[PATCH] virtio-blk: make the queue depth configurable

["Theodore Ts'o" <tytso@mit.edu>]

The current virtio block sets a queue depth of 64.  With a
sufficiently fast device, using a queue depth of 256 can double the
IOPS which can be sustained.  So make the queue depth something which
can be set at module load time or via a kernel boot-time parameter.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: virtio-dev@lists.oasis-open.org
Cc: virtualization@lists.linux-foundation.org
Cc: fes@google.com
---
 drivers/block/virtio_blk.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 6a680d4..0c9e57f 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -481,6 +481,9 @@ static struct blk_mq_ops virtio_mq_ops = {
 	.free_hctx	= blk_mq_free_single_hw_queue,
 };
 
+static int queue_depth = 64;
+module_param(queue_depth, int, 444);
+
 static struct blk_mq_reg virtio_mq_reg = {
 	.ops		= &virtio_mq_ops,
 	.nr_hw_queues	= 1,
@@ -551,6 +554,7 @@ static int virtblk_probe(struct virtio_device *vdev)
 		goto out_free_vq;
 	}
 
+	virtio_mq_reg.queue_depth = queue_depth;
 	virtio_mq_reg.cmd_size =
 		sizeof(struct virtblk_req) +
 		sizeof(struct scatterlist) * sg_elems;
-- 
1.9.0