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

[Huang, Ying]

Johannes Weiner <hannes@xxxxxxxxxxx> writes:

> 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
> fragmentation management, although the effects would be harder to
> measure. Settle for simpler and faster code as justification here.

I suspected the PCP allocating/freeing path may be influenced (that is,
allocating/freeing batch is less than PCP high).  So I tested
one-process will-it-scale/page_fault1 with sysctl
percpu_pagelist_high_fraction=8.  So pages will be allocated/freed
from/to PCP only.  The test results are as follows,

Before:
will-it-scale.1.processes                        618364.3      (+-  0.075%)
perf-profile.children.get_pfnblock_flags_mask         0.13     (+-  9.350%)

After:
will-it-scale.1.processes	                 616512.0      (+-  0.057%)
perf-profile.children.get_pfnblock_flags_mask	      0.41     (+-  22.44%)

The change isn't large: -0.3%.  Perf profiling shows the cycles% of
get_pfnblock_flags_mask() increases.

--
Best Regards,
Huang, Ying