Date: Thu, 2 Oct 2008 10:22:13 +0200
From: Jens Axboe <jens.axboe@oracle.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Arjan van de Ven <arjan@infradead.org>, linux-kernel@vger.kernel.org,
       Alan Cox <alan@lxorguk.ukuu.org.uk>
Subject: Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority
Message-ID: <20081002082212.GY19428@kernel.dk>
References: <20081001200034.65eb67d6@infradead.org> <20081001215638.3a65134c.akpm@linux-foundation.org> <20081002062736.GR19428@kernel.dk> <20081001235501.2b7f50fe.akpm@linux-foundation.org> <20081002074523.GW19428@kernel.dk> <20081002010315.1cda8147.akpm@linux-foundation.org>
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On Thu, Oct 02 2008, Andrew Morton wrote:
> On Thu, 2 Oct 2008 09:45:24 +0200 Jens Axboe <jens.axboe@oracle.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.

If diddling with io priorities makes a big difference (and Arjan said it
does), it's clearly stuck inside the io scheduler waiting to be
dispatched. If it was marked sync, it would be going through much
faster. As async, it'll get mixed in with other async writeout and that
doesn't get a lot of attention in case of sync activity.

-- 
Jens Axboe

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