cfq-iosched: give busy sync queue no dispatch limit

If there are a sync and an async queue and the sync queue's think time is small, we can ignore the sync queue's dispatch quantum. Because the sync queue will always preempt the async queue, we don't need to care about async's latency. This can fix a performance regression of aiostress test, which is introduced by commit f8ae6e3. The issue should exist even without the commit, but the commit amplifies the impact. The initial post does the same optimization for RT queue too, but since I have no real workload for it, Vivek suggests to drop it. Signed-off-by: Shaohua Li <[email protected]> Reviewed-by: Gui Jianfeng <[email protected]> Signed-off-by: Jens Axboe <[email protected]>
CamonZ · Mar 7, 2011 · ef8a41d · ef8a41d
1 parent 93803e0
commit ef8a41d
Showing 1 changed file with 24 additions and 2 deletions.
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
@@ -237,6 +237,7 @@ struct cfq_data {
 	struct rb_root prio_trees[CFQ_PRIO_LISTS];
 
 	unsigned int busy_queues;
+	unsigned int busy_sync_queues;
 
 	int rq_in_driver;
 	int rq_in_flight[2];
@@ -1344,6 +1345,8 @@ static void cfq_add_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	BUG_ON(cfq_cfqq_on_rr(cfqq));
 	cfq_mark_cfqq_on_rr(cfqq);
 	cfqd->busy_queues++;
+	if (cfq_cfqq_sync(cfqq))
+		cfqd->busy_sync_queues++;
 
 	cfq_resort_rr_list(cfqd, cfqq);
 }
@@ -1370,6 +1373,8 @@ static void cfq_del_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	cfq_group_service_tree_del(cfqd, cfqq->cfqg);
 	BUG_ON(!cfqd->busy_queues);
 	cfqd->busy_queues--;
+	if (cfq_cfqq_sync(cfqq))
+		cfqd->busy_sync_queues--;
 }
 
 /*
@@ -2377,22 +2382,39 @@ static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	 * Does this cfqq already have too much IO in flight?
 	 */
 	if (cfqq->dispatched >= max_dispatch) {
+		bool promote_sync = false;
 		/*
 		 * idle queue must always only have a single IO in flight
 		 */
 		if (cfq_class_idle(cfqq))
 			return false;
 
+		/*
+		 * If there is only one sync queue, and its think time is
+		 * small, we can ignore async queue here and give the sync
+		 * queue no dispatch limit. The reason is a sync queue can
+		 * preempt async queue, limiting the sync queue doesn't make
+		 * sense. This is useful for aiostress test.
+		 */
+		if (cfq_cfqq_sync(cfqq) && cfqd->busy_sync_queues == 1) {
+			struct cfq_io_context *cic = RQ_CIC(cfqq->next_rq);
+
+			if (sample_valid(cic->ttime_samples) &&
+				cic->ttime_mean < cfqd->cfq_slice_idle)
+				promote_sync = true;
+		}
+
 		/*
 		 * We have other queues, don't allow more IO from this one
 		 */
-		if (cfqd->busy_queues > 1 && cfq_slice_used_soon(cfqd, cfqq))
+		if (cfqd->busy_queues > 1 && cfq_slice_used_soon(cfqd, cfqq) &&
+				!promote_sync)
 			return false;
 
 		/*
 		 * Sole queue user, no limit
 		 */
-		if (cfqd->busy_queues == 1)
+		if (cfqd->busy_queues == 1 || promote_sync)
 			max_dispatch = -1;
 		else
 			/*