linux-kernel - Re: [PATCH v3 0/5] Scan for an idle sibling in a single pass

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Date:   Mon, 25 Jan 2021 12:29:47 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Qais Yousef <qais.yousef@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 0/5] Scan for an idle sibling in a single pass

On 2021/1/22 21:22, Vincent Guittot wrote:
> On Fri, 22 Jan 2021 at 11:14, Mel Gorman <mgorman@...hsingularity.net> wrote:
>>
>> On Fri, Jan 22, 2021 at 10:30:52AM +0100, Vincent Guittot wrote:
>>> Hi Mel,
>>>
>>> On Tue, 19 Jan 2021 at 13:02, Mel Gorman <mgorman@...hsingularity.net> wrote:
>>>>
>>>> On Tue, Jan 19, 2021 at 12:33:04PM +0100, Vincent Guittot wrote:
>>>>> On Tue, 19 Jan 2021 at 12:22, Mel Gorman <mgorman@...hsingularity.net> wrote:
>>>>>>
>>>>>> Changelog since v2
>>>>>> o Remove unnecessary parameters
>>>>>> o Update nr during scan only when scanning for cpus
>>>>>
>>>>> Hi Mel,
>>>>>
>>>>> I haven't looked at your previous version mainly because I'm chasing a
>>>>> performance regression on v5.11-rcx which prevents me from testing the
>>>>> impact of your patchset on my !SMT2 system.
>>>>> Will do this as soon as this problem is fixed
>>>>>
>>>>
>>>> Thanks, that would be appreciated as I do not have access to a !SMT2
>>>> system to do my own evaluation.
>>>
>>> I have been able to run tests with your patchset on both large arm64
>>> SMT4 system and small arm64 !SMT system and patch 3 is still a source
>>> of regression on both. Decreasing min number of loops to 2 instead of
>>> 4 and scaling it with smt weight doesn't seem to be a good option as
>>> regressions disappear when I remove them as I tested with the patch
>>> below
>>>
>>> hackbench -l 2560 -g 1 on 8 cores arm64
>>> v5.11-rc4 : 1.355 (+/- 7.96)
>>> + sis improvement : 1.923 (+/- 25%)
>>> + the patch below : 1.332 (+/- 4.95)
>>>
>>> hackbench -l 2560 -g 256 on 8 cores arm64
>>> v5.11-rc4 : 2.116 (+/- 4.62%)
>>> + sis improvement : 2.216 (+/- 3.84%)
>>> + the patch below : 2.113 (+/- 3.01%)
>>>

4 benchmarks reported out during weekend, with patch 3 on a x86 4s system
with 24 cores per socket and 2 HT per core, total 192 CPUs.

It looks like mid-load has notable changes on my side:
- netperf 50% num of threads in TCP mode has 27.25% improved
- tbench 50% num of threads has 9.52% regression

Details below:

hackbench: 10 iterations, 10000 loops, 40 fds per group
======================================================

- pipe process

	group	base	%std	patch	%std
  	6	1	5.27	1.0469	8.53
  	12	1	1.03	1.0398	1.44
	24	1	2.36	1.0275	3.34

- pipe thread

	group	base	%std	patch	%std
	6       1	7.48	1.0747	5.25
	12	1	0.97	1.0432	1.95
	24	1	7.01	1.0299	6.81

- socket process

	group	base	%std	patch	%std
	6       1       1.01	0.9656	1.09
	12      1       0.35	0.9853	0.49
	24      1       1.33	0.9877	1.20

- socket thread

	group	base	%std	patch	%std
	6       1       2.52	0.9346	2.75
	12      1       0.86	0.9830	0.66
	24      1       1.17	0.9791	1.23

netperf: 10 iterations x 100 seconds, transactions rate / sec
=============================================================

- tcp request/response performance

	thread	base	%std	patch	%std
	50%     1       3.98    1.2725   7.52
	100%    1       2.73    0.9446   2.86
	200%    1       39.36   0.9955  29.45

- udp request/response performance

	thread	base	%std	patch	%std
	50%     1       6.18    1.0704  11.99
	100%    1       47.85   0.9637  45.83
	200%    1       45.74   1.0162  36.99

tbench: 10 iterations x 100 seconds, throughput / sec
=====================================================

	thread	base	%std	patch	%std
	50%	1	1.38 	0.9048 	2.46 
	100%	1	1.05 	0.9640 	0.68 
	200%	1	6.76 	0.9886 	2.86 

schbench: 10 iterations x 100 seconds, 99th percentile latency
==============================================================

	mthread	base	%std	patch	%std
	6	1	29.07	0.8714	25.73
	12	1	15.32	1.0000	12.39
	24	1	 0.08	0.9996	 0.01

>>> So starting with a min of 2 loops instead of 4 currently and scaling
>>> nr loop with smt weight doesn't seem to be a good option and we should
>>> remove it for now
>>>
>> Note that this is essentially reverting the patch. As you remove "nr *=
>> sched_smt_weight", the scan is no longer proportional to cores, it's
> 
> Yes. My goal above was to narrow the changes only to lines that
> generate the regressions but i agree that removing patch 3 is the
> right solution> 
>> proportial to logical CPUs and the rest of the patch and changelog becomes
>> meaningless. On that basis, I'll queue tests over the weekend that remove
>> this patch entirely and keep the CPU scan as a single pass.
>>
>> --
>> Mel Gorman
>> SUSE Labs