource.com
Errors-To: xen-devel-bounces@lists.xensource.com
To: =?ISO-8859-1?Q?T=F5nu_Raitviir?= <jussuf@eenet.ee>
Cc: xen-devel@lists.xensource.com
List-Id: xen-devel@lists.xenproject.org

This is a multi-part message in MIME format.

--===============0122836927==
Content-Type: multipart/alternative;
	boundary="------------000708090400020406070200"

This is a multi-part message in MIME format.

--------------000708090400020406070200
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: quoted-printable

Clock skew is a known problem in virtualized systems.

VMWare wrote a whitepaper a while back which goes into some of the=20
problems that they faced while dealing with this same issue...
http://www.vmware.com/pdf/vmware_timekeeping.pdf

The Xen project is not immune to these problems - however, some efforts=20
have been made recently on the list to address some of these issues

See these threads:
http://lists.xensource.com/archives/html/xen-devel/2007-10/msg01010.html
http://lists.xensource.com/archives/html/xen-devel/2007-11/msg00554.html

What it basically comes down to is that it is necessary to keep the=20
clock skew under 0.05% for NTP to work - so that was the design goal.

The code introduced by some recent patches claims to be accurate to=20
0.02% (for at least HVM guests)


Hope this helps.

Ben


T=F5nu Raitviir wrote:
>
>
> Hello!
> I have understood that when /proc/sys/xen/independent_wallclock is 0,=20
> then domU clock stays in sync with dom0 clock. Now I have a case with=20
> Xen 3.1.2 (also occured with 3.1.1, but I haven't documented it) where=20
> domU clock runs significantly faster - after 65 hours domU clock is 20=20
> seconds ahead of dom0 clock.
> In this case dom0 is not running ntpd. When ntpd is running on dom0,=20
> domU clock also stays correct. Is this normal behaviour?
>
> Some examples to show clock differences:
>
> domU# uptime
>  14:54:09 up 2 days, 17:05,  1 user,  load average: 0.75, 0.49, 0.43
>
> domU# ntpdate -q ntp.eenet.ee
> server 193.40.133.142, stratum 1, offset -20.856732, delay 0.02579
> 19 Nov 14:54:13 ntpdate[8467]: step time server 193.40.133.142 offset=20
> -20.856732 sec
>
> dom0# uptime
>  14:55:02 up 2 days, 17:06,  1 user,  load average: 0.07, 0.02, 0.00
>
> dom0# ntpdate -q ntp.eenet.ee
> server 193.40.133.142, stratum 1, offset -0.438055, delay 0.02589
> 19 Nov 14:55:17 ntpdate[32284]: adjust time server 193.40.133.142=20
> offset -0.438055 sec
>
>
> Xen version used:
>
> Xen version 3.1.2 (jussuf@localdomain) (gcc version 4.1.2 20061115=20
> (prerelease) (Debian 4.1.1-21)) Fri Nov 16 17:52:59 EET 2007
> Latest ChangeSet: Wed Nov 14 23:35:43 2007 +0000 15502:c6776b6da8ee
>
> Linux xen-2 2.6.18.8-xen0 #1 SMP Fri Nov 16 17:39:31 EET 2007 x86_64=20
> GNU/Linux
>
> -----------------------------------------------------------------------=
-
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
>  =20


--------------000708090400020406070200
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
  <meta content="text/html;charset=ISO-8859-1" http-equiv="Content-Type">
</head>
<body bgcolor="#ffffff" text="#000000">
Clock skew is a known problem in virtualized systems.<br>
<p>VMWare wrote a whitepaper a while back which goes into some of the
problems that they faced while dealing with this same issue... <br>
<a href="http://www.vmware.com/pdf/vmware_timekeeping.pdf"
 target="_blank">http://www.vmware.com/pdf/vmware_timekeeping.pdf</a></p>
The Xen project is not immune to these problems - however, some efforts
have been made recently on the list to address some of these issues<br>
<br>
See these threads:<br>
<a class="moz-txt-link-freetext" href="http://lists.xensource.com/archives/html/xen-devel/2007-10/msg01010.html">http://lists.xensource.com/archives/html/xen-devel/2007-10/msg01010.html</a><br>
<a class="moz-txt-link-freetext" href="http://lists.xensource.com/archives/html/xen-devel/2007-11/msg00554.html">http://lists.xensource.com/archives/html/xen-devel/2007-11/msg00554.html</a><br>
<br>
What it basically comes down to is that it is necessary to keep the
clock skew under 0.05% for NTP to work - so that was the design goal.<br>
<br>
The code introduced by some recent patches claims to be accurate to
0.02% (for at least HVM guests)<br>
<br>
<br>
Hope this helps.<br>
<br>
Ben<br>
<br>
<br>
T&otilde;nu Raitviir wrote:
<blockquote cite="mid:20071119162849.U32489@zhuryrwn.rrarg.rr"
 type="cite"><br>
  <br>
Hello!
  <br>
I have understood that when /proc/sys/xen/independent_wallclock is 0,
then domU clock stays in sync with dom0 clock. Now I have a case with
Xen 3.1.2 (also occured with 3.1.1, but I haven't documented it) where
domU clock runs significantly faster - after 65 hours domU clock is 20
seconds ahead of dom0 clock.
  <br>
In this case dom0 is not running ntpd. When ntpd is running on dom0,
domU clock also stays correct. Is this normal behaviour?
  <br>
  <br>
Some examples to show clock differences:
  <br>
  <br>
domU# uptime
  <br>
&nbsp;14:54:09 up 2 days, 17:05,&nbsp; 1 user,&nbsp; load average: 0.75, 0.49, 0.43
  <br>
  <br>
domU# ntpdate -q ntp.eenet.ee
  <br>
server 193.40.133.142, stratum 1, offset -20.856732, delay 0.02579
  <br>
19 Nov 14:54:13 ntpdate[8467]: step time server 193.40.133.142 offset
-20.856732 sec
  <br>
  <br>
dom0# uptime
  <br>
&nbsp;14:55:02 up 2 days, 17:06,&nbsp; 1 user,&nbsp; load average: 0.07, 0.02, 0.00
  <br>
  <br>
dom0# ntpdate -q ntp.eenet.ee
  <br>
server 193.40.133.142, stratum 1, offset -0.438055, delay 0.02589
  <br>
19 Nov 14:55:17 ntpdate[32284]: adjust time server 193.40.133.142
offset -0.438055 sec
  <br>
  <br>
  <br>
Xen version used:
  <br>
  <br>
Xen version 3.1.2 (jussuf@localdomain) (gcc version 4.1.2 20061115
(prerelease) (Debian 4.1.1-21)) Fri Nov 16 17:52:59 EET 2007
  <br>
Latest ChangeSet: Wed Nov 14 23:35:43 2007 +0000 15502:c6776b6da8ee
  <br>
  <br>
Linux xen-2 2.6.18.8-xen0 #1 SMP Fri Nov 16 17:39:31 EET 2007 x86_64
GNU/Linux
  <br>
  <br>
  <pre wrap="">
<hr size="4" width="90%">
_______________________________________________
Xen-devel mailing list
<a class="moz-txt-link-abbreviated" href="mailto:Xen-devel@lists.xensource.com">Xen-devel@lists.xensource.com</a>
<a class="moz-txt-link-freetext" href="http://lists.xensource.com/xen-devel">http://lists.xensource.com/xen-devel</a>
  </pre>
</blockquote>
<br>
</body>
</html>

--------------000708090400020406070200--


--===============0122836927==
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Disposition: inline

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

--===============0122836927==--