Xen devel - Feb 2009 - comprehensive documentation of clock-related options for HVM guests

Hello list.

Despite using ntp, we''re facing heavy clock skews on our HVM guests, 
which happen to be almost all x86_64 linux. I''ve been browsing the list
archive, I found reference to two different xen guest configuration options:
- timer_mode
- hpet

However, I couldn''t find neither reference documentation, neither on 
their default values.

According to this tread, the default for timer_mode was changed in some 
part of the source tree from 0 to 1:
http://lists.xensource.com/archives/html/xen-devel/2008-10/msg00261.html

But according to this one, there was a discussion for changing 
timer_mode from 0 to 2, and hpet from 0 to 1:
http://lists.xensource.com/archives/html/xen-devel/2008-06/msg00095.html

And according to XendConfig.py code (3.3.1), the default value for both 
of them is actually 0...

This is a bit confusing :(
-- 
BOFH excuse #243:

The computer fleetly, mouse and all.

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I agree it is confusing.  There is no good default.  Your
best plan is to specify timer_mode=2 in the config file
for hvm x86_64 linux guests.

I think the defaults in XendConfig.py are to ensure that
old saved guests don''t get confused by a different default.
> -----Original Message-----
> From: Guillaume Rousse [mailto:Guillaume.Rousse@inria.fr]
> Sent: Monday, February 16, 2009 3:40 AM
> To: xen-devel@lists.xensource.com
> Subject: [Xen-devel] comprehensive documentation of clock-related
> options for HVM guests
> 
> 
> Hello list.
> 
> Despite using ntp, we''re facing heavy clock skews on our HVM
guests,
> which happen to be almost all x86_64 linux. I''ve been 
> browsing the list 
> archive, I found reference to two different xen guest 
> configuration options:
> - timer_mode
> - hpet
> 
> However, I couldn''t find neither reference documentation, neither
on
> their default values.
> 
> According to this tread, the default for timer_mode was 
> changed in some 
> part of the source tree from 0 to 1:
> http://lists.xensource.com/archives/html/xen-devel/2008-10/msg
00261.html

But according to this one, there was a discussion for changing 
timer_mode from 0 to 2, and hpet from 0 to 1:
http://lists.xensource.com/archives/html/xen-devel/2008-06/msg00095.html

And according to XendConfig.py code (3.3.1), the default value for both 
of them is actually 0...

This is a bit confusing :(
-- 
BOFH excuse #243:

The computer fleetly, mouse and all.

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

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It''s also worth pointing out that NTP synchronisation in dom0 does not
affect HVM guest synchronisation at all. And in general that would be very
hard to do in a way that isn''t fragile. The only solutions I can see
here
for absolute time synchronisation are a PV driver or daemon for time
handling (which we do not currently have for any type of HVM guest) or
perform NTP synchronisation in the guest itself (our time handling should be
accurate enough for an ntp daemon to latch onto it).

 -- Keir

On 16/02/2009 15:18, "Dan Magenheimer"
<dan.magenheimer@oracle.com> wrote:
> I agree it is confusing.  There is no good default.  Your
> best plan is to specify timer_mode=2 in the config file
> for hvm x86_64 linux guests.
> 
> I think the defaults in XendConfig.py are to ensure that
> old saved guests don''t get confused by a different default.
> 
>> -----Original Message-----
>> From: Guillaume Rousse [mailto:Guillaume.Rousse@inria.fr]
>> Sent: Monday, February 16, 2009 3:40 AM
>> To: xen-devel@lists.xensource.com
>> Subject: [Xen-devel] comprehensive documentation of clock-related
>> options for HVM guests
>> 
>> 
>> Hello list.
>> 
>> Despite using ntp, we''re facing heavy clock skews on our HVM
guests,
>> which happen to be almost all x86_64 linux. I''ve been
>> browsing the list
>> archive, I found reference to two different xen guest
>> configuration options:
>> - timer_mode
>> - hpet
>> 
>> However, I couldn''t find neither reference documentation,
neither on
>> their default values.
>> 
>> According to this tread, the default for timer_mode was
>> changed in some 
>> part of the source tree from 0 to 1:
>> http://lists.xensource.com/archives/html/xen-devel/2008-10/msg
> 00261.html
> 
> But according to this one, there was a discussion for changing
> timer_mode from 0 to 2, and hpet from 0 to 1:
> http://lists.xensource.com/archives/html/xen-devel/2008-06/msg00095.html
> 
> And according to XendConfig.py code (3.3.1), the default value for both
> of them is actually 0...
> 
> This is a bit confusing :(


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Keir Fraser a écrit :
> It''s also worth pointing out that NTP synchronisation in dom0 does
not
> affect HVM guest synchronisation at all. And in general that would be very
> hard to do in a way that isn''t fragile. The only solutions I can
see here
> for absolute time synchronisation are a PV driver or daemon for time
> handling (which we do not currently have for any type of HVM guest) or
> perform NTP synchronisation in the guest itself (our time handling should
be
> accurate enough for an ntp daemon to latch onto it).
That''s what (ntp sync in guests) we do, sorry if I wasn''t
clear. I''ve
tried timer_mode=1 yesterday, it seem to have reduced the deviation to 
an acceptable level for every host but one. I''ll try to switch to 
timer_mode=2 today.
-- 
BOFH excuse #40:

not enough memory, go get system upgrade

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On 17/02/2009 08:38, "Guillaume Rousse"
<Guillaume.Rousse@inria.fr> wrote:
> Keir Fraser a écrit :
>> It''s also worth pointing out that NTP synchronisation in dom0
does not
>> affect HVM guest synchronisation at all. And in general that would be
very
>> hard to do in a way that isn''t fragile. The only solutions I
can see here
>> for absolute time synchronisation are a PV driver or daemon for time
>> handling (which we do not currently have for any type of HVM guest) or
>> perform NTP synchronisation in the guest itself (our time handling
should be
>> accurate enough for an ntp daemon to latch onto it).
> That''s what (ntp sync in guests) we do, sorry if I wasn''t
clear. I''ve
> tried timer_mode=1 yesterday, it seem to have reduced the deviation to
> an acceptable level for every host but one. I''ll try to switch to
> timer_mode=2 today.
Good idea. Timer_mode=0 is in general pretty bad. 1 and 2 seem to cover most
guests okay.

 -- Keir



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