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RE: [Xen-devel] pv_ops (2.6.31) and CONFIG_NO_HZ

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Xen-Devel (E-mail)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] pv_ops (2.6.31) and CONFIG_NO_HZ
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 2 Nov 2009 14:29:23 -0800 (PST)
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
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> Periodic ticks are disabled for non-running PV guests, so I 
> don't think
> no-hz has as much potential win as you might think.

In this case, I was mostly interested in HV guests, but
happened to have a 2.6.31 build in a handy PV guest so
thought I would give it a try.

But as long as we are on the PV topic: Does this mean that
a PV guest that is idle will never get scheduled due to
needing to process a tick?  I do see that an idle
PV domain is doing about 250 kernel rdtsc/second, so
is this just because the Xen scheduler has nothing
better to do with a pcpu so it schedules an "idle"
vcpu on it?

> On 02/11/2009 21:45, "Dan Magenheimer" 
> <dan.magenheimer@xxxxxxxxxx> wrote:
> > Has anyone looked at using CONFIG_NO_HZ with an upstream
> > pv_ops kernel?  This seems like an intelligent combination...
> > since pvclock is a good timesource, it seems like there
> > should be no reason to have a regular tick, and one
> > could pack more idle domains into one physical machine.
> > 
> > I tried enabling CONFIG_NO_HZ from an otherwise working .config
> > file for 2.6.31 and the new kernel (running as a PV domU) 
> panics early
> > in boot ("Kernel panic -- not syncing: Attempted to kill init!"
> > with a call trace of syscall_call->sys_exit_group->do_group_exit->
> > do_exit->panic).  So I'm wondering if I did something stupid
> > or it is known to not work.
> > 
> > Thanks,
> > Dan
> > 
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel

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