Re: [PATCH 03/10] mm, pcp: reduce lock contention for draining high-order pages

[Huang, Ying]

Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> writes:

> On Wed, Sep 20, 2023 at 02:18:49PM +0800, Huang Ying wrote:
>> In commit f26b3fa04611 ("mm/page_alloc: limit number of high-order
>> pages on PCP during bulk free"), the PCP (Per-CPU Pageset) will be
>> drained when PCP is mostly used for high-order pages freeing to
>> improve the cache-hot pages reusing between page allocating and
>> freeing CPUs.
>> 
>> On system with small per-CPU data cache, pages shouldn't be cached
>> before draining to guarantee cache-hot.  But on a system with large
>> per-CPU data cache, more pages can be cached before draining to reduce
>> zone lock contention.
>> 
>> So, in this patch, instead of draining without any caching, "batch"
>> pages will be cached in PCP before draining if the per-CPU data cache
>> size is more than "4 * batch".
>> 
>> On a 2-socket Intel server with 128 logical CPU, with the patch, the
>> network bandwidth of the UNIX (AF_UNIX) test case of lmbench test
>> suite with 16-pair processes increase 72.2%.  The cycles% of the
>> spinlock contention (mostly for zone lock) decreases from 45.8% to
>> 21.2%.  The number of PCP draining for high order pages
>> freeing (free_high) decreases 89.8%.  The cache miss rate keeps 0.3%.
>> 
>> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>
> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>
> However, the flag should also have been documented to make it clear that
> it preserves some pages on the PCP if the cache is large enough.

Sure.  Will do this.

> Similar
> to the previous patch, it would have been easier to reason about in the
> general case if the decision had only been based on the LLC without
> having to worry if any intermediate layer has a meaningful impact that
> varies across CPU implementations.

Sure.  Will do this.

--
Best Regards,
Huang, Ying