linux-kernel - Re: [RFC 1/7] mm: introduce MADV

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Date:   Tue, 21 May 2019 12:05:37 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.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>, linux-api@...r.kernel.org
Subject: Re: [RFC 1/7] mm: introduce MADV_COOL

On Tue 21-05-19 18:11:34, Minchan Kim wrote:
> On Tue, May 21, 2019 at 08:04:43AM +0200, Michal Hocko wrote:
> > On Tue 21-05-19 07:54:19, Minchan Kim wrote:
> > > On Mon, May 20, 2019 at 10:16:21AM +0200, Michal Hocko wrote:
> > [...]
> > > > > Internally, it works via deactivating memory from active list to
> > > > > inactive's head so when the memory pressure happens, they will be
> > > > > reclaimed earlier than other active pages unless there is no
> > > > > access until the time.
> > > > 
> > > > Could you elaborate about the decision to move to the head rather than
> > > > tail? What should happen to inactive pages? Should we move them to the
> > > > tail? Your implementation seems to ignore those completely. Why?
> > > 
> > > Normally, inactive LRU could have used-once pages without any mapping
> > > to user's address space. Such pages would be better candicate to
> > > reclaim when the memory pressure happens. With deactivating only
> > > active LRU pages of the process to the head of inactive LRU, we will
> > > keep them in RAM longer than used-once pages and could have more chance
> > > to be activated once the process is resumed.
> > 
> > You are making some assumptions here. You have an explicit call what is
> > cold now you are assuming something is even colder. Is this assumption a
> > general enough to make people depend on it? Not that we wouldn't be able
> > to change to logic later but that will always be risky - especially in
> > the area when somebody want to make a user space driven memory
> > management.
> 
> Think about MADV_FREE. It moves those pages into inactive file LRU's head.
> See the get_scan_count which makes forceful scanning of inactive file LRU
> if it has enough size based on the memory pressure.
> The reason is it's likely to have used-once pages in inactive file LRU,
> generally. Those pages has been top-priority candidate to be reclaimed
> for a long time.

OK, fair enough. Being consistent with MADV_FREE is reasonable. I just
forgot we do rotate like this there.

-- 
Michal Hocko
SUSE Labs