block: Skip I/O merges when disabled

The block I/O + elevator + I/O scheduler code spend a lot of time trying
to merge I/Os -- rightfully so under "normal" circumstances. However,
if one were to know that the incoming I/O stream was /very/ random in
nature, the cycles are wasted.

This patch adds a per-request_queue tunable that (when set) disables
merge attempts (beyond the simple one-hit cache check), thus freeing up
a non-trivial amount of CPU cycles.

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

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 08df1ea8bac4..c09696a90d6a 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -408,6 +408,7 @@ struct request_queue
 #define QUEUE_FLAG_PLUGGED      7       /* queue is plugged */
 #define QUEUE_FLAG_ELVSWITCH    8       /* don't use elevator, just do FIFO */
 #define QUEUE_FLAG_BIDI         9       /* queue supports bidi requests */
+#define QUEUE_FLAG_NOMERGES    10       /* disable merge attempts */
 
 static inline void queue_flag_set_unlocked(unsigned int flag,
                                            struct request_queue *q)
@@ -476,6 +477,7 @@ enum {
 #define blk_queue_plugged(q)    test_bit(QUEUE_FLAG_PLUGGED, &(q)->queue_flags)
 #define blk_queue_tagged(q)     test_bit(QUEUE_FLAG_QUEUED, &(q)->queue_flags)
 #define blk_queue_stopped(q)    test_bit(QUEUE_FLAG_STOPPED, &(q)->queue_flags)
+#define blk_queue_nomerges(q)   test_bit(QUEUE_FLAG_NOMERGES, &(q)->queue_flags)
 #define blk_queue_flushing(q)   ((q)->ordseq)
 
 #define blk_fs_request(rq)      ((rq)->cmd_type == REQ_TYPE_FS)