linux-kernel - Re: [RESEND][RFC] BFQ I/O Scheduler

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Message-ID: <20080415180833.GB32602@gandalf.sssup.it>
Date:	Tue, 15 Apr 2008 20:08:33 +0200
From:	Fabio Checconi <fchecconi@...il.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org, paolo.valente@...more.it
Subject: Re: [RESEND][RFC] BFQ I/O Scheduler

> From: Jens Axboe <jens.axboe@...cle.com>
> Date: Tue, Apr 15, 2008 02:42:48PM +0200
>
> On Tue, Apr 15 2008, Fabio Checconi wrote:
> > of course the hlist_sched_*() functions are much better than what was
> > in the patch (the idea behind the patch was to not touch at all cfq code).
> 
> As long as the changes at that point are straight forward and 'obviously
> correct', there's no harm done. Have you thought about merging bfq and
> cfq?
> 

Well, I'm maintaining bfq as a modified version of cfq, and I use a
script to generate the bfq-iosched.c file.  It allows keeping the common
code in sync.

I've posted this version to allow comparisons between the two schedulers
and because I consider cfq a reference, more tested/stable scheduler.  If you
are interested in it I can clean up the cfq patch in the next few days and
post it here for discussion.


> > the ->bfq_ioprio_changed was there to avoid that a process/ioc doing
> > i/o on multiple devices managed by cfq and bfq would see ioprio
> > changes only for one of the two schedulers.
> > 
> > cfq_ioc_set_ioprio() (and its bfq counterpart bfq_ioc_set_ioprio())
> > unconditionally assign 0 to (bfq_)ioprio_changed, so the first
> > scheduler that sees the ioprio change eats the new priority values.
> > of course I may be wrong, but I think it (or some better mechanism
> > to avoid the problem) is necessary.
> 
> I see. If we can and will merge bfq and cfq, then it's not really an
> issue. Otherwise, I'd suggest using bits 0 of ioprio_changed for cfq and
> 1 for bfq and so on. That avoids adding another int to the io context.
> 

Yes, that's a better solution, at least for now.
[I'm sorry I cannot post a patch correcting it right now because I don't
have access to my dev and test boxes.]
 

> > > The code is now in the 'bfq' branch of the block git repo.
> > > 
> > 
> > of course we'll be glad to help in testing in any way you can find useful.
> 
> I'll push it to the for-akpm branch as well, so it should show up in the
> next -mm and get some testing there.
> 

Ok, thank you very much.
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