block, bfq: boost the throughput with random I/O on NCQ-capable HDDs

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>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 6 insertions, 10 deletions

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 208178478dc4..549f030509ef 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -6439,20 +6439,15 @@ 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,
@@ -6460,7 +6455,8 @@ 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,