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

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Date:	Fri, 3 Oct 2008 09:58:49 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jens Axboe <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 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...

I should resurrect the XFS patches I had an retest them....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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