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

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Message-ID: <1882a6da-a599-b820-6257-11bbac02b220@kernel.dk>
Date:   Mon, 23 Sep 2019 09:36:28 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Dave Chinner <david@...morbit.com>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2] mm: implement write-behind policy for sequential file
 writes

On 9/20/19 5:10 PM, Linus Torvalds wrote:
> On Fri, Sep 20, 2019 at 4:05 PM Linus Torvalds
> <torvalds@...ux-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.

-- 
Jens Axboe