linux-kernel - Re: [RFCv2 1/6] mm: introduce MADV

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Message-ID: <20190604070228.GD4669@dhcp22.suse.cz>
Date:   Tue, 4 Jun 2019 09:02:28 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>, jannh@...gle.com,
        oleg@...hat.com, christian@...uner.io, oleksandr@...hat.com,
        hdanton@...a.com
Subject: Re: [RFCv2 1/6] mm: introduce MADV_COLD

On Tue 04-06-19 13:26:51, Minchan Kim wrote:
> On Mon, Jun 03, 2019 at 09:16:07AM +0200, Michal Hocko wrote:
[...]
> > Right. But there is still the page cache reclaim. Is it expected that
> > an explicitly cold memory doesn't get reclaimed because we have a
> > sufficient amount of page cache (a very common case) and we never age
> > anonymous memory because of that?
> 
> If there are lots of used-once pages in file-LRU, I think there is no
> need to reclaim anonymous pages because it needs bigger overhead due to
> IO. It has been true for a long time in current VM policy.

You are making an assumption which is not universally true. If I _know_
that there is a considerable amount of idle anonymous memory then I
would really prefer if it goes to the swap rather than make a pressure
on caching. Inactive list is not guaranteed to contain only used-once
pages, right?

Anyway, as already mentioned, we can start with a simpler implementation
for now and explicitly note that pagecache biased reclaim is known to be
a problem potentially. I am pretty sure somebody will come sooner or
later and we can address the problem then with some good numbers to back
the additional complexity.

-- 
Michal Hocko
SUSE Labs