block, bfq: do not plug I/O of in-service queue when harmful (ac8b0cb4) · Commits · 戴 / test

If the in-service bfq_queue is sync and remains temporarily idle, then I/O dispatching (from other queues) may be plugged. It may be dome for two reasons: either to boost throughput, or to preserve the bandwidth share of the in-service queue. In the first case, if the I/O of the in-service queue, when it finally arrives, consists only of one small I/O request, then it makes sense to plug even the I/O of the in-service queue. In fact, serving such a small request immediately is likely to lower throughput instead of boosting it, whereas waiting a little bit is likely to let that request grow, thanks to request merging, and become more profitable in terms of throughput (this is likely to happen exactly because the I/O of the queue has been detected to boost throughput). On the opposite end, if I/O dispatching is being plugged only to preserve the bandwidth of the in-service queue, then it would be better not to plug also the I/O of the in-service queue, because such a plugging is likely to cause only loss of bandwidth for the queue. Unfortunately, no distinction is made between the two cases, and the I/O of the in-service queue is always plugged in case just a small I/O request arrives. This commit draws this missing distinction and does not perform harmful plugging. Signed-off-by:

Paolo Valente <paolo.valente@linaro.org> Signed-off-by:

Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

+17 −14

Original line number	Diff line number	Diff line
		@@ -4599,28 +4599,31 @@ static void bfq_rq_enqueued(struct bfq_data bfqd, struct bfq_queue bfqq,
		bool budget_timeout = bfq_bfqq_budget_timeout(bfqq);

		/*
		* There is just this request queued: if the request
		* is small and the queue is not to be expired, then
		* just exit.
		* There is just this request queued: if
		* - the request is small, and
		* - we are idling to boost throughput, and
		* - the queue is not to be expired,
		* then just exit.
		*
		* In this way, if the device is being idled to wait
		* for a new request from the in-service queue, we
		* avoid unplugging the device and committing the
		* device to serve just a small request. On the
		* contrary, we wait for the block layer to decide
		* when to unplug the device: hopefully, new requests
		* will be merged to this one quickly, then the device
		* will be unplugged and larger requests will be
		* dispatched.
		*/
		if (small_req && !budget_timeout)
		* device to serve just a small request. In contrast
		* we wait for the block layer to decide when to
		* unplug the device: hopefully, new requests will be
		* merged to this one quickly, then the device will be
		* unplugged and larger requests will be dispatched.
		*/
		if (small_req && idling_boosts_thr_without_issues(bfqd, bfqq) &&
		!budget_timeout)
		return;

		/*
		* A large enough request arrived, or the queue is to
		* be expired: in both cases disk idling is to be
		* stopped, so clear wait_request flag and reset
		* timer.
		* A large enough request arrived, or idling is being
		* performed to preserve service guarantees, or
		* finally the queue is to be expired: in all these
		* cases disk idling is to be stopped, so clear
		* wait_request flag and reset timer.
		*/
		bfq_clear_bfqq_wait_request(bfqq);
		hrtimer_try_to_cancel(&bfqd->idle_slice_timer);

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