Re: [PATCH 7/8] mq-deadline: add blk-mq adaptation of the deadline IO scheduler

Il giorno 17 gen 2017, alle ore 03:47, Jens Axboe [off-list ref] ha =

scritto:

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On 12/22/2016 09:49 AM, Paolo Valente wrote:

quoted

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quoted

Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe [off-list ref] ha =

scritto:

quoted

quoted

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This is basically identical to deadline-iosched, except it registers
as a MQ capable scheduler. This is still a single queue design.
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One last question (for today ...):in mq-deadline there are no
"schedule dispatch" or "unplug work" functions.  In blk, CFQ and BFQ
do these schedules/unplugs in a lot of cases.  What's the right
replacement?  Just doing nothing?

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You just use blk_mq_run_hw_queue() or variants thereof to kick off =

queue

runs.
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Hi Jens,
I'm working on this right now.  I have a pair of quick questions about
performance.

In the blk version of bfq, if the in-service bfq_queue happen to have
no more budget when the bfq dispatch function is invoked, then bfq:
returns no request (NULL), immediately expires the in-service
bfq_queue, and schedules a new dispatch.  The third step is taken so
that, if other bfq_queues have requests, then a new in-service
bfq_queue will be selected on the upcoming new dispatch, and a new
request will be provided right away.

My questions are: is this dispatch-schedule step still needed with
blk-mq, to avoid a stall?  If it is not needed to avoid a stall, would
it still be needed to boost throughput, because it would force an
immediate, next dispatch?

BTW, bfq-mq survived its first request completion.  I will provide you
with a link to a github branch as soon as bfq-mq seems able to stand
up with a minimal workload.

Thanks,
Paolo

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Jens Axboe
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