RE: [Xen-devel] timekeepeing in XEN - request fo comments

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

Hi Pasi --

The timekeeping problems I am referring to are
only(?) for hvm guests.  HVM guests with different
OS's and different versions of a Linux kernel (32-bit
vs 64-bit, even kernel updates between EL5uX and
EL5uX+1) behave differently when ticks are delivered
at imperfect intervals (which happens frequently
for Xen and other hypervisors).  Xen distros
and "virtual appliance" distributors may specify
certain settings of timer_mode and certain kernel
parameters (e.g. "clock=pit") to ensure time
is kept as accurately as possible.  There is no
"one size fits all" solution unfortunately.

For PV guests, note that there have been patches
to dom0 (2.6.18-xen) that improve timekeeping
in dom0 and this may affect pv guests with
independent_wallclock==0.  I don't know if the
patches have been applied to pv_ops dom0, or
if they are even necessary.  In fact, I don't
know if there is a concept of independent_wallclock
in pv_ops.

Dan

> -----Original Message-----
> From: Pasi Kärkkäinen [mailto:pasik@xxxxxx]
> Sent: Monday, November 09, 2009 11:50 AM
> To: Dan Magenheimer
> Cc: Bartosz Lis; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] timekeepeing in XEN - request fo comments
> 
> 
> On Mon, Nov 09, 2009 at 08:50:52AM -0800, Dan Magenheimer wrote:
> > What kernels are running in your domUs?  Are they
> > also (all) 2.6.31.5?  If your domU's are older kernels
> > (e.g. SLES10 or older, RHEL5 or older) there are
> > many issues that cannot be entirely solved by
> > Xen or dom0.
> > 
> 
> Could you tell more about these timekeeping problems with 
> SLES10/RHEL5 PV kernels?
> 
> -- Pasi
> 
> > > -----Original Message-----
> > > From: Bartosz Lis [mailto:bartoszl@xxxxxxxxxxxxx]
> > > Sent: Monday, November 09, 2009 5:21 AM
> > > To: xen-devel@xxxxxxxxxxxxxxxxxxx
> > > Subject: [Xen-devel] timekeepeing in XEN - request fo comments
> > > 
> > > 
> > > Dear developers,
> > > 
> > > I have some issues with time synchronization among dom0 and 
> > > domUs. In my case 
> > > helped reconfiguring kernel with HZ=1000 for dom0 and HZ=100 
> > > for domUs and 
> > > running ntpd also on domUs. I think, running ntpd on domU is 
> > > not a good idea, 
> > > but it works acceptably good as a workaround for my 
> > > configuration: XEN 3.4.1 + 
> > > kernel 3.6.31.5 with pv_ops (taken from git last week). When 
> > > I don't run ntpd 
> > > on domU clock slowly drifts. I have choosen xen clocksource 
> > > in both dom0 and 
> > > domU.
> > > 
> > > I'm not the only one to have time synchronization issues. 
> > > Please write some 
> > > document specifying how time synchronization between dom0 and 
> > > domU should be 
> > > achieved. I have a number of special questions, but probably 
> > > there are more 
> > > questions that should be aswered:
> > > 
> > > 1. What is the mechanics behind the clocksource named xen? 
> > > How does it depend 
> > > on phisical time sources available on different platforms?
> > > 
> > > 2. How gets domU's clock initialized when domU boots? (I 
> > > observe that just 
> > > after domU has started its clock differs from dom0. The 
> > > difference is about 
> > > 10s.)
> > > 
> > > 3. What are the consequences of not selecting xen clocksource 
> > > (by selecting 
> > > one of hpet, acpi_pm, etc.) for dom0 and for domU?
> > > 
> > > 4. What are the best practices to achieve clock synchronization?
> > > 
> > > 5. What has changed recently in timekeeping? What is going to 
> > > change? Are 
> > > there any improvements/bugfixes added to redhat or suse 
> > > kernels? Are these 
> > > bugfixes planned to be added upstream?
> > > 
> > > Thank you in advance,
> > > 
> > > -- 
> > > Bartosz Lis @ Inst. of Information Technology, Technical 
> > > Univ. of Lodz, Poland
> > >    bartoszl @ ics.p.lodz.pl
> > > 
> > > _______________________________________________
> > > Xen-devel mailing list
> > > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > > http://lists.xensource.com/xen-devel
> > > 
> > >
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
>

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