linux-kernel - Re: [PATCH 4/4] mm: zero-seek shrinkers

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Date:   Wed, 10 Oct 2018 01:03:50 +0000
From:   Rik van Riel <riel@...com>
To:     Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     Rik van Riel <riel@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH 4/4] mm: zero-seek shrinkers

On Tue, 2018-10-09 at 14:47 -0400, Johannes Weiner wrote:

> These workloads also deal with tens of thousands of open files and
> use
> /proc for introspection, which ends up growing the proc_inode_cache
> to
> absurdly large sizes - again at the cost of valuable cache space,
> which isn't a reasonable trade-off, given that proc inodes can be
> re-created without involving the disk.
> 
> This patch implements a "zero-seek" setting for shrinkers that
> results
> in a target ratio of 0:1 between their objects and IO-backed
> caches. This allows such virtual caches to grow when memory is
> available (they do cache/avoid CPU work after all), but effectively
> disables them as soon as IO-backed objects are under pressure.
> 
> It then switches the shrinkers for procfs and sysfs metadata, as well
> as excess page cache shadow nodes, to the new zero-seek setting.

This patch looks like a great step in the right
direction, though I do not know whether it is
aggressive enough.

Given that internal slab fragmentation will
prevent the slab cache from returning a slab to
the VM if just one object in that slab is still
in use, there may well be workloads where we
should just put a hard cap on the number of
freeable items these slabs, and reclaim them
preemptively.

However, I do not know for sure, and this patch
seems like a big improvement over what we had
before, so ...

> Reported-by: Domas Mituzas <dmituzas@...com>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Reviewed-by: Rik van Riel <riel@...riel.com>