[PATCH] queue congestion threshold hysteresis

We need to open the gap between congestion on/off a little bit, or we
risk burning many cycles continually putting processes on a wait queue
only to wake them up again immediately. This was observed with CFQ at
least, which showed way excessive sys time.

Patch is from Arjan.
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/block/ll_rw_blk.c

@@ -100,7 +100,7 @@ static void blk_queue_congestion_threshold(struct request_queue *q)
 		nr = q->nr_requests;
 	q->nr_congestion_on = nr;
 
-	nr = q->nr_requests - (q->nr_requests / 8) - 1;
+	nr = q->nr_requests - (q->nr_requests / 8) - (q->nr_requests / 16) - 1;
 	if (nr < 1)
 		nr = 1;
 	q->nr_congestion_off = nr;