linux-kernel - Re: [PATCH v1 5/9] block: loop: convert to blk-mq

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Date:	Wed, 27 Aug 2014 21:19:36 +0400
From:	Maxim Patlasov <mpatlasov@...allels.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
CC:	Ming Lei <ming.lei@...onical.com>, Jens Axboe <axboe@...nel.dk>,
	"Christoph Hellwig" <hch@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Zach Brown <zab@...bo.net>,
	"Kent Overstreet" <kmo@...erainc.com>, open list:
	AIO <linux-aio@...ck.org>,
	"Linux FS Devel" <linux-fsdevel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>, ;
Subject: Re: [PATCH v1 5/9] block: loop: convert to blk-mq

On 08/27/2014 08:29 PM, Benjamin LaHaise wrote:
> On Wed, Aug 27, 2014 at 08:08:59PM +0400, Maxim Patlasov wrote:
> ...
>> 1) /dev/loop0 of 3.17.0-rc1 with Ming's patches applied -- 11K iops
>> 2) the same as above, but call loop_queue_work() directly from
>> loop_queue_rq() -- 270K iops
>> 3) /dev/nullb0 of 3.17.0-rc1 -- 380K iops
>>
>> Taking into account so big difference (11K vs. 270K), would it be worthy
>> to implement pure non-blocking version of aio_kernel_submit() returning
>> error if blocking needed? Then loop driver (or any other in-kernel user)
>> might firstly try that non-blocking submit as fast-path, and, only if
>> it's failed, fall back to queueing.
> What filesystem is the backing file for loop0 on?  O_DIRECT access as
> Ming's patches use should be non-blocking, and if not, that's something
> to fix.

I used loop0 directly on top of null_blk driver (because my goal was to 
measure the overhead of processing requests in a separate thread).

In case of real-life filesystems, e.g. ext4, aio_kernel_submit() may 
easily block on something like bh_submit_read(), when fs reads file 
metadata to calculate the offset on block device by position in the file.

Thanks,
Maxim
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