Upon an I/O-dispatch attempt, BFQ may detect that it was better to
plug I/O dispatch, and to wait for a new request to arrive for the
currently in-service queue. But the arrival of a new request for an
empty bfq_queue, and thus the switch from idle to busy of the
bfq_queue, may cause the scenario to change, and make plugging no
longer needed for service guarantees, or more convenient for
throughput. In this case, keeping I/O-dispatch plugged would certainly
lower throughput.
To address this issue, this commit makes such a check, and stops
plugging I/O if it is better to stop plugging I/O.
Tested-by: Jan Kara <j...@suse.cz>
Signed-off-by: Paolo Valente <paolo.vale...@linaro.org>
---
block/bfq-iosched.c | 24 +++++++++++++++++++-----
1 file changed, 19 insertions(+), 5 deletions(-)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index db393f5d70ba..6a02a12ff553 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -1649,6 +1649,8 @@ static bool bfq_bfqq_higher_class_or_weight(struct
bfq_queue *bfqq,
return bfqq_weight > in_serv_weight;
}
+static bool bfq_better_to_idle(struct bfq_queue *bfqq);
+
static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
struct bfq_queue *bfqq,
int old_wr_coeff,
@@ -1750,10 +1752,10 @@ static void bfq_bfqq_handle_idle_busy_switch(struct
bfq_data *bfqd,
bfq_add_bfqq_busy(bfqd, bfqq);
/*
- * Expire in-service queue only if preemption may be needed
- * for guarantees. In particular, we care only about two
- * cases. The first is that bfqq has to recover a service
- * hole, as explained in the comments on
+ * Expire in-service queue if preemption may be needed for
+ * guarantees or throughput. As for guarantees, we care
+ * explicitly about two cases. The first is that bfqq has to
+ * recover a service hole, as explained in the comments on
* bfq_bfqq_update_budg_for_activation(), i.e., that
* bfqq_wants_to_preempt is true. However, if bfqq does not
* carry time-critical I/O, then bfqq's bandwidth is less
@@ -1780,11 +1782,23 @@ static void bfq_bfqq_handle_idle_busy_switch(struct
bfq_data *bfqd,
* timestamps of the in-service queue would need to be
* updated, and this operation is quite costly (see the
* comments on bfq_bfqq_update_budg_for_activation()).
+ *
+ * As for throughput, we ask bfq_better_to_idle() whether we
+ * still need to plug I/O dispatching. If bfq_better_to_idle()
+ * says no, then plugging is not needed any longer, either to
+ * boost throughput or to perserve service guarantees. Then
+ * the best option is to stop plugging I/O, as not doing so
+ * would certainly lower throughput. We may end up in this
+ * case if: (1) upon a dispatch attempt, we detected that it
+ * was better to plug I/O dispatch, and to wait for a new
+ * request to arrive for the currently in-service queue, but
+ * (2) this switch of bfqq to busy changes the scenario.
*/
if (bfqd->in_service_queue &&
((bfqq_wants_to_preempt &&
bfqq->wr_coeff >= bfqd->in_service_queue->wr_coeff) ||
- bfq_bfqq_higher_class_or_weight(bfqq, bfqd->in_service_queue)) &&
+ bfq_bfqq_higher_class_or_weight(bfqq, bfqd->in_service_queue) ||
+ !bfq_better_to_idle(bfqd->in_service_queue)) &&
next_queue_may_preempt(bfqd))
bfq_bfqq_expire(bfqd, bfqd->in_service_queue,
false, BFQQE_PREEMPTED);
--
2.20.1
