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Date:   Wed, 26 Feb 2020 18:41:04 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Sultan Alsawaf <sultan@kerneltoast.com>,
        Mel Gorman <mgorman@suse.de>,
        Dave Hansen <dave.hansen@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH] mm: Stop kswapd early when nothing's waiting for it to
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On Wed 26-02-20 09:00:57, Shakeel Butt wrote:
> On Wed, Feb 26, 2020 at 1:08 AM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Tue 25-02-20 14:30:03, Shakeel Butt wrote:
> > > On Tue, Feb 25, 2020 at 1:10 AM Michal Hocko <mhocko@kernel.org> wrote:
> > > >
> > > [snip]
> > > >
> > > > The proper fix should, however, check the amount of reclaimable pages
> > > > and back off if they cannot meet the target IMO. We cannot rely on the
> > > > general reclaimability here because that could really be thrashing.
> > > >
> > >
> > > "check the amount of reclaimable pages" vs "cannot rely on the general
> > > reclaimability"? Can you clarify?
> >
> > kswapd targets the high watermark and if your reclaimable memory (aka
> > zone_reclaimable_pages) is lower than the high wmark then it cannot
> > simply satisfy that target, right? Keeping reclaim in that situations
> > seems counter productive to me because you keep evicting pages that
> > might be reused without any feedback mechanism on the actual usage.
> > Please see my other reply.
> >
> 
> I understand and agree with the argument that if reclaimable pages are
> less than high wmark then no need to reclaim. Regarding not depending
> on general reclaimability, I thought you meant that even if
> reclaimable pages are over high wmark, we might not want to continue
> the reclaim to not cause thrashing. Is that right?

That is a completely different story. I would stick with the pathological
problem reported here.  General threshing problem is much more complex
and harder to provide a solution for without introducing a lot of policy
into the reclaim.
-- 
Michal Hocko
SUSE Labs