linux-kernel - Re: [PATCH] [RFC] timekeeping: Rework frequency adjustments to work better w/ nohz

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Date:	Wed, 29 Jan 2014 09:56:20 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Miroslav Lichvar <mlichvar@...hat.com>,
	Prarit Bhargava <prarit@...hat.com>
Subject: Re: [PATCH] [RFC] timekeeping: Rework frequency adjustments to work
 better w/ nohz

On Tue, Jan 28, 2014 at 9:58 AM, Richard Cochran
<richardcochran@...il.com> wrote:
> I tested for a regression using the patched kernel with the nohz=off
> command line option...
>
> On Mon, Jan 13, 2014 at 10:15:18AM -0800, John Stultz wrote:
>> On 01/13/2014 09:51 AM, Richard Cochran wrote:
>> >
>> > - Linux 3.12.7-nohz-plain-20140106                nohz-plain.log
>> > - Linux 3.12.7-nohz-plain-20140106 NOHZ=OFF       periodic-plain.log
>> > - Linux 3.12.7-nohz-fix-20140106-00001-gd753140   nohz-fix.log
>
> Regression check:
>
>  - Linux homeboy 3.12.7-nohz-fix-20140106-00001-gd753140 NOHZ=OFF
>    nohz-fix-periodic.log
>
>> > The performance in the log files as reflected in the clock offset is
>> > summarized in this table. The values are in nanoseconds.
>> >
>> >  |         | periodic-plain |      nohz-fix |    nohz-plain |
>> >  |---------+----------------+---------------+---------------|
>> >  | minimum |  -1.599000e+03 | -1.051000e+03 | -5.373700e+04 |
>> >  | maximum |  +1.311000e+03 | +1.048000e+03 | +6.389500e+04 |
>> >  | mean    |  +9.880240e-02 | -7.747305e+01 | +1.597904e+01 |
>> >  | stddev  |  +4.610021e+02 | +3.960978e+02 | +1.491263e+04 |
>
> Comparing the nohz=off case with and without the patch, the three hour
> test looks like this.
>
>   |         | periodic-plain | nohz-fix-periodic |
>   |---------+----------------+-------------------+
>   | minimum |  -1.599000e+03 |     -1.427000e+03 |
>   | maximum |  +1.311000e+03 |     +1.279000e+03 |
>   | mean    |  +9.880240e-02 |     -2.710778e+01 |
>   | stddev  |  +4.610021e+02 |     +3.974372e+02 |
>
>> >    http://linuxptp.sourceforge.net/nohz-fix/current_nohz.png
>> >    http://linuxptp.sourceforge.net/nohz-fix/periodic_vs_fix.png
>
> I also made a third graph showing before and after the patch.
>
>     http://linuxptp.sourceforge.net/nohz-fix/nohz_regression.png
>
>> If you do get a chance to look again, I'd also be interested if running
>> with nohz=off w/ the fix doesn't show any regression compared to the
>> unmodified nohz=off case.
>
> Looks like there is no regression.

That's great to hear! Looks like we might be able to queue it for 3.15...

Thanks so much again for the detailed testing! I really appreciate it!

thanks
-john
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