Re: [PATCH v4 2/3] mm/free_pcppages_bulk: do not hold lock when picking pages to free

[Huang\, Ying]

Michal Hocko <mhocko@xxxxxxxxxx> writes:

> On Thu 01-03-18 14:28:44, Aaron Lu wrote:
>> When freeing a batch of pages from Per-CPU-Pages(PCP) back to buddy,
>> the zone->lock is held and then pages are chosen from PCP's migratetype
>> list. While there is actually no need to do this 'choose part' under
>> lock since it's PCP pages, the only CPU that can touch them is us and
>> irq is also disabled.
>> 
>> Moving this part outside could reduce lock held time and improve
>> performance. Test with will-it-scale/page_fault1 full load:
>> 
>> kernel      Broadwell(2S)  Skylake(2S)   Broadwell(4S)  Skylake(4S)
>> v4.16-rc2+  9034215        7971818       13667135       15677465
>> this patch  9536374 +5.6%  8314710 +4.3% 14070408 +3.0% 16675866 +6.4%
>> 
>> What the test does is: starts $nr_cpu processes and each will repeatedly
>> do the following for 5 minutes:
>> 1 mmap 128M anonymouse space;
>> 2 write access to that space;
>> 3 munmap.
>> The score is the aggregated iteration.
>
> Iteration count I assume. I am still quite surprised that this would
> have such a large impact.

The test is run with full load, this means near or more than 100
processes will allocate memory in parallel.  According to Amdahl's law,
the performance of a parallel program will be dominated by the serial
part.  For this case, the part protected by zone->lock.  So small
changes to code under zone->lock could make bigger changes to overall
score.

Best Regards,
Huang, Ying