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From: Jens Axboe <axboe@kernel.dk>
To: Jan Kara <jack@suse.cz>
Cc: linux-block@vger.kernel.org, Ming Lei <ming.lei@redhat.com>,
	Paolo Valente <paolo.valente@linaro.org>
Subject: Re: [PATCH v2] block: Do not pull requests from the scheduler when we cannot dispatch them
Date: Thu, 3 Jun 2021 12:01:56 -0600	[thread overview]
Message-ID: <8dc3c313-e2cf-ed45-b299-809c332458ea@kernel.dk> (raw)
In-Reply-To: <20210603104721.6309-1-jack@suse.cz>

On 6/3/21 4:47 AM, Jan Kara wrote:
> Provided the device driver does not implement dispatch budget accounting
> (which only SCSI does) the loop in __blk_mq_do_dispatch_sched() pulls
> requests from the IO scheduler as long as it is willing to give out any.
> That defeats scheduling heuristics inside the scheduler by creating
> false impression that the device can take more IO when it in fact
> cannot.
> 
> For example with BFQ IO scheduler on top of virtio-blk device setting
> blkio cgroup weight has barely any impact on observed throughput of
> async IO because __blk_mq_do_dispatch_sched() always sucks out all the
> IO queued in BFQ. BFQ first submits IO from higher weight cgroups but
> when that is all dispatched, it will give out IO of lower weight cgroups
> as well. And then we have to wait for all this IO to be dispatched to
> the disk (which means lot of it actually has to complete) before the
> IO scheduler is queried again for dispatching more requests. This
> completely destroys any service differentiation.
> 
> So grab request tag for a request pulled out of the IO scheduler already
> in __blk_mq_do_dispatch_sched() and do not pull any more requests if we
> cannot get it because we are unlikely to be able to dispatch it. That
> way only single request is going to wait in the dispatch list for some
> tag to free.

Applied to for-5.14/block, thanks.

-- 
Jens Axboe


  reply	other threads:[~2021-06-03 18:02 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-03 10:47 [PATCH v2] block: Do not pull requests from the scheduler when we cannot dispatch them Jan Kara
2021-06-03 18:01 ` Jens Axboe [this message]
2021-06-07 10:05 ` Hannes Reinecke
2021-06-07 11:41   ` Jan Kara
2021-06-16 15:40 ` Jan Kara
2021-06-16 15:43   ` Jens Axboe
2021-06-16 15:51     ` Jan Kara

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