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SubjectRe: [PATCH] virtio-blk: make the queue depth the max supportable by the hypervisor
Date writes:
> On Mon, Mar 17, 2014 at 11:12:15AM +1030, Rusty Russell wrote:
>> Note that with indirect descriptors (which is supported by Almost
>> Everyone), we can actually use the full index, so this value is a bit
>> pessimistic. But it's OK as a starting point.
> So is this something that can go upstream with perhaps a slight
> adjustment in the commit description?

Well, I rewrote it again, see below.

> Do you think we need to be able
> to dynamically adjust the queue depth after the module has been loaded
> or the kernel has been booted?

That would be nice, sure, but...

> If so, anyone a hint about the best
> way to do that would be much appreciated.

... I share your wonder and mystery at the ways of the block layer.

Subject: virtio-blk: base queue-depth on virtqueue ringsize or module param

Venkatash spake thus:

virtio-blk set the default queue depth to 64 requests, which was
insufficient for high-IOPS devices. Instead set the blk-queue depth to
the device's virtqueue depth divided by two (each I/O requires at least
two VQ entries).

But behold, Ted added a module parameter:

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.

And I rewrote it substantially, mainly to take

As QEMU sets the vq size for PCI to 128, Venkatash's patch wouldn't
have made a change. This version does (since QEMU also offers

Inspired-by: "Theodore Ts'o" <>
Based-on-the-true-story-of: Venkatesh Srinivas <>
Cc: "Michael S. Tsirkin" <>
Cc: Frank Swiderski <>
Signed-off-by: Rusty Russell <>

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index a2db9ed288f2..c101bbc72095 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -491,10 +491,11 @@ static struct blk_mq_ops virtio_mq_ops = {
static struct blk_mq_reg virtio_mq_reg = {
.ops = &virtio_mq_ops,
.nr_hw_queues = 1,
- .queue_depth = 64,
+ .queue_depth = 0, /* Set in virtblk_probe */
.numa_node = NUMA_NO_NODE,
+module_param_named(queue_depth, virtio_mq_reg.queue_depth, uint, 0444);

static void virtblk_init_vbr(void *data, struct blk_mq_hw_ctx *hctx,
struct request *rq, unsigned int nr)
@@ -558,6 +559,13 @@ static int virtblk_probe(struct virtio_device *vdev)
goto out_free_vq;

+ /* Default queue sizing is to fill the ring. */
+ if (!virtio_mq_reg.queue_depth) {
+ virtio_mq_reg.queue_depth = vblk->vq->num_free;
+ /* ... but without indirect descs, we use 2 descs per req */
+ if (!virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC))
+ virtio_mq_reg.queue_depth /= 2;
+ }
virtio_mq_reg.cmd_size =
sizeof(struct virtblk_req) +
sizeof(struct scatterlist) * sg_elems;

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