linux-kernel - Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler

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Date:   Fri, 28 Oct 2016 18:17:44 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Linus Walleij <linus.walleij@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Paolo Valente <paolo.valente@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        linux-block@...r.kernel.org,
        Linux-Kernal <linux-kernel@...r.kernel.org>,
        Hannes Reinecke <hare@...e.de>,
        Grant Likely <grant.likely@...retlab.ca>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra
 scheduler

On Fri, Oct 28, 2016 at 06:05:35PM +0200, Arnd Bergmann wrote:
> On Friday, October 28, 2016 9:30:07 AM CEST Jens Axboe wrote:

> > Also, 4.8 and newer have support for BLK_MQ_F_BLOCKING, if you need to
> > block in ->queue_rq(). That could eliminate the need to offload to a
> > kthread manually.

> I think the main reason for the kthread is that on ARM and other
> architectures, the dma mapping operations are fairly slow (for
> cache flushes or bounce buffering) and we want to minimize the
> time between subsequent requests being handled by the hardware.

> This is not unique to MMC in any way, MMC just happens to be
> common on ARM and it is limited by its lack of hardware
> command queuing.

Plus the fact that MMC (and SD) have some *relatively* high performance
implementations which amplify the effects of desaturating the hardware -
the faster the hardware is the more noticable the overhead of stalling
it becomes.

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