block: Added in stricter no merge semantics for block I/O

Updated 'nomerges' tunable to accept a value of '2' - indicating that _no_
merges at all are to be attempted (not even the simple one-hit cache).

The following table illustrates the additional benefit - 5 minute runs of
a random I/O load were applied to a dozen devices on a 16-way x86_64 system.

nomerges        Throughput      %System         Improvement (tput / %sys)
--------        ------------    -----------     -------------------------
0               12.45 MB/sec    0.669365609
1               12.50 MB/sec    0.641519199     0.40% / 2.71%
2               12.52 MB/sec    0.639849750     0.56% / 2.96%

Signed-off-by: Alan D. Brunelle <alan.brunelle@hp.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/block/elevator.c b/block/elevator.c
index 9ad5ccc4c5ee..ee3a883840f2 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -474,6 +474,15 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
         int ret;
 
         /*
+         * Levels of merges:
+         *      nomerges:  No merges at all attempted
+         *      noxmerges: Only simple one-hit cache try
+         *      merges:    All merge tries attempted
+         */
+        if (blk_queue_nomerges(q))
+                return ELEVATOR_NO_MERGE;
+
+        /*
          * First try one-hit cache.
          */
         if (q->last_merge) {
@@ -484,7 +493,7 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
                 }
         }
 
-        if (blk_queue_nomerges(q))
+        if (blk_queue_noxmerges(q))
                 return ELEVATOR_NO_MERGE;
 
         /*