linux-kernel - Re: [PATCH v3 4/5] mm: introduce MADV

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Message-ID: <20190710194719.GS29695@dhcp22.suse.cz>
Date:   Wed, 10 Jul 2019 21:47:19 +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>, oleksandr@...hat.com,
        hdanton@...a.com, lizeb@...gle.com,
        Dave Hansen <dave.hansen@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 4/5] mm: introduce MADV_PAGEOUT

On Wed 10-07-19 20:53:56, Minchan Kim wrote:
> On Wed, Jul 10, 2019 at 01:16:22PM +0200, Michal Hocko wrote:
> > On Wed 10-07-19 19:48:09, Minchan Kim wrote:
> > > On Tue, Jul 09, 2019 at 11:55:19AM +0200, Michal Hocko wrote:
> > [...]
> > > > I am still not convinced about the SWAP_CLUSTER_MAX batching and the
> > > > udnerlying OOM argument. Is one pmd worth of pages really an OOM risk?
> > > > Sure you can have many invocations in parallel and that would add on
> > > > but the same might happen with SWAP_CLUSTER_MAX. So I would just remove
> > > > the batching for now and think of it only if we really see this being a
> > > > problem for real. Unless you feel really strong about this, of course.
> > > 
> > > I don't have the number to support SWAP_CLUSTER_MAX batching for hinting
> > > operations. However, I wanted to be consistent with other LRU batching
> > > logic so that it could affect altogether if someone try to increase
> > > SWAP_CLUSTER_MAX which is more efficienty for batching operation, later.
> > > (AFAIK, someone tried it a few years ago but rollback soon, I couldn't
> > > rebemeber what was the reason at that time, anyway).
> > 
> > Then please drop this part. It makes the code more complex while any
> > benefit is not demonstrated.
> 
> The history says the benefit.
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/patch/?id=d37dd5dcb955dd8c2cdd4eaef1f15d1b7ecbc379

Limiting the number of isolated pages is fine. All I am saying is that
SWAP_CLUSTER_MAX is an arbitrary number same as 512 pages for one PMD as
a unit of work. Both can lead to the same effect if there are too many
parallel tasks doing the same thing.

I do not want you to change that in the reclaim path. All I am asking
for is to add a bathing without any actual data to back that because
that makes the code more complex without any gains.
-- 
Michal Hocko
SUSE Labs