linux-kernel - Re: [PATCH 02/11] block: kill QUEUE_ORDERED_BY

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Date:	Fri, 13 Aug 2010 15:06:57 +0200
From:	Christoph Hellwig <hch@....de>
To:	Vladislav Bolkhovitin <vst@...b.net>
Cc:	Tejun Heo <tj@...nel.org>, jaxboe@...ionio.com,
	linux-fsdevel@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, hch@....de, James.Bottomley@...e.de,
	tytso@....edu, chris.mason@...cle.com, swhiteho@...hat.com,
	konishi.ryusuke@....ntt.co.jp, dm-devel@...hat.com, jack@...e.cz,
	rwheeler@...hat.com, hare@...e.de,
	Christoph Hellwig <hch@...radead.org>,
	Nick Piggin <npiggin@...nel.dk>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...source.com>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH 02/11] block: kill QUEUE_ORDERED_BY_TAG

On Fri, Aug 13, 2010 at 04:56:32PM +0400, Vladislav Bolkhovitin wrote:
> Tejun Heo, on 08/12/2010 04:41 PM wrote:
> >Nobody is making meaningful use of ORDERED_BY_TAG now and queue
> >draining for barrier requests will be removed soon which will render
> >the advantage of tag ordering moot.
> 
> Have you seen Hannes Reinecke's and my measurements in 
> http://marc.info/?l=linux-scsi&m=128110662528485&w=2 and 
> http://marc.info/?l=linux-scsi&m=128111995217405&w=2 correspondingly?
> 
> If yes, what else evidences do you need to see that the tag ordering is 
> a big performance win?

It's not tag odering that is a win but big queue depth.  That's what you
measured and what I fully agree on.  I haven't been able to get out of
Hannes what he actually measured.

And if you'd actually look at the patchset allowing deep queues is
exactly what it allows us, and while I haven't done testing on this
patchset but only on my previous version it does get us back to use
the full potential of large arrays exactly because of that.
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