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

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Date:   Wed, 10 Oct 2018 11:15:49 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Rik van Riel <riel@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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 Wed, Oct 10, 2018 at 01:03:50AM +0000, Rik van Riel wrote:
> 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 ...

Fully agreed, fragmentation is still a concern. I'm still working on
that part, but artificial caps and pro-active reclaim are trickier to
get right than prioritization, and since these patches here are useful
on their own I didn't want to hold them back.

> > Reported-by: Domas Mituzas <dmituzas@...com>
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> Reviewed-by: Rik van Riel <riel@...riel.com>

Thanks!