cfq-iosched: remove limit of dispatch depth of max 4 times quantum

This basically limits the hardware queue depth to 4*quantum at any
point in time, which is 16 with the default settings. As CFQ uses
other means to shrink the hardware queue when necessary in the first
place, there's really no need for this extra heuristic. Additionally,
it ends up hurting performance in some cases.

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

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index adaf93a..ee8a90c 100644 (file)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1136,12 +1136,8 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
                if (cfq_class_idle(cfqq))
                        max_dispatch = 1;
 
-               if (cfqq->dispatched >= max_dispatch) {
-                       if (cfqd->busy_queues > 1)
-                               break;
-                       if (cfqq->dispatched >= 4 * max_dispatch)
-                               break;
-               }
+               if (cfqq->dispatched >= max_dispatch && cfqd->busy_queues > 1)
+                       break;
 
                if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq))
                        break;