linux-kernel - Re: [PATCH v2] mm: implement write-behind policy for sequential file writes

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Message-ID: <20190923151912.GG2233839@devbig004.ftw2.facebook.com>
Date:   Mon, 23 Sep 2019 08:19:12 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Michal Hocko <mhocko@...e.com>,
        Dave Chinner <david@...morbit.com>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] mm: implement write-behind policy for sequential file
 writes

Hello,

On Mon, Sep 23, 2019 at 06:06:46PM +0300, Konstantin Khlebnikov wrote:
> On 23/09/2019 17.52, Tejun Heo wrote:
> >Hello, Konstantin.
> >
> >On Fri, Sep 20, 2019 at 10:39:33AM +0300, Konstantin Khlebnikov wrote:
> >>With vm.dirty_write_behind 1 or 2 files are written even faster and
> >
> >Is the faster speed reproducible?  I don't quite understand why this
> >would be.
> 
> Writing to disk simply starts earlier.

I see.

> >Generic write-behind would definitely have other benefits and also a
> >bunch of regression possibilities.  I'm not trying to say that
> >write-behind isn't a good idea but it'd be useful to consider that a
> >good portion of the benefits can already be obtained fairly easily.
> >
> 
> I'm afraid this could end badly if each simple task like file copying
> will require own systemd job and container with manual tuning.

At least the write window size part of it is pretty easy - the range
of acceptable values is fiarly wide - and setting up a cgroup and
running a command in it isn't that expensive.  It's not like these
need full-on containers.  That said, yes, there sure are benefits to
the kernel being able to detect and handle these conditions
automagically.

Thanks.

-- 
tejun