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    SubjectRe: [PATCH v4 2/3] mm/free_pcppages_bulk: do not hold lock when picking pages to free
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    Michal Hocko <mhocko@kernel.org> 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

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