From: Yu Kuai <yukuai3@xxxxxxxxxx>
The maximal default nr_requests is 256, and if disk can handle more than
256 requests concurrently, use elevator in this case is useless, on the
one hand it limits the number of requests to 256, on the other hand,
it can't merge or sort IO because requests are dispatched to disk
immediately and the elevator is just empty.
For example, for nvme megaraid with 512 queue_depth by default, we have
to change default elevator to none, otherwise deadline will lose a lot of
performance.
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
block/elevator.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/block/elevator.c b/block/elevator.c
index 7c3ba80e5ff4..4cce1e7c47d5 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -568,6 +568,17 @@ static struct elevator_type *elevator_get_default(struct request_queue *q)
!blk_mq_is_shared_tags(q->tag_set->flags))
return NULL;
+ /*
+ * If nr_queues will be less than disk ability, requests will be
+ * dispatched to disk immediately, it's useless to use elevator. User
+ * should set a bigger nr_requests or limit disk ability manually if
+ * they really want to use elevator.
+ */
+ if (q->queue_depth && q->queue_depth >= BLKDEV_DEFAULT_RQ * 2)
+ return NULL;
+ if (!q->queue_depth && q->tag_set->queue_depth >= BLKDEV_DEFAULT_RQ * 2)
+ return NULL;
+
return elevator_find_get("mq-deadline");
}
--
2.39.2
