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SubjectRe: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority
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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