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

Li, Aubrey
2021-01-25T04:31:55Z

On 2021/1/22 21:22, Vincent Guittot wrote:
> On Fri, 22 Jan 2021 at 11:14, Mel Gorman <mgorman@techsingularity.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@techsingularity.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@techsingularity.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