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Re: [Xen-devel] A clocksource question

To: Joanna Rutkowska <joanna@xxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] A clocksource question
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 09 Mar 2010 15:36:09 -0800
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On 03/09/2010 02:47 AM, Joanna Rutkowska wrote:
I would appreciate if somebody could shed some light on the currently
preferred setting of the clocksource for the pvops0 kernels, used both
in Dom0 and DomU (I'm using pvops0 in DomU because it provides backends,
e.g. network backend).

clocksource=xen is supposed to be the ideal clocksource, with no downsides.

I have a bad experience with running the Dom0 kernel
(xen/stabale- with the default clocksource=xen, as it
results in the system getting temporary "hangs", very short ones, but
annoying. E.g. when one types fast on a keyboard, then every 10 seconds
or so, the keystroke processing seems to be slowing down significantly
for a second o so.

There have been a number of reports about hiccups of varying degrees of severity when using clocksource=xen (and, interestingly, in KVM when using its very similar pv clock interface). I haven't seen anyone mention keyboard interactive performance; the failure mode I've seen is disk IO hiccuping or even getting completely wedged. Unfortunately I haven't made much headway in debugging it (or even reliably reproducing it).

What is the hardware platform? How many CPUs are you using? Does pinning the vcpus to pcpus help?

Running the kernel with clocksource=jiffies eliminates the above problem
but has a disadvantage of the clock drift in Dom0. This is not
acceptable on my setup, where I don't have any networking in Dom0, which
means I cannot correct it via NTP.

Interestingly the above problem didn't seem to affect the Dom0 kernel
based on xen/stable-2.6.32.

That's interesting; there's no difference in the Xen-specific timekeeping parts of the kernel, but perhaps something else has changed in the way time is handled.

I don't remember if 3.4.2 ended up with tsc emulation, but if it does you might try enabling it.

  Unfortunately I assume I'm no longer able to
run this kernel when using Xen 3.4.2.

Yes.  3.4.3 will support the newer kernels, but that isn't released yet.


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