Re: [PATCH 1/6] mm: page_alloc: remove pcppage migratetype caching

[Zi Yan]

On 11 Sep 2023, at 15:41, Johannes Weiner wrote:

> The idea behind the cache is to save get_pageblock_migratetype()
> lookups during bulk freeing. A microbenchmark suggests this isn't
> helping, though. The pcp migratetype can get stale, which means that
> bulk freeing has an extra branch to check if the pageblock was
> isolated while on the pcp.
>
> While the variance overlaps, the cache write and the branch seem to
> make this a net negative. The following test allocates and frees
> batches of 10,000 pages (~3x the pcp high marks to trigger flushing):
>
> Before:
>           8,668.48 msec task-clock                       #   99.735 CPUs utilized               ( +-  2.90% )
>                 19      context-switches                 #    4.341 /sec                        ( +-  3.24% )
>                  0      cpu-migrations                   #    0.000 /sec
>             17,440      page-faults                      #    3.984 K/sec                       ( +-  2.90% )
>     41,758,692,473      cycles                           #    9.541 GHz                         ( +-  2.90% )
>    126,201,294,231      instructions                     #    5.98  insn per cycle              ( +-  2.90% )
>     25,348,098,335      branches                         #    5.791 G/sec                       ( +-  2.90% )
>         33,436,921      branch-misses                    #    0.26% of all branches             ( +-  2.90% )
>
>          0.0869148 +- 0.0000302 seconds time elapsed  ( +-  0.03% )
>
> After:
>           8,444.81 msec task-clock                       #   99.726 CPUs utilized               ( +-  2.90% )
>                 22      context-switches                 #    5.160 /sec                        ( +-  3.23% )
>                  0      cpu-migrations                   #    0.000 /sec
>             17,443      page-faults                      #    4.091 K/sec                       ( +-  2.90% )
>     40,616,738,355      cycles                           #    9.527 GHz                         ( +-  2.90% )
>    126,383,351,792      instructions                     #    6.16  insn per cycle              ( +-  2.90% )
>     25,224,985,153      branches                         #    5.917 G/sec                       ( +-  2.90% )
>         32,236,793      branch-misses                    #    0.25% of all branches             ( +-  2.90% )
>
>          0.0846799 +- 0.0000412 seconds time elapsed  ( +-  0.05% )
>
> A side effect is that this also ensures that pages whose pageblock
> gets stolen while on the pcplist end up on the right freelist and we
> don't perform potentially type-incompatible buddy merges (or skip
> merges when we shouldn't), whis is likely beneficial to long-term

s/whis/this

> fragmentation management, although the effects would be harder to
> measure. Settle for simpler and faster code as justification here.
>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>  mm/page_alloc.c | 61 ++++++++++++-------------------------------------
>  1 file changed, 14 insertions(+), 47 deletions(-)
>

Acked-by: Zi Yan <ziy@xxxxxxxxxx>

--
Best Regards,
Yan, Zi

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