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

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Message-ID: <53FE029B.1030200@parallels.com>
Date:	Wed, 27 Aug 2014 20:08:59 +0400
From:	Maxim Patlasov <mpatlasov@...allels.com>
To:	Ming Lei <ming.lei@...onical.com>, Jens Axboe <axboe@...nel.dk>
CC:	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>, Benjamin LaHaise <bcrl@...ck.org>,
	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 08/21/2014 09:44 AM, Ming Lei wrote:
> On Wed, Aug 20, 2014 at 4:50 AM, Jens Axboe <axboe@...nel.dk> wrote:
>
>>
>> Reworked a bit more:
>>
>> http://git.kernel.dk/?p=linux-block.git;a=commit;h=a323185a761b9a54dc340d383695b4205ea258b6
> One big problem of the commit is that it is basically a serialized workqueue
> because of single &hctx->run_work, and per-req work_struct has to be
> used for concurrent implementation.  So looks the approach isn't flexible
> enough compared with doing that in driver, or any idea about how to fix
> that?
>

I'm interested what's the price of handling requests in a separate 
thread at large. I used the following fio script:

     [global]
     direct=1
     bsrange=512-512
     timeout=10
     numjobs=1
     ioengine=sync

     filename=/dev/loop0 # or /dev/nullb0

     [f1]
     rw=randwrite

to compare the performance of:

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.

Thanks,
Maxim
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