linux-kernel - Re: [RFD] I/O scheduling in blk-mq

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Date:	Mon, 8 Aug 2016 08:26:05 -0700
From:	Bart Van Assche <bvanassche@....org>
To:	Paolo <paolo.valente@...aro.org>, Jens Axboe <axboe@...nel.dk>,
	Tejun Heo <tj@...nel.org>,
	Christoph Hellwig <hch@...radead.org>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Linus Walleij <linus.walleij@...aro.org>, broonie@...nel.org
Subject: Re: [RFD] I/O scheduling in blk-mq

On 08/08/16 07:09, Paolo wrote:
> 2) To provide per-process service guarantees, an I/O scheduler must
> create per-process internal queues. BFQ and CFQ use I/O contexts to
> achieve this goal. Is something like that (or exactly the same)
> available also in blk-mq? If so, do you have any suggestion, or link to
> documentation/code on how to use what is available in blk-mq?

Hello Paolo,

I/O contexts are, by definition, data structures that are shared by 
multiple I/O queues. blk-mq reaches high performance by keeping each 
per-CPU queue independent. This means that using I/O contexts in a 
blk-mq I/O scheduler would introduce a contention point and probably 
also a performance bottleneck. So I would appreciate it if multiqueue 
schedulers would avoid constructs similar to I/O contexts.

Thanks,

Bart.