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

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Date:	Wed, 19 Mar 2014 16:58:29 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	tytso@....edu
Cc:	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	Venkatesh Srinivas <venkateshs@...gle.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	virtio-dev@...ts.oasis-open.org,
	virtualization@...ts.linux-foundation.org,
	Frank Swiderski <fes@...gle.com>
Subject: Re: [PATCH] virtio-blk: make the queue depth the max supportable by the hypervisor

tytso@....edu 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
VIRTIO_RING_F_INDIRECT_DESC into account.

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
VIRTIO_RING_F_INDIRECT_DESC.

Inspired-by: "Theodore Ts'o" <tytso@....edu>
Based-on-the-true-story-of: Venkatesh Srinivas <venkateshs@...gle.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>
Cc: virtio-dev@...ts.oasis-open.org
Cc: virtualization@...ts.linux-foundation.org
Cc: Frank Swiderski <fes@...gle.com>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>

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,
 	.flags		= BLK_MQ_F_SHOULD_MERGE,
 };
+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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