linux-kernel - Re: [PATCH 0/2] cfq-iosched: fixing RQ

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Message-ID: <AANLkTimxrPLaAoH28PuxggHr99G9Xq8GlHaDJEd_QhNT@mail.gmail.com>
Date:	Tue, 20 Jul 2010 21:10:56 +0200
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Jeff Moyer <jmoyer@...hat.com>, axboe@...nel.dk,
	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] cfq-iosched: fixing RQ_NOIDLE handling.

On Tue, Jul 20, 2010 at 4:26 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Tue, Jul 20, 2010 at 10:11:03AM -0400, Christoph Hellwig wrote:
>> Didn't you guys have a previous iteration of the fixes that gets
>> rid of REQ_NODILE by improving the heuristics inside cfq?  That
>> would be much, much preffered from the filesystem point of view.
I think the previous iteration required more complex heuristics, while
this one uses existing ones to handle one more class of problems.
I understand that you still see the complexity from the fs side, but
Vivek's proposal may help also there. It only needs to be tested thoroughly.

>
> Actually in this patch, I was thinking we can probably get rid of
> RQ_NOIDLE flag and just check for WRITE_SYNC. Any WRITE_SYNC queue
> gets served on sync-noidle tree. I am wondering will we not face jbd
> thread issues with direct writes also? If yes, then not special casing
> direct IO writes and treat them same as O_SYNC writes will make sense.

Probably it is better to submit this first, since it is already
tested, and then have a different patch that can finish the work
This will help when bisecting for possible regressions, since I'm not
sure why the other writes are not already marked with RQ_NOIDLE (maybe
it was introduced for some good reason to distinguish the two sets,
and we won't know unless we find the workload where it helped).
I'll resend the current patch with Jeff's reviewed and tested tags.

Corrado

>
> I really wished that we had some blktrace of some standard workloads
> stored somewhere which we could simply replay using "btreplay" and come
> to some kind of conclusion whenever we are faced with taking such
> decisions.
>
> Thanks
> Vivek
>
>



-- 
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dott. Corrado Zoccolo                          mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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