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

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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: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Luca Miccio <lucmiccio@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Paolo Valente 2017-08-04 07:35:11 +02:00 committed by Jens Axboe
parent d5be3fefc9
commit edaf94285b
1 changed files with 19 additions and 10 deletions
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29
block/bfq-iosched.c
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@ -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,