linux-kernel - Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler

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Message-ID: <12386463.fJy0cVexVD@wuerfel>
Date:   Wed, 26 Oct 2016 17:13:07 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Bart Van Assche <bart.vanassche@...disk.com>
Cc:     Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
        Paolo Valente <paolo.valente@...aro.org>,
        Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        ulf.hansson@...aro.org, linus.walleij@...aro.org,
        broonie@...nel.org, hare@...e.de, grant.likely@...retlab.ca,
        James.Bottomley@...senpartnership.com
Subject: Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler

On Wednesday, October 26, 2016 8:05:11 AM CEST Bart Van Assche wrote:
> On 10/26/2016 04:34 AM, Jan Kara wrote:
> > On Wed 26-10-16 03:19:03, Christoph Hellwig wrote:
> >> Just as last time:
> >>
> >> big NAK for introducing giant new infrastructure like a new I/O scheduler
> >> for the legacy request structure.
> >>
> >> Please direct your engergy towards blk-mq instead.
> >
> > Christoph, we will probably talk about this next week but IMO rotating
> > disks and SATA based SSDs are going to stay with us for another 15 years,
> > likely more. For them blk-mq is no win, relatively complex IO scheduling
> > like CFQ or BFQ does is a big win for them in some cases. So I think IO
> > scheduling (and thus place for something like BFQ) is going to stay with us
> > for quite a long time still. So are we going to add hooks in blk-mq to
> > support full-blown IO scheduling at least for single queue devices? Or how
> > else do we want to support that HW?
> 
> Hello Jan,
> 
> Having two versions (one for non-blk-mq, one for blk-mq) of every I/O 
> scheduler would be a maintenance nightmare. Has anyone already analyzed 
> whether it would be possible to come up with an API for I/O schedulers 
> that makes it possible to use the same I/O scheduler for both blk-mq and 
> the traditional block layer?

The question to ask first is whether to actually have pluggable
schedulers on blk-mq at all, or just have one that is meant to
do the right thing in every case (and possibly can be bypassed
completely).

	Arnd