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>
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index fc41d83..e85c401 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -135,6 +135,25 @@
return queue_var_show(max_hw_sectors_kb, (page));
}
+static ssize_t queue_nomerges_show(struct request_queue *q, char *page)
+{
+ return queue_var_show(blk_queue_nomerges(q), page);
+}
+
+static ssize_t queue_nomerges_store(struct request_queue *q, const char *page,
+ size_t count)
+{
+ unsigned long nm;
+ ssize_t ret = queue_var_store(&nm, page, count);
+
+ if (nm)
+ set_bit(QUEUE_FLAG_NOMERGES, &q->queue_flags);
+ else
+ clear_bit(QUEUE_FLAG_NOMERGES, &q->queue_flags);
+
+ return ret;
+}
+
static struct queue_sysfs_entry queue_requests_entry = {
.attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
@@ -170,6 +189,12 @@
.show = queue_hw_sector_size_show,
};
+static struct queue_sysfs_entry queue_nomerges_entry = {
+ .attr = {.name = "nomerges", .mode = S_IRUGO | S_IWUSR },
+ .show = queue_nomerges_show,
+ .store = queue_nomerges_store,
+};
+
static struct attribute *default_attrs[] = {
&queue_requests_entry.attr,
&queue_ra_entry.attr,
@@ -177,6 +202,7 @@
&queue_max_sectors_entry.attr,
&queue_iosched_entry.attr,
&queue_hw_sector_size_entry.attr,
+ &queue_nomerges_entry.attr,
NULL,
};
diff --git a/block/elevator.c b/block/elevator.c
index 7253fa0..ac5310e 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -488,6 +488,9 @@
}
}
+ if (blk_queue_nomerges(q))
+ return ELEVATOR_NO_MERGE;
+
/*
* See if our hash lookup can find a potential backmerge.
*/
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 08df1ea..c09696a 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -408,6 +408,7 @@
#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 @@
#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)