linux-kernel - Re: [PATCH v2] mm/slub: disable slab merging in the default configuration

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Date:   Tue, 18 Jul 2023 14:08:07 +0200
From:   "Julian Pidancet" <julian.pidancet@...cle.com>
To:     "David Rientjes" <rientjes@...gle.com>
Cc:     "Christoph Lameter" <cl@...ux.com>,
        "Lameter, Christopher" <cl@...amperecomputing.com>,
        "Pekka Enberg" <penberg@...nel.org>,
        "Joonsoo Kim" <iamjoonsoo.kim@....com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Roman Gushchin" <roman.gushchin@...ux.dev>,
        "Hyeonggon Yoo" <42.hyeyoo@...il.com>, <linux-mm@...ck.org>,
        "Jonathan Corbet" <corbet@....net>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        "Matthew Wilcox" <willy@...radead.org>,
        "Kees Cook" <keescook@...omium.org>,
        "Rafael Aquini" <aquini@...hat.com>
Subject: Re: [PATCH v2] mm/slub: disable slab merging in the default
 configuration

On Mon Jul 10, 2023 at 04:40, David Rientjes wrote:
> On Sun, 9 Jul 2023, David Rientjes wrote:
>
> > There are some substantial performance degradations, most notably 
> > context_switch1_per_thread_ops which regressed ~21%.  I'll need to repeat
> > that test to confirm it and can also try on cascadelake if it reproduces.
> > 
>
> So the regression on skylake for will-it-scale appears to be real:
>
>                LABEL              | COUNT |    MIN     |    MAX     |    MEAN    |   MEDIAN   | STDDEV | DIRECTION  
> ----------------------------------+-------+------------+------------+------------+------------+--------+------------
>   context_switch1_per_thread_ops  |       |            |            |            |            |        |            
>   (A) v6.1.30                     | 1     | 314507.000 | 314507.000 | 314507.000 | 314507.000 | 0      |            
>   (B) v6.1.30 slab_nomerge        | 1     | 257403.000 | 257403.000 | 257403.000 | 257403.000 | 0      |            
>     !! REGRESSED !!               |       | -18.16%    | -18.16%    | -18.16%    | -18.16%    | ---    | + is good  
>
> but I can't reproduce this on cascadelake:
>
>                LABEL              | COUNT |    MIN     |    MAX     |    MEAN    |   MEDIAN   | STDDEV | DIRECTION  
> ----------------------------------+-------+------------+------------+------------+------------+--------+------------
>   context_switch1_per_thread_ops  |       |            |            |            |            |        |            
>   (A) v6.1.30                     | 1     | 301128.000 | 301128.000 | 301128.000 | 301128.000 | 0      |            
>   (B) v6.1.30 slab_nomerge        | 1     | 301282.000 | 301282.000 | 301282.000 | 301282.000 | 0      |            
>                                   |       | +0.05%     | +0.05%     | +0.05%     | +0.05%     | ---    | + is good  
>
> So I'm a bit baffled at the moment.
>
> I'll try to dig deeper and see what slab caches this benchmark exercises
> that apparently no other benchmarks do.  (I'm really hoping that the only
> way to recover this performance is by something like
> kmem_cache_create(SLAB_MERGE).)

Hi David,

Many thanks for running all these tests. The amount of attention you've
given this change is simply amazing. I wish I could have been able to
assist you by doing more tests, but I've been lacking the necessary
resources to do so.

I'm as surprised as you are regarding the skylake regression. 20% is
quite a large number, but perhaps it's less worrying than it looks given
that benchmarks are usually very different from real-world workloads?

As Kees Cook was suggesting in his own reply, have you given a thought
about including this change in -next and see if there are regressions
showing up in CI performance tests results?

Regards,

-- 
Julian

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