linux-kernel - Re: [PATCH v5 2/2] mm/memcontrol.c: Reduce reclaim retries in mem_cgroup_resize

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Message-ID: <20180222140932.GL30681@dhcp22.suse.cz>
Date:   Thu, 22 Feb 2018 15:09:32 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: [PATCH v5 2/2] mm/memcontrol.c: Reduce reclaim retries in
 mem_cgroup_resize_limit()

On Thu 22-02-18 16:50:33, Andrey Ryabinin wrote:
> On 02/21/2018 11:17 PM, Andrew Morton wrote:
> > On Fri, 19 Jan 2018 16:11:18 +0100 Michal Hocko <mhocko@...nel.org> wrote:
> > 
> >> And to be honest, I do not really see why keeping retrying from
> >> mem_cgroup_resize_limit should be so much faster than keep retrying from
> >> the direct reclaim path. We are doing SWAP_CLUSTER_MAX batches anyway.
> >> mem_cgroup_resize_limit loop adds _some_ overhead but I am not really
> >> sure why it should be that large.
> > 
> > Maybe restarting the scan lots of times results in rescanning lots of
> > ineligible pages at the start of the list before doing useful work?
> > 
> > Andrey, are you able to determine where all that CPU time is being spent?
> > 
> 
> I should have been more specific about the test I did. The full script looks like this:
> 
> mkdir -p /sys/fs/cgroup/memory/test
> echo $$ > /sys/fs/cgroup/memory/test/tasks
> cat 4G_file > /dev/null
> while true; do cat 4G_file > /dev/null; done &
> loop_pid=$!
> perf stat echo 50M > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
> echo -1 > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
> kill $loop_pid
> 
> 
> I think the additional loops add some overhead and it's not that big by itself, but
> this small overhead allows task to refill slightly more pages, increasing
> the total amount of pages that mem_cgroup_resize_limit() need to reclaim.
> 
> By using the following commands to show the the amount of reclaimed pages:
> perf record -e vmscan:mm_vmscan_memcg_reclaim_end echo 50M > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
> perf script|cut -d '=' -f 2| paste -sd+ |bc
> 
> I've got 1259841 pages (4.9G) with the patch vs 1394312 pages (5.4G) without it.

So how does the picture changes if you have multiple producers?

-- 
Michal Hocko
SUSE Labs