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SubjectRe: [PATCH v4 2/3] mm/free_pcppages_bulk: do not hold lock when picking pages to free
On Thu,  1 Mar 2018 14:28:44 +0800 Aaron Lu <aaron.lu@intel.com> 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.

But it's a loss for uniprocessor systems: it adds more code and adds an
additional pass across a list.

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