linux-kernel - Re: SLUB: percpu partial object count is highly inaccurate, causing some memory wastage and maybe also worse tail latencies?

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Message-ID: <20210118110319.GC14336@dhcp22.suse.cz>
Date:   Mon, 18 Jan 2021 12:03:19 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Christoph Lameter <cl@...ux.com>, Jann Horn <jannh@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Roman Gushchin <guro@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Minchan Kim <minchan@...nel.org>
Subject: Re: SLUB: percpu partial object count is highly inaccurate, causing
 some memory wastage and maybe also worse tail latencies?

On Thu 14-01-21 10:27:40, Vlastimil Babka wrote:
> On 1/12/21 5:35 PM, Christoph Lameter wrote:
> > On Tue, 12 Jan 2021, Jann Horn wrote:
> > 
> >> [This is not something I intend to work on myself. But since I
> >> stumbled over this issue, I figured I should at least document/report
> >> it, in case anyone is willing to pick it up.]
> > 
> > Well yeah all true. There is however a slabinfo tool that has an -s option
> > to shrink all slabs.
> > 
> > 	slabinfo -s
> > 
> > So you could put that somewhere that executes if the system is
> > idle or put it into cron or so.
> 
> Hm this would be similar to recommending a periodical echo > drop_caches
> operation. We actually discourage from that (and yeah, some tools do that, and
> we now report those in dmesg). I believe the kernel should respond to memory
> pressure and not OOM prematurely by itself, including SLUB.

Absolutely agreed! Partial caches are a very deep internal
implementation detail of the allocator and admin has no bussiness into
fiddling with that. This would only lead to more harm than good.
Comparision to drop_caches is really exact!

-- 
Michal Hocko
SUSE Labs