Commit edaf9428 authored by Paolo Valente's avatar Paolo Valente Committed by Jens Axboe
Browse files

block, bfq: boost throughput with flash-based non-queueing devices 



When a queue associated with a process remains empty, there are cases
where throughput gets boosted if the device is idled to await the
arrival of a new I/O request for that queue. Currently, BFQ assumes
that one of these cases is when the device has no internal queueing
(regardless of the properties of the I/O being served). Unfortunately,
this condition has proved to be too general. So, this commit refines it
as "the device has no internal queueing and is rotational".

This refinement provides a significant throughput boost with random
I/O, on flash-based storage without internal queueing. For example, on
a HiKey board, throughput increases by up to 125%, growing, e.g., from
6.9MB/s to 15.6MB/s with two or three random readers in parallel.

Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarLuca Miccio <lucmiccio@gmail.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent d5be3fef

block/bfq-iosched.c

+19 −10
Original line number Diff line number Diff line
@@ -3114,7 +3114,10 @@ static bool bfq_may_expire_for_budg_timeout(struct bfq_queue *bfqq) 
static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)

{

	struct bfq_data *bfqd = bfqq->bfqd;

	bool idling_boosts_thr, idling_boosts_thr_without_issues,

	bool rot_without_queueing =

		!blk_queue_nonrot(bfqd->queue) && !bfqd->hw_tag,

		bfqq_sequential_and_IO_bound,

		idling_boosts_thr, idling_boosts_thr_without_issues,

		idling_needed_for_service_guarantees,

		asymmetric_scenario;
@@ -3133,28 +3136,34 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq) 
	    bfq_class_idle(bfqq))

		return false;



	bfqq_sequential_and_IO_bound = !BFQQ_SEEKY(bfqq) &&

		bfq_bfqq_IO_bound(bfqq) && bfq_bfqq_has_short_ttime(bfqq);



	/*

	 * The next variable takes into account the cases where idling

	 * boosts the throughput.

	 *

	 * The value of the variable is computed considering, first, that

	 * idling is virtually always beneficial for the throughput if:

	 * (a) the device is not NCQ-capable, or

	 * (b) regardless of the presence of NCQ, the device is rotational

	 *     and the request pattern for bfqq is I/O-bound and sequential.

	 * (a) the device is not NCQ-capable and rotational, or

	 * (b) regardless of the presence of NCQ, the device is rotational and

	 *     the request pattern for bfqq is I/O-bound and sequential, or

	 * (c) regardless of whether it is rotational, the device is

	 *     not NCQ-capable and the request pattern for bfqq is

	 *     I/O-bound and sequential.

	 *

	 * Secondly, and in contrast to the above item (b), idling an

	 * NCQ-capable flash-based device would not boost the

	 * throughput even with sequential I/O; rather it would lower

	 * the throughput in proportion to how fast the device

	 * is. Accordingly, the next variable is true if any of the

	 * above conditions (a) and (b) is true, and, in particular,

	 * happens to be false if bfqd is an NCQ-capable flash-based

	 * device.

	 * above conditions (a), (b) or (c) is true, and, in

	 * particular, happens to be false if bfqd is an NCQ-capable

	 * flash-based device.

	 */

	idling_boosts_thr = !bfqd->hw_tag ||

		(!blk_queue_nonrot(bfqd->queue) && bfq_bfqq_IO_bound(bfqq) &&

		 bfq_bfqq_has_short_ttime(bfqq));

	idling_boosts_thr = rot_without_queueing ||

		((!blk_queue_nonrot(bfqd->queue) || !bfqd->hw_tag) &&

		 bfqq_sequential_and_IO_bound);



	/*

	 * The value of the next variable,

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