linux-kernel - Re: [PATCH 1/3 v2] slub: prefetch next freelist pointer in __slab

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Message-ID: <CAAmzW4NmJs0nanYt+gMPuRCzTqByg6ufpQZBjQ3w80bapqLwFA@mail.gmail.com>
Date:	Thu, 5 Jul 2012 01:24:18 +0900
From:	JoonSoo Kim <js1304@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH 1/3 v2] slub: prefetch next freelist pointer in __slab_alloc()

2012/7/5 Eric Dumazet <eric.dumazet@...il.com>:
> On Thu, 2012-07-05 at 00:48 +0900, JoonSoo Kim wrote:
>> 2012/7/5 Eric Dumazet <eric.dumazet@...il.com>:
>> > Its the slow path. I am not convinced its useful on real workloads (not
>> > a benchmark)
>> >
>> > I mean, if a workload hits badly slow path, some more important work
>> > should be done to avoid this at a higher level.
>> >
>>
>> In hackbench test, fast path allocation is about to 93%.
>> Is it insufficient?
>
> 7% is insufficient I am afraid.
>
> One prefetch() in the fast path serves 93% of the allocations,
> so added icache pressure is ok.
>
> One prefetch() in slow path serves 7% of the allocations, do you see the
> difference ?
>
> Adding a prefetch() is usually a win when a benchmark uses the path one
> million times per second.
>
> But adding prefetches also increases kernel size and it hurts globally.
> (Latency of the kernel depends on its size, when cpu caches are cold)
>

Okay.
Thanks for comments which is very helpful to me.
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