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>

2 files changed, 29 insertions, 0 deletions

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index fc41d83be22b..e85c4013e8a2 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -135,6 +135,25 @@ static ssize_t queue_max_hw_sectors_show(struct request_queue *q, char *page)
         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 @@ static struct queue_sysfs_entry queue_hw_sector_size_entry = {
         .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 @@ static struct attribute *default_attrs[] = {
         &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 7253fa05db0a..ac5310ef8270 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -488,6 +488,9 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
                 }
         }
 
+        if (blk_queue_nomerges(q))
+                return ELEVATOR_NO_MERGE;
+
         /*
          * See if our hash lookup can find a potential backmerge.
          */