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SubjectRe: [PATCH 1/3 v2] slub: prefetch next freelist pointer in __slab_alloc()
From
> 2012/7/4 Pekka Enberg <penberg@kernel.org>:
>> Well, can you show improvement in any benchmark or workload?
>> Prefetching is not always an obvious win and the reason we merged
>> Eric's patch was that he was able to show an improvement in hackbench.

On Wed, Jul 4, 2012 at 5:30 PM, JoonSoo Kim <js1304@gmail.com> wrote:
> I thinks that this patch is perfectly same effect as Eric's patch, so
> doesn't include benchmark result.
> Eric's patch which add "prefetch instruction" in fastpath works for
> second ~ last object of cpu slab.
> This patch which add "prefetch instrunction" in slowpath works for
> first object of cpu slab.

Prefetching can also have negative effect on overall performance:

http://lwn.net/Articles/444336/

> But, I do test "./perf stat -r 20 ./hackbench 50 process 4000 >
> /dev/null" and gain following outputs.
>
> ***** vanilla *****
>
> Performance counter stats for './hackbench 50 process 4000' (20 runs):
>
> 114189.571311 task-clock # 7.924 CPUs utilized
> ( +- 0.29% )
> 2,978,515 context-switches # 0.026 M/sec
> ( +- 3.45% )
> 102,635 CPU-migrations # 0.899 K/sec
> ( +- 5.63% )
> 123,948 page-faults # 0.001 M/sec
> ( +- 0.16% )
> 422,477,120,134 cycles # 3.700 GHz
> ( +- 0.29% )
> <not supported> stalled-cycles-frontend
> <not supported> stalled-cycles-backend
> 251,943,851,074 instructions # 0.60 insns per
> cycle ( +- 0.14% )
> 46,214,207,979 branches # 404.715 M/sec
> ( +- 0.15% )
> 215,342,095 branch-misses # 0.47% of all
> branches ( +- 0.53% )
>
> 14.409990448 seconds time elapsed
> ( +- 0.30% )
>
> Performance counter stats for './hackbench 50 process 4000' (20 runs):
>
> 114576.053284 task-clock # 7.921 CPUs utilized
> ( +- 0.35% )
> 2,810,138 context-switches # 0.025 M/sec
> ( +- 3.21% )
> 85,641 CPU-migrations # 0.747 K/sec
> ( +- 5.05% )
> 124,299 page-faults # 0.001 M/sec
> ( +- 0.18% )
> 423,906,539,517 cycles # 3.700 GHz
> ( +- 0.35% )
> <not supported> stalled-cycles-frontend
> <not supported> stalled-cycles-backend
> 251,354,351,283 instructions # 0.59 insns per
> cycle ( +- 0.13% )
> 46,098,601,012 branches # 402.341 M/sec
> ( +- 0.13% )
> 213,448,657 branch-misses # 0.46% of all
> branches ( +- 0.50% )
>
> 14.464325969 seconds time elapsed
> ( +- 0.34% )
>
>
> ***** patch applied *****
>
> Performance counter stats for './hackbench 50 process 4000' (20 runs):
>
> 112935.199731 task-clock # 7.926 CPUs utilized
> ( +- 0.29% )
> 2,810,157 context-switches # 0.025 M/sec
> ( +- 2.95% )
> 104,278 CPU-migrations # 0.923 K/sec
> ( +- 6.83% )
> 123,999 page-faults # 0.001 M/sec
> ( +- 0.17% )
> 417,834,406,420 cycles # 3.700 GHz
> ( +- 0.29% )
> <not supported> stalled-cycles-frontend
> <not supported> stalled-cycles-backend
> 251,291,523,926 instructions # 0.60 insns per
> cycle ( +- 0.11% )
> 46,083,091,476 branches # 408.049 M/sec
> ( +- 0.12% )
> 213,714,228 branch-misses # 0.46% of all
> branches ( +- 0.43% )
>
> 14.248980376 seconds time elapsed
> ( +- 0.29% )
>
> Performance counter stats for './hackbench 50 process 4000' (20 runs):
>
> 113640.944855 task-clock # 7.926 CPUs utilized
> ( +- 0.28% )
> 2,776,983 context-switches # 0.024 M/sec
> ( +- 5.66% )
> 95,962 CPU-migrations # 0.844 K/sec
> ( +- 10.69% )
> 123,849 page-faults # 0.001 M/sec
> ( +- 0.15% )
> 420,446,572,595 cycles # 3.700 GHz
> ( +- 0.28% )
> <not supported> stalled-cycles-frontend
> <not supported> stalled-cycles-backend
> 251,174,259,429 instructions # 0.60 insns per
> cycle ( +- 0.21% )
> 46,060,683,039 branches # 405.318 M/sec
> ( +- 0.23% )
> 213,480,999 branch-misses # 0.46% of all
> branches ( +- 0.75% )
>
> 14.336843534 seconds time elapsed
> ( +- 0.28% )

That doesn't seem like that obvious win to me... Eric, Christoph?


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