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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