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Date: Wed, 1 Oct 2008 23:55:01 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Jens Axboe <jens.axboe@oracle.com>
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
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On Thu, 2 Oct 2008 08:27:37 +0200 Jens Axboe <jens.axboe@oracle.com> wrote:

> On Wed, Oct 01 2008, Andrew Morton wrote:
> > On Wed, 1 Oct 2008 20:00:34 -0700 Arjan van de Ven <arjan@infradead.org> wrote:
> > 
> > > Subject: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority
> > 
> > You proposed this a while back and it didn't happen and I forget
> > why and the changelog doesn't mention any of that?
> 
> I think you called for benchmark results, which I don't think happened.
> The patch definitely makes sense, so we should just make sure that we
> don't regress elsewhere. Honestly, I'd be surprised if we did...

Now I think about it, didn't the earlier patch tweak CPU priority and
not IO priority?  I forget.  <kicks the changelog again>

> How about I just toss it into the 2.6.28 testing mix, plenty of time for
> testing and such?

Many performance regressions don't get noticed for six or twelve
months, by which time everyone is suffering from them (see
kernel/sched.c).

kjournald does huge amounts of not-terribly-important writeback.  One
obvious risk is that by making all that bulk writeback high-priority,
read-latency-sensitive applications might suffer latency spikes.



Now, kjournald is _supposed_ to be mostly asynchronous wrt foreground
operations.  And once upon a time (seven years ago) it mostly was.  But
there was some horrid race which I ended up fixing by introducing one
point where synchronous userspace actions had to block behind kjournald
activity.  I spent quite some time on it but couldn't come up with
anything better.  It had fairly bad effects on some workloads.

I've forgotten where that code is now, but I don't think it was ever
revisited.  It should be.

So.  Where are these atime updaters getting blocked?
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