linux-kernel - Re: [PATCH RFC - TAKE TWO - 00/12] New version of the BFQ I/O Scheduler

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Date:	Mon, 2 Jun 2014 14:57:30 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
Cc:	Paolo Valente <paolo.valente@...more.it>,
	Li Zefan <lizefan@...wei.com>,
	Fabio Checconi <fchecconi@...il.com>,
	Arianna Avanzini <avanzini.arianna@...il.com>,
	Paolo Valente <posta_paolo@...oo.it>,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org
Subject: Re: [PATCH RFC - TAKE TWO - 00/12] New version of the BFQ I/O
 Scheduler

On Mon, Jun 02 2014, Tejun Heo wrote:
> Hello,
> 
> On Mon, Jun 02, 2014 at 11:46:36AM -0600, Jens Axboe wrote:
> > But blk-mq will potentially drive anything, so it might not be out of
> > the question with a more expensive scheduling variant, if it makes any
> > sense to do of course. At least until there's no more rotating stuff out
> > there :-). But it's not a priority at all to me yet. As long as we have
> > coexisting IO paths, it'd be trivial to select the needed one based on
> > the device characteristics.
> 
> Hmmm... yeah, moving rotating devices over to blk-mq doesn't really
> seem beneficial to me.  I think there are fundamental behavioral
> differences for rotating rusts and newer solid state devices to share
> single code path for things like scheduling and selecting the
> appropriate path depending on the actual devices sounds like a much
> better plan even in the long term.

It's not so much about it being more beneficial to run in blk-mq, as it
is about not having two code paths. But yes, we're likely going to
maintain that code for a long time, so it's not going anywhere anytime
soon.

And for scsi-mq, it's already opt-in, though on a per-host basis. Doing
finer granularity than that is going to be difficult, unless we let
legacy-block and blk-mq share a tag map (though that would not be too
hard).

-- 
Jens Axboe

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