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Subject: Re: [PATCH v2] mm: implement write-behind policy for sequential file
 writes
To:     Jens Axboe <axboe@kernel.dk>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@suse.com>,
        Dave Chinner <david@fromorbit.com>,
        Mel Gorman <mgorman@suse.de>,
        Johannes Weiner <hannes@cmpxchg.org>, Tejun Heo <tj@kernel.org>
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From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Message-ID: <a5e769db-823d-9a48-b5f9-a4b406710c6f@yandex-team.ru>
Date:   Mon, 23 Sep 2019 19:05:55 +0300
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On 23/09/2019 18.36, Jens Axboe wrote:
> On 9/20/19 5:10 PM, Linus Torvalds wrote:
>> On Fri, Sep 20, 2019 at 4:05 PM Linus Torvalds
>> <torvalds@linux-foundation.org> wrote:
>>>
>>>
>>> Now, I hear you say "those are so small these days that it doesn't
>>> matter". And maybe you're right. But particularly for slow media,
>>> triggering good streaming write behavior has been a problem in the
>>> past.
>>
>> Which reminds me: the writebehind trigger should likely be tied to the
>> estimate of the bdi write speed.
>>
>> We _do_ have that avg_write_bandwidth thing in the bdi_writeback
>> structure, it sounds like a potentially good idea to try to use that
>> to estimate when to do writebehind.
>>
>> No?
> 
> I really like the feature, and agree it should be tied to the bdi write
> speed. How about just making the tunable acceptable time of write behind
> dirty? Eg if write_behind_msec is 1000, allow 1s of pending dirty before
> starting writbeack.
> 

I haven't digged into it yet.

But IIRR writeback speed estimation has some problems:

There is no "slow start" - initial speed is 100MiB/s.
This is especially bad for slow usb disks - right after plugging
we'll accumulate too much dirty cache before starting writeback.

And I've seen problems with cgroup-writeback:
each cgroup has own estimation, doesn't work well for short-living cgroups.