On Mon, Sep 11, 2023 at 03:41:42PM -0400, 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
> fragmentation management, although the effects would be harder to
> measure. Settle for simpler and faster code as justification here.
>
> Signed-off-by: Johannes Weiner <[email protected]>
I've no specific objection and other minor corrections have already been
suggested. I don't recall specifically but I think
get_pageblock_migratetype might have been called redundantly once upon a
time when there were concerns about page allocator overhead for high
speed network. Now that there is bulk allocation and the flow has
changed significantly, it's feasible to simply avoid calling
get_pageblock_migratetype unnecessarily.
Acked-by: Mel Gorman <[email protected]>
--
Mel Gorman
SUSE Labs