linux-kernel - Re: [PATCH] Give kjournald a IOPRIO_CLASS

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Message-Id: <20081002170646.2a532680.akpm@linux-foundation.org>
Date:	Thu, 2 Oct 2008 17:06:46 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	jens.axboe@...cle.com, arjan@...radead.org, andi@...stfloor.org,
	linux-kernel@...r.kernel.org, alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority

On Fri, 3 Oct 2008 09:58:49 +1000
Dave Chinner <david@...morbit.com> wrote:

> On Thu, Oct 02, 2008 at 02:37:13PM -0700, Andrew Morton wrote:
> > On Thu, 2 Oct 2008 21:22:23 +0200 Jens Axboe <jens.axboe@...cle.com> wrote:
> > > Can we agree on this patch?
> > 
> > This change will cause _all_ kjournald writeout to have elevated
> > priority.  The majority of that writeout (in data=ordered mode) is file
> > data, which we didn't intend to change.
> > 
> > The risk here is that this will *worsen* latency for plain old read(),
> > because now kjournald writeout will be favoured.
> > 
> > There is in fact a good argument for _reducing_ kjournald's IO
> > priority, not increasing it!
> > 
> > A better approach might be to mark the relevant buffers/bios as needing
> > higher priority at submit_bh() time (if that's possible).  At least
> > that way we don't accidentally elevate the priority of the bulk data.
> 
> You can do that for submit_bio() by calling bio_set_prio() before
> submision - I did that for elevating only the XFS journal I/O.
> submit_bh() doesn't have any way of passing a priority through to it
> right now...

Yup.  There are plenty of spare bits in buffer_head.b_state. 
set_buffer_kludge()?

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