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RE: [Xen-devel] Re: [Xen-users] About profiling xen

To: Marco Tizzoni <marco.tizzoni@xxxxxxxxx>
Subject: RE: [Xen-devel] Re: [Xen-users] About profiling xen
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Wed, 30 Sep 2009 09:04:39 -0700 (PDT)
Cc: xen <xen-devel@xxxxxxxxxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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> On Wed, Sep 30, 2009 at 4:21 PM, Dan Magenheimer
> <dan.magenheimer@xxxxxxxxxx> wrote:
> > Because of save/restore and migrate, no hardware timer is
> > suitable for a pv domain.  The code that is handling the
> > clock is the paravirtualized clock code (aka pvclock).
> > Nobody ever changed the sysfs reporting mechanism in
> > pv versions of released Red Hat kernels, so clocksource
> > gets reported as "jiffies", and even though "pit"
> > appears to be available, it is not (since it wouldn't
> > work).  I suspect this is also the case for other
> > distros' pv kernels.
> How can I get a different, and I hope better, clocksource? In time.c I
> found acpi and hpet are available.

You can't.  Paravirtualization disables all other
clocksources.  You can modify the source to change
HZ and rebuild (which you apparently already did),
or you can use an HVM instead (but that will change
many things, so you will be comparing apples and

I'm no networking expert, but if your packets are
only being processed once per HZ, that seems like
a completely different problem.  Even if you are
able to get a hardware high resolution timer, the
resolution has nothing to do with the clock
interrupt frequency (HZ).

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