linux-kernel - Re: [PATCH] mm,slab,vmscan: accumulate gradual pressure on small slabs

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Date:   Mon, 28 Jan 2019 16:31:18 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-team@...com, Chris Mason <clm@...com>,
        Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm,slab,vmscan: accumulate gradual pressure on small
 slabs

On Mon, Jan 28, 2019 at 02:35:35PM -0500, Rik van Riel wrote:
> There are a few issues with the way the number of slab objects to
> scan is calculated in do_shrink_slab.  First, for zero-seek slabs,
> we could leave the last object around forever. That could result
> in pinning a dying cgroup into memory, instead of reclaiming it.
> The fix for that is trivial.
> 
> Secondly, small slabs receive much more pressure, relative to their
> size, than larger slabs, due to "rounding up" the minimum number of
> scanned objects to batch_size.
> 
> We can keep the pressure on all slabs equal relative to their size
> by accumulating the scan pressure on small slabs over time, resulting
> in sometimes scanning an object, instead of always scanning several.
> 
> This results in lower system CPU use, and a lower major fault rate,
> as actively used entries from smaller caches get reclaimed less
> aggressively, and need to be reloaded/recreated less often.
> 
> Fixes: 4b85afbdacd2 ("mm: zero-seek shrinkers")
> Fixes: 172b06c32b94 ("mm: slowly shrink slabs with a relatively small number of objects")
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Chris Mason <clm@...com>
> Cc: Roman Gushchin <guro@...com>
> Cc: kernel-team@...com
> Tested-by: Chris Mason <clm@...com>

Acked-by: Johannes Weiner <hannes@...xchg.org>