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

[Mark Brown]

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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