linux-kernel - Re: [GIT PULL] Queue free fix (was Re: [PATCH] block: Free queue resources at blk_release

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Message-ID: <CADOQvuM2VP1TsLMAOL+NC-zEypDS2NJGmiN6RFCxmjUT6LqaPA@mail.gmail.com>
Date:	Wed, 28 Sep 2011 12:05:10 -0700
From:	Eric Seppanen <eric@...estorage.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>, Hannes Reinecke <hare@...e.de>,
	James Bottomley <James.Bottomley@...allels.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] Queue free fix (was Re: [PATCH] block: Free queue
 resources at blk_release_queue())

On Wed, Sep 28, 2011 at 11:16 AM, Christoph Hellwig <hch@...radead.org> wrote:
> Right now on high iops device queue_lock is the major killer for
> performance.  It's one major reason (*) why a lot of the high iops devices
> are all moving to ->make_request, which has other issues.
>
> (*) others are struct request allocation and the pointless merge hash

I agree: queue lock is the worst performance killer when hw can do
>100K IOPS per block device.

Rather than just being chased away from the request queue due to
performance issues, I could argue there's very little point to having
a queue for devices that
(a) have no seek penalty (and always use noop elevator)
(b) have hardware queues at least as deep as the default request queue
(c) don't benefit from merging

(c) is maybe debatable, but if a device can saturate its bus bandwidth
on 4KB IO, the latency is probably not worth it.
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