linux-kernel - Re: scsi-mq V2

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Message-ID: <20140710133609.GO12478@kvack.org>
Date:	Thu, 10 Jul 2014 09:36:09 -0400
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	"Elliott, Robert (Server Storage)" <Elliott@...com>,
	Jens Axboe <axboe@...nel.dk>,
	"dgilbert@...erlog.com" <dgilbert@...erlog.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Bart Van Assche <bvanassche@...ionio.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: scsi-mq V2

On Wed, Jul 09, 2014 at 11:20:40PM -0700, Christoph Hellwig wrote:
> On Thu, Jul 10, 2014 at 12:53:36AM +0000, Elliott, Robert (Server Storage) wrote:
> > the problem still occurs - fio results in low or 0 IOPS, with perf top 
> > reporting unusual amounts of time spent in do_io_submit and io_submit.
> 
> The diff between the two version doesn't show too much other possible
> interesting commits, the most interesting being some minor block
> updates.
> 
> I guess we'll have to a manual bisect, I've pushed out a
> scsi-mq.3-bisect-1 branch that is rebased to just before the merge of
> the block tree, and a scsi-mq.3-bisect-2 branch that is just after
> the merge of the block tree to get started.

There is one possible concern that could be exacerbated by other changes in 
the system: if the application is running close to the bare minimum number 
of requests allocated in io_setup(), the per cpu reference counters will 
have a hard time batching things.  It might be worth testing with an 
increased number of requests being allocated if this is the case.

		-ben
-- 
"Thought is the essence of where you are now."
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