linux-kernel - Re: [RFC] Some questions and an idea on SLUB/SLAB

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Date:   Sat, 9 Oct 2021 01:33:43 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [RFC] Some questions and an idea on SLUB/SLAB

On Sat, Oct 09, 2021 at 12:19:03AM +0000, Hyeonggon Yoo wrote:
>  - Is there a reason that SLUB does not implement cache coloring?
>    it will help utilizing hardware cache. Especially in block layer,
>    they are literally *squeezing* its performance now.

Have you tried turning off cache colouring in SLAB and seeing if
performance changes?  My impression is that it's useful for caches
with low associativity (direct mapped / 2-way / 4-way), but loses
its effectiveness for caches with higher associativity.  For example,
my laptop:

 L1 Data Cache: 48KB, 12-way associative, 64 byte line size
 L1 Instruction Cache: 32KB, 8-way associative, 64 byte line size
 L2 Unified Cache: 1280KB, 20-way associative, 64 byte line size
 L3 Unified Cache: 12288KB, 12-way associative, 64 byte line size

I very much doubt that cache colouring is still useful for this machine.