[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>

diff --git a/drivers/block/noop-iosched.c b/drivers/block/noop-iosched.c
index 888c477e02b314b75b0b06d3658b784bc7cf9d28..b1730b62c37e4793a2401df975680c1231903d16 100644 (file)
--- a/drivers/block/noop-iosched.c
+++ b/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;
+       return ret;
 }
 
 static void elevator_noop_merge_requests(request_queue_t *q, struct request *req,