linux-kernel - Re: [PATCH] slub: prefetch next freelist pointer in slab

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Date:	Sun, 18 Dec 2011 14:47:35 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Pekka Enberg <penberg@...nel.org>,
	"Alex,Shi" <alex.shi@...el.com>, Shaohua Li <shaohua.li@...el.com>,
	Matt Mackall <mpm@...enic.com>
Subject: Re: [PATCH] slub: prefetch next freelist pointer in slab_alloc()

On Fri, 16 Dec 2011, Eric Dumazet wrote:

> Recycling a page is a problem, since freelist link chain is hot on
> cpu(s) which freed objects, and possibly very cold on cpu currently
> owning slab.
> 
> Adding a prefetch of cache line containing the pointer to next object in
> slab_alloc() helps a lot in many workloads, in particular on assymetric
> ones (allocations done on one cpu, frees on another cpus). Added cost is
> three machine instructions only.
> 
> Examples on my dual socket quad core ht machine (Intel CPU E5540
> @2.53GHz) (16 logical cpus, 2 memory nodes), 64bit kernel.
> 
> Before patch :
> 
> # perf stat -r 32 hackbench 50 process 4000 >/dev/null
> 
>  Performance counter stats for 'hackbench 50 process 4000' (32 runs):
> 
>      327577,471718 task-clock                #   15,821 CPUs utilized            ( +-  0,64% )
>         28 866 491 context-switches          #    0,088 M/sec                    ( +-  1,80% )
>          1 506 929 CPU-migrations            #    0,005 M/sec                    ( +-  3,24% )
>            127 151 page-faults               #    0,000 M/sec                    ( +-  0,16% )
>    829 399 813 448 cycles                    #    2,532 GHz                      ( +-  0,64% )
>    580 664 691 740 stalled-cycles-frontend   #   70,01% frontend cycles idle     ( +-  0,71% )
>    197 431 700 448 stalled-cycles-backend    #   23,80% backend  cycles idle     ( +-  1,03% )
>    503 548 648 975 instructions              #    0,61  insns per cycle        
>                                              #    1,15  stalled cycles per insn  ( +-  0,46% )
>     95 780 068 471 branches                  #  292,389 M/sec                    ( +-  0,48% )
>      1 426 407 916 branch-misses             #    1,49% of all branches          ( +-  1,35% )
> 
>       20,705679994 seconds time elapsed                                          ( +-  0,64% )
> 
> After patch :
> 
> # perf stat -r 32 hackbench 50 process 4000 >/dev/null
> 
>  Performance counter stats for 'hackbench 50 process 4000' (32 runs):
> 
>      286236,542804 task-clock                #   15,786 CPUs utilized            ( +-  1,32% )
>         19 703 372 context-switches          #    0,069 M/sec                    ( +-  4,99% )
>          1 658 249 CPU-migrations            #    0,006 M/sec                    ( +-  6,62% )
>            126 776 page-faults               #    0,000 M/sec                    ( +-  0,12% )
>    724 636 593 213 cycles                    #    2,532 GHz                      ( +-  1,32% )
>    499 320 714 837 stalled-cycles-frontend   #   68,91% frontend cycles idle     ( +-  1,47% )
>    156 555 126 809 stalled-cycles-backend    #   21,60% backend  cycles idle     ( +-  2,22% )
>    463 897 792 661 instructions              #    0,64  insns per cycle        
>                                              #    1,08  stalled cycles per insn  ( +-  0,94% )
>     87 717 352 563 branches                  #  306,451 M/sec                    ( +-  0,99% )
>        941 738 280 branch-misses             #    1,07% of all branches          ( +-  3,35% )
> 
>       18,132070670 seconds time elapsed                                          ( +-  1,30% )
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> CC: Pekka Enberg <penberg@...nel.org>
> CC: Matt Mackall <mpm@...enic.com> 
> CC: David Rientjes <rientjes@...gle.com>
> CC: "Alex,Shi" <alex.shi@...el.com>
> CC: Shaohua Li <shaohua.li@...el.com>

Acked-by: David Rientjes <rientjes@...gle.com>
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