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Subject: Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option
From:   Dario Faggioli <dfaggioli@suse.com>
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>, thibodux@gmail.com,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Cc:     oleksandr_andrushchenko@epam.com, tglx@linutronix.de,
        jgross@suse.com, ryan.thibodeaux@starlab.io,
        luca abeni <luca.abeni@santannapisa.it>
Date:   Sat, 23 Mar 2019 03:58:16 +0100
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On Fri, 2019-03-22 at 18:10 -0400, Boris Ostrovsky wrote:
> On 3/22/19 2:29 PM, thibodux@gmail.com wrote:
> > From: Ryan Thibodeaux <ryan.thibodeaux@starlab.io>
> >=20
> > The original timer slop value has not changed since the
> > introduction
> > of the Xen-aware Linux kernel code. This commit provides users an
> > opportunity to tune timer performance given the refinements to
> > hardware and the Xen event channel processing. It also mirrors
> > a feature in the Xen hypervisor - the "timer_slop" Xen command line
> > option.
>=20
> Is there any data that shows effects of using this new parameter?
>=20
Yes, I've done some research and experiments on this. I did it together
with a friend, which I'm Cc-ing, as I'm not sure we're ready/capable to
share the results, yet (Luca?).

What I think I can anticipate is that having such a high value for
timer slop in the kernel, for the Xen clockevent device is (together
with the also quite high default value of timer_slop in Xen itself)
responsible for really high vcpu wakeup latencies.

Lowering those two values, reduces such latencies dramatically.

Regards,
Dario
--=20
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Software Engineer @ SUSE https://www.suse.com/

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