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

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Date:   Wed, 28 Jun 2023 09:44:51 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Julian Pidancet <julian.pidancet@...cle.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        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] mm/slub: disable slab merging in the default
 configuration

On Tue, Jun 27, 2023 at 12:32:15PM -0700, David Rientjes wrote:
> On Tue, 27 Jun 2023, Julian Pidancet wrote:
> 
> > Make CONFIG_SLAB_MERGE_DEFAULT default to n unless CONFIG_SLUB_TINY is
> > enabled. Benefits of slab merging is limited on systems that are not
> > memory constrained: the overhead is negligible and evidence of its
> > effect on cache hotness is hard to come by.
> > 
> 
> I don't have an objection to this, I think it makes sense.

+1

I believe the overhead was much larger when we had per-memcg slab caches,
but now it should be fairly small on most systems.

But I wonder if we need a new flag (SLAB_MERGE?) to explicitly force merging
on per-slab cache basis. I believe there are some cases when slab caches can
be created in noticeable numbers and in those cases the memory footprint might
be noticeable.

Thanks!