linux-kernel - Re: [PATCH v2 4/4] block: loop: support to submit I/O via kernel aio based

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Date:	Wed, 18 Mar 2015 11:28:55 -0700
From:	Maxim Patlasov <mpatlasov@...allels.com>
To:	Ming Lei <ming.lei@...onical.com>
CC:	<linux-kernel@...r.kernel.org>,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Jens Axboe <axboe@...nel.dk>, Zach Brown <zab@...bo.net>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Benjamin LaHaise <bcrl@...ck.org>
Subject: Re: [PATCH v2 4/4] block: loop: support to submit I/O via kernel
 aio based

On 01/13/2015 07:44 AM, Ming Lei wrote:
> Part of the patch is based on Dave's previous post.
>
> This patch submits I/O to fs via kernel aio, and we
> can obtain following benefits:
>
> 	- double cache in both loop file system and backend file
> 	gets avoided
> 	- context switch decreased a lot, and finally CPU utilization
> 	is decreased
> 	- cached memory got decreased a lot
>
> One main side effect is that throughput is decreased when
> accessing raw loop block(not by filesystem) with kernel aio.
>
> This patch has passed xfstests test(./check -g auto), and
> both test and scratch devices are loop block, file system is ext4.
>
> Follows two fio tests' result:
>
> 1. fio test inside ext4 file system over loop block
> 1) How to run
> 	- linux kernel base: 3.19.0-rc3-next-20150108(loop-mq merged)
> 	- loop over SSD image 1 in ext4
> 	- linux psync, 16 jobs, size 200M, ext4 over loop block
> 	- test result: IOPS from fio output
>
> 2) Throughput result:
> 	-------------------------------------------------------------
> 	test cases          |randread   |read   |randwrite  |write  |
> 	-------------------------------------------------------------
> 	base                |16799      |59508  |31059      |58829
> 	-------------------------------------------------------------
> 	base+kernel aio     |15480      |64453  |30187      |57222
> 	-------------------------------------------------------------

Ming, it's important to understand the overhead of aio_kernel_() 
implementation. So could you please add test results for raw SSD device 
to the table above next time (in v3 of your patches).

Jens, if you have some fast storage at hand, could you please measure 
IOPS for Ming's patches vs. raw block device -- to ensure that the 
patches do not impose too low limit on performance.

Thanks,
Maxim
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