From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
To: Dave Chinner <david@fromorbit.com>
Cc: Tejun Heo <tj@kernel.org>,
linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
Michal Hocko <mhocko@suse.com>, Mel Gorman <mgorman@suse.de>,
Johannes Weiner <hannes@cmpxchg.org>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2] mm: implement write-behind policy for sequential file writes
Date: Tue, 24 Sep 2019 12:00:17 +0300 [thread overview]
Message-ID: <edafed8a-5269-1e54-fe31-7ba87393eb34@yandex-team.ru> (raw)
In-Reply-To: <20190924073940.GM6636@dread.disaster.area>
On 24/09/2019 10.39, Dave Chinner wrote:
> 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.
>
> Stupid question: how is this any different to simply winding down
> our dirty writeback and throttling thresholds like so:
>
> # echo $((100 * 1000 * 1000)) > /proc/sys/vm/dirty_background_bytes
>
> to start background writeback when there's 100MB of dirty pages in
> memory, and then:
>
> # echo $((200 * 1000 * 1000)) > /proc/sys/vm/dirty_bytes
>
> So that writers are directly throttled at 200MB of dirty pages in
> memory?
>
> This effectively gives us global writebehind behaviour with a
> 100-200MB cache write burst for initial writes.
Global limits affect all dirty pages including memory-mapped and
randomly touched. Write-behind aims only into sequential streams.
>
> ANd, really such strict writebehind behaviour is going to cause all
> sorts of unintended problesm with filesystems because there will be
> adverse interactions with delayed allocation. We need a substantial
> amount of dirty data to be cached for writeback for fragmentation
> minimisation algorithms to be able to do their job....
I think most sequentially written files never change after close.
Except of knowing final size of huge files (>16Mb in my patch)
there should be no difference for delayed allocation.
Probably write behind could provide hint about streaming pattern:
pass something like "MSG_MORE" into writeback call.
>
> Cheers,
>
> Dave.
>
next prev parent reply other threads:[~2019-09-24 9:00 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-20 7:35 [PATCH v2] mm: implement write-behind policy for sequential file writes Konstantin Khlebnikov
2019-09-20 7:39 ` Konstantin Khlebnikov
2019-09-23 14:52 ` Tejun Heo
2019-09-23 15:06 ` Konstantin Khlebnikov
2019-09-23 15:19 ` Tejun Heo
2019-09-24 7:39 ` Dave Chinner
2019-09-24 9:00 ` Konstantin Khlebnikov [this message]
2019-09-25 7:18 ` Dave Chinner
2019-09-25 8:15 ` Konstantin Khlebnikov
2019-09-25 23:25 ` Dave Chinner
2019-09-25 12:54 ` Theodore Y. Ts'o
2019-09-24 19:08 ` Linus Torvalds
2019-09-25 8:00 ` Dave Chinner
2019-09-20 23:05 ` Linus Torvalds
2019-09-20 23:10 ` Linus Torvalds
2019-09-23 15:36 ` Jens Axboe
2019-09-23 16:05 ` Konstantin Khlebnikov
2019-09-24 9:29 ` Konstantin Khlebnikov
2019-09-22 7:47 ` kbuild test robot
2019-09-23 0:36 ` [mm] e0e7df8d5b: will-it-scale.per_process_ops -7.3% regression kernel test robot
2019-09-23 19:11 ` Konstantin Khlebnikov
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