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Subject: Re: [PATCH v2] mm: implement write-behind policy for sequential file
 writes
To:     Tejun Heo <tj@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
        Michal Hocko <mhocko@suse.com>,
        Dave Chinner <david@fromorbit.com>,
        Mel Gorman <mgorman@suse.de>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
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 <20190923145242.GF2233839@devbig004.ftw2.facebook.com>
From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Message-ID: <ed5d930c-88c6-c8e4-4a6c-529701caa993@yandex-team.ru>
Date:   Mon, 23 Sep 2019 18:06:46 +0300
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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.

> 
>> during copying amount of dirty memory always stays around at 16MiB.
> 
> The following is the test part of a slightly modified version of your
> test script which should run fine on any modern systems.
> 
>    for mode in 0 1; do
> 	  if [ $mode == 0 ]; then
> 		  prefix=''
> 	  else
> 		  prefix='systemd-run --user --scope -p MemoryMax=64M'
> 	  fi
> 
> 	  echo COPY
> 	  time $prefix cp -r dummy copy
> 
> 	  grep Dirty /proc/meminfo
> 
> 	  echo SYNC
> 	  time sync
> 
> 	  rm -fr copy
>    done
> 
> and the result looks like the following.
> 
>    $ ./test-writebehind.sh
>    SIZE
>    3.3G    dummy
>    COPY
> 
>    real    0m2.859s
>    user    0m0.015s
>    sys     0m2.843s
>    Dirty:           3416780 kB
>    SYNC
> 
>    real    0m34.008s
>    user    0m0.000s
>    sys     0m0.008s
>    COPY
>    Running scope as unit: run-r69dca5326a9a435d80e036435ff9e1da.scope
> 
>    real    0m32.267s
>    user    0m0.032s
>    sys     0m4.186s
>    Dirty:             14304 kB
>    SYNC
> 
>    real    0m1.783s
>    user    0m0.000s
>    sys     0m0.006s
> 
> This is how we are solving the massive dirtier problem.  It's easy,
> works pretty well and can easily be tailored to the specific
> requirements.
> 
> 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.