Re: [PATCH] virtio-blk: make the queue depth the max supportable by the hypervisor

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On March 14, 2014 11:34:31 PM EDT, Theodore Ts'o <tytso@mit.edu> wrote:
>The current virtio block sets a queue depth of 64, which is
>insufficient for very fast devices.  It has been demonstrated that
>with a high IOPS device, using a queue depth of 256 can double the
>IOPS which can be sustained.
>
>As suggested by Venkatash Srinivas, set the queue depth by default to
>be one half the the device's virtqueue, which is the maximum queue
>depth that can be supported by the channel to the host OS (each I/O
>request requires at least two VQ entries).
>
>Also allow the queue depth to be something which can be set at module
>load time or via a kernel boot-time parameter, for
>testing/benchmarking purposes.
>
>Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
>Signed-off-by: Venkatesh Srinivas <venkateshs@google.com>
>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: Frank Swiderski <fes@google.com>
>---
>
>This is a combination of my patch and Vekatash's patch.  I agree that
>setting the default automatically is better than requiring the user to
>set the value by hand.
>
> drivers/block/virtio_blk.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
>diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>index 6a680d4..0f70c01 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 = -1;
>+module_param(queue_depth, int, 0444);

?

>+
> static struct blk_mq_reg virtio_mq_reg = {
> 	.ops		= &virtio_mq_ops,
> 	.nr_hw_queues	= 1,
>@@ -551,9 +554,14 @@ static int virtblk_probe(struct virtio_device
>*vdev)
> 		goto out_free_vq;
> 	}
> 
>+	virtio_mq_reg.queue_depth = queue_depth > 0 ? queue_depth :
>+		(vblk->vq->num_free / 2);
> 	virtio_mq_reg.cmd_size =
> 		sizeof(struct virtblk_req) +
> 		sizeof(struct scatterlist) * sg_elems;
>+	virtblk_name_format("vd", index, vblk->disk->disk_name,
>DISK_NAME_LEN);
>+	pr_info("%s: using queue depth %d\n", vblk->disk->disk_name,
>+		virtio_mq_reg.queue_depth);

Isn't that visible from sysfs?
> 
> 	q = vblk->disk->queue = blk_mq_init_queue(&virtio_mq_reg, vblk);
> 	if (!q) {
>@@ -565,8 +573,6 @@ static int virtblk_probe(struct virtio_device
>*vdev)
> 
> 	q->queuedata = vblk;
> 
>-	virtblk_name_format("vd", index, vblk->disk->disk_name,
>DISK_NAME_LEN);
>-
> 	vblk->disk->major = major;
> 	vblk->disk->first_minor = index_to_minor(index);
> 	vblk->disk->private_data = vblk;