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

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Date:   Tue, 24 Sep 2019 12:00:17 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Dave Chinner <david@...morbit.com>
Cc:     Tejun Heo <tj@...nel.org>, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, Michal Hocko <mhocko@...e.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

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.
>