netdev - Re: [PATCH RFC] mm+net: allow to set kmem_cache create flag for SLAB_NEVER

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Date:   Tue, 17 Jan 2023 15:54:34 +0100 (CET)
From:   Christoph Lameter <cl@...two.de>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
cc:     netdev@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, penberg@...nel.org,
        penberg@...nel.org, vbabka@...e.cz,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, edumazet@...gle.com,
        pabeni@...hat.com
Subject: Re: [PATCH RFC] mm+net: allow to set kmem_cache create flag for
 SLAB_NEVER_MERGE

On Tue, 17 Jan 2023, Jesper Dangaard Brouer wrote:

> When running different network performance microbenchmarks, I started
> to notice that performance was reduced (slightly) when machines had
> longer uptimes. I believe the cause was 'skbuff_head_cache' got
> aliased/merged into the general slub for 256 bytes sized objects (with
> my kernel config, without CONFIG_HARDENED_USERCOPY).

Well that is a common effect that we see in multiple subsystems. This is
due to general memory fragmentation. Depending on the prior load the
performance could actually be better after some runtime if the caches are
populated avoiding the page allocator etc.

The merging could actually be beneficial since there may be more partial
slabs to allocate from and thus avoiding expensive calls to the page
allocator.

I wish we had some effective way of memory defragmentation.