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

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Date:	Thu, 2 Oct 2008 01:03:15 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority

On Thu, 2 Oct 2008 09:45:24 +0200 Jens Axboe <jens.axboe@...cle.com> wrote:

> > So.  Where are these atime updaters getting blocked?
> 
> Behind other IO activity I suppose, since it's marked async. A more
> appropriate fix may be to mark atime updates as sync IO.

No, they might be getting blocked at a higher level.

An async atime update gets recorded into the current transaction. 
kjournald is working on the committing transaction.  We try to keep
those separated, to prevent user processes from getting blocked behind
kjournald activity.

But sometimes that doesn't work (including the place where I knowingly
broke it).  If we can find and fix the offending piece of jbd logic (a
big if) then all is peachy.

If the above theory turns out to be true then diddling IO priorities
is but a workaround.
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