[PATCH] noop-iosched: kill O(N) merge scan

Profiling hit rates on merging shows that the last merge hint works
extremely well for most work loads.  So lets kill the linear merge scan in
noop-iosched, so it provides O(1) run time for any operation.

Testing credits go to Ken Chen from Intel.
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/block/noop-iosched.c

@@ -13,34 +13,13 @@
 static int elevator_noop_merge(request_queue_t *q, struct request **req,
 			       struct bio *bio)
 {
-	struct list_head *entry = &q->queue_head;
-	struct request *__rq;
 	int ret;
 
-	if ((ret = elv_try_last_merge(q, bio))) {
+	ret = elv_try_last_merge(q, bio);
+	if (ret != ELEVATOR_NO_MERGE)
 		*req = q->last_merge;
-		return ret;
-	}
-
-	while ((entry = entry->prev) != &q->queue_head) {
-		__rq = list_entry_rq(entry);
-
-		if (__rq->flags & (REQ_SOFTBARRIER | REQ_HARDBARRIER))
-			break;
-		else if (__rq->flags & REQ_STARTED)
-			break;
 
-		if (!blk_fs_request(__rq))
-			continue;
-
-		if ((ret = elv_try_merge(__rq, bio))) {
-			*req = __rq;
-			q->last_merge = __rq;
 	return ret;
-		}
-	}
-
-	return ELEVATOR_NO_MERGE;
 }
 
 static void elevator_noop_merge_requests(request_queue_t *q, struct request *req,