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From:   Joonsoo Kim <js1304@gmail.com>
Date:   Mon, 20 Jul 2020 15:53:25 +0900
Message-ID: <CAAmzW4OeCdcAnj2yUJWw_XuC8eVBF-wWjA+7vJ3J1Oju68kfJA@mail.gmail.com>
Subject: Re: [PATCH v6 2/6] mm/vmscan: protect the workingset on anonymous LRU
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        Hugh Dickins <hughd@google.com>,
        Minchan Kim <minchan@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>, kernel-team@lge.com,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
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2020=EB=85=84 7=EC=9B=94 17=EC=9D=BC (=EA=B8=88) =EC=98=A4=ED=9B=84 10:59, =
Johannes Weiner <hannes@cmpxchg.org>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=B1:
>
> On Wed, Jun 17, 2020 at 02:26:19PM +0900, js1304@gmail.com wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >
> > In current implementation, newly created or swap-in anonymous page
> > is started on active list. Growing active list results in rebalancing
> > active/inactive list so old pages on active list are demoted to inactiv=
e
> > list. Hence, the page on active list isn't protected at all.
> >
> > Following is an example of this situation.
> >
> > Assume that 50 hot pages on active list. Numbers denote the number of
> > pages on active/inactive list (active | inactive).
> >
> > 1. 50 hot pages on active list
> > 50(h) | 0
> >
> > 2. workload: 50 newly created (used-once) pages
> > 50(uo) | 50(h)
> >
> > 3. workload: another 50 newly created (used-once) pages
> > 50(uo) | 50(uo), swap-out 50(h)
> >
> > This patch tries to fix this issue.
> > Like as file LRU, newly created or swap-in anonymous pages will be
> > inserted to the inactive list. They are promoted to active list if
> > enough reference happens. This simple modification changes the above
> > example as following.
> >
> > 1. 50 hot pages on active list
> > 50(h) | 0
> >
> > 2. workload: 50 newly created (used-once) pages
> > 50(h) | 50(uo)
> >
> > 3. workload: another 50 newly created (used-once) pages
> > 50(h) | 50(uo), swap-out 50(uo)
> >
> > As you can see, hot pages on active list would be protected.
> >
> > Note that, this implementation has a drawback that the page cannot
> > be promoted and will be swapped-out if re-access interval is greater th=
an
> > the size of inactive list but less than the size of total(active+inacti=
ve).
> > To solve this potential issue, following patch will apply workingset
> > detection that is applied to file LRU some day before.
> >
> > v6: Before this patch, all anon pages (inactive + active) are considere=
d
> > as workingset. However, with this patch, only active pages are consider=
ed
> > as workingset. So, file refault formula which uses the number of all
> > anon pages is changed to use only the number of active anon pages.
>
> I can see that also from the code, but it doesn't explain why.
>
> And I'm not sure this is correct. I can see two problems with it.
>
> After your patch series, there is still one difference between anon
> and file: cache trim mode. If the "use-once" anon dominate most of
> memory and you have a small set of heavily thrashing files, it would
> not get recognized. File refaults *have* to compare their distance to
> the *entire* anon set, or we could get trapped in cache trimming mode
> even as file pages with access frequencies <=3D RAM are thrashing.
>
> On the anon side, there is no cache trimming mode. But even if we're
> not in cache trimming mode and active file is already being reclaimed,
> we have to recognize thrashing on the anon side when reuse frequencies
> are within available RAM. Otherwise we treat an inactive file that is
> not being reused as having the same value as an anon page that is
> being reused. And then we may reclaim file and anon at the same rate
> even as anon is thrashing and file is not. That's not right.
>
> We need to activate everything with a reuse frequency <=3D RAM. Reuse
> frequency is refault distance plus size of the inactive list the page
> was on. This means anon distances should be compared to active anon +
> inactive file + active file, and file distances should be compared to
> active file + inactive_anon + active anon.

You're right. Maybe, I'm confused about something at that time. I will chan=
ge
it as you suggested.

Thanks.