Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v2] mm: implement write-behind policy for sequential file
 writes
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
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>
References: <156896493723.4334.13340481207144634918.stgit@buzz>
 <CAHk-=whmCZvYcR10Pe9fEy912fc8xywbiP9mn054Jg_9+0TqCg@mail.gmail.com>
 <CAHk-=wi+G1MfSu79Ayi-yxbmhdyuLnZ5e1tmBTc69v9Zvd-NKg@mail.gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <1882a6da-a599-b820-6257-11bbac02b220@kernel.dk>
Date:   Mon, 23 Sep 2019 09:36:28 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
MIME-Version: 1.0
In-Reply-To: <CAHk-=wi+G1MfSu79Ayi-yxbmhdyuLnZ5e1tmBTc69v9Zvd-NKg@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

-- 
Jens Axboe