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[PATCH V2 13/16] block, bfq: boost the throughput with random I/O on NCQ-capable HDDs

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This patch is basically the counterpart, for NCQ-capable rotational
devices, of the previous patch. Exactly as the previous patch does on
flash-based devices and for any workload, this patch disables device
idling on rotational devices, but only for random I/O. In fact, only
with these queues disabling idling boosts the throughput on
NCQ-capable rotational devices. To not break service guarantees,
idling is disabled for NCQ-enabled rotational devices only when the
same symmetry conditions considered in the previous patches hold.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
---
 block/bfq-iosched.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index ca1ae24..738fd5f 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -6423,20 +6423,15 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
 	 * The next variable takes into account the cases where idling
 	 * boosts the throughput.
 	 *
-	 * The value of the variable is computed considering that
-	 * idling is usually beneficial for the throughput if:
+	 * 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 (possible
-	 *     throughput losses caused by granting idling to seeky queues
-	 *     are mitigated by the fact that, in all scenarios where
-	 *     boosting throughput is the best thing to do, i.e., in all
-	 *     symmetric scenarios, only a minimal idle time is allowed to
-	 *     seeky queues).
+	 *     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 intense I/O; rather it would lower
+	 * 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,
@@ -6444,7 +6439,8 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
 	 * device.
 	 */
 	idling_boosts_thr = !bfqd->hw_tag ||
-		(!blk_queue_nonrot(bfqd->queue) && bfq_bfqq_IO_bound(bfqq));
+		(!blk_queue_nonrot(bfqd->queue) && bfq_bfqq_IO_bound(bfqq) &&
+		 bfq_bfqq_idle_window(bfqq));
 
 	/*
 	 * The value of the next variable,
-- 
2.10.0

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[PATCH V2 00/16] Introduce the BFQ I/O scheduler Paolo Valente <paolo.valente@linaro.org> - 2017-03-31 15:00 +0200
  [PATCH V2 05/16] block, bfq: add more fairness with writes and slow processes Paolo Valente <paolo.valente@linaro.org> - 2017-03-31 15:00 +0200
  [PATCH V2 04/16] block, bfq: modify the peak-rate estimator Paolo Valente <paolo.valente@linaro.org> - 2017-03-31 15:00 +0200
    Re: [PATCH V2 04/16] block, bfq: modify the peak-rate estimator Paolo Valente <paolo.valente@linaro.org> - 2017-04-04 12:50 +0200
      Re: [PATCH V2 04/16] block, bfq: modify the peak-rate estimator Bart Van Assche <Bart.VanAssche@sandisk.com> - 2017-04-04 17:30 +0200
        Re: [PATCH V2 04/16] block, bfq: modify the peak-rate estimator Paolo Valente <paolo.valente@linaro.org> - 2017-04-06 21:40 +0200
  [PATCH V2 11/16] block, bfq: reduce idling only in symmetric scenarios Paolo Valente <paolo.valente@linaro.org> - 2017-03-31 15:00 +0200
    Re: [PATCH V2 11/16] block, bfq: reduce idling only in symmetric scenarios Paolo Valente <paolo.valente@linaro.org> - 2017-04-07 09:50 +0200
  [PATCH V2 09/16] block, bfq: reduce latency during request-pool saturation Paolo Valente <paolo.valente@linaro.org> - 2017-03-31 15:00 +0200
  [PATCH V2 13/16] block, bfq: boost the throughput with random I/O on NCQ-capable HDDs Paolo Valente <paolo.valente@linaro.org> - 2017-03-31 15:00 +0200
  [PATCH V2 07/16] block, bfq: reduce I/O latency for soft real-time applications Paolo Valente <paolo.valente@linaro.org> - 2017-03-31 15:00 +0200
  [PATCH V2 15/16] block, bfq: remove all get and put of I/O contexts Paolo Valente <paolo.valente@linaro.org> - 2017-03-31 15:00 +0200

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