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

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Date:	Wed, 30 Nov 2011 11:18:42 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Eric Seppanen <eric@...estorage.com>
CC:	Christoph Hellwig <hch@...radead.org>,
	Vivek Goyal <vgoyal@...hat.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 2011-09-28 21:05, Eric Seppanen wrote:
> 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.

I agree on a+b, but c is definitely more than debatable. I have yet to
see a device saturate its bandwidth on 4KB IOS. So merging on the write
side is always going to be a win.

-- 
Jens Axboe

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