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Re: [Xen-users] dom0 hangs when doing heavy I/O on domU

To: Joost Roeleveld <joost@xxxxxxxxxxxx>

From: Kiefer Chang <zapchang@xxxxxxxxx>

Date: Thu, 21 Apr 2011 23:02:31 +0800

Delivery-date: Thu, 21 Apr 2011 08:05:23 -0700

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List-id: Xen user discussion <xen-users.lists.xensource.com>

I am sure dom0 is pinned to first core (configuring xen parameters in grub menu) and domUs won't use first core by specifying VCPU and CPU relation in VM's xml file.

Initially we didn't dedicate specific core to dom0, so XEN might allocate cores to dom0 and domU dynamically.

When we dedicate a single core to dom0 we find the symptom is much easier to reproduce.

Thanks.

Kiefer Chang

2011/4/21 Joost Roeleveld <joost@xxxxxxxxxxxx>

On Thursday 21 April 2011 12:59:17 Florian Heigl wrote:
> Hi Chang,
>
> 2011/4/20 Kiefer Chang <zapchang@xxxxxxxxx>:
> > Hi all,
> > We are using XEN as hypervisor to setup our private cloud.
> > The framework is Eucalyptus and using CentOS 5.4 as dom0 OS.
> > Sometimes we find some machines' dom0 become unresponsive, the symptoms
> > are: (1) We can't log into dom0 via ssh. After typing password, it just
> > stops there.
> > (2) We can ping dom0 successfully.
> > (3) We can log into domU without problem.
> > The unresponsive dom0 eventually "alive" after a period of time. Maybe
> > half hour or even several hours.
>
> So one of your domUs is trashing the disks and dom0 can't get enough
> performance, right?
> - are they sharing a disk?
> - can you check what I/O scheduler you are using?
> (with cfq you can then use ionice to lower prio on all blkback
> threads a little. that way dom0 will "win the race")
>
> In general, your dom0 is privileged in terms of IO access rights, but
> not in IO peformance. So if one domU goes crazy, it will affect
> anything.
> ... until you take measures :)
> I'd suggest you switch to deadline scheduler and re-test.
> dom0 on a different disk media is also very advisable imho.

Another possible cause for this is if dom0 is not pinned to a "private" cpu-
core.
I noticed similar issues before I pinned dom0 to cpu0 and configured all the
other domUs to use the other 3 cores.

--
Joost

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