linux-kernel - Re: [PATCH] virtio-ring: Use threshold for switching to indirect descriptors

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Date:	Sun, 04 Dec 2011 20:23:59 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	markmc@...hat.com
Subject: Re: [PATCH] virtio-ring: Use threshold for switching to indirect
 descriptors

On Sun, 2011-12-04 at 19:39 +0200, Sasha Levin wrote:
> On Sun, 2011-12-04 at 19:37 +0200, Avi Kivity wrote:
> > On 12/04/2011 07:34 PM, Sasha Levin wrote:
> > > > 
> > > > I'm confused. didn't you see a bigger benefit for guest->host by
> > > > switching indirect off?
> > >
> > > The 5% improvement is over the 'regular' indirect on, not over indirect
> > > off. Sorry for the confusion there.
> > >
> > > I suggested this change regardless of the outcome of indirect descriptor
> > > threshold discussion, since it would help anyways.
> > 
> > For net, this makes sense.  For block, it reduces the effective queue
> > depth, so it's not a trivial change.  It probably makes sense there too,
> > though.
> 
> It doesn't have to be limited at that number, anything above that can go
> through the regular kmalloc() path.

Something like the following patch:

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c7a2c20..3166ca0 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -82,6 +82,7 @@ struct vring_virtqueue
 
 	/* Host supports indirect buffers */
 	bool indirect;
+	struct kmem_cache *indirect_cache;
 
 	/* Host publishes avail event idx */
 	bool event;
@@ -110,6 +111,9 @@ struct vring_virtqueue
 
 #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
 
+static unsigned int ind_alloc_thresh = 0;
+module_param(ind_alloc_thresh, uint, S_IRUGO);
+
 /* Set up an indirect table of descriptors and add it to the queue. */
 static int vring_add_indirect(struct vring_virtqueue *vq,
 			      struct scatterlist sg[],
@@ -121,7 +125,10 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
 	unsigned head;
 	int i;
 
-	desc = kmalloc((out + in) * sizeof(struct vring_desc), gfp);
+	if ((out + in) <= ind_alloc_thresh)
+		desc = kmem_cache_alloc(vq->indirect_cache, gfp);
+	else
+		desc = kmalloc((out + in) * sizeof(struct vring_desc), gfp);
 	if (!desc)
 		return -ENOMEM;
 
@@ -479,6 +486,9 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
 	vq->broken = false;
 	vq->last_used_idx = 0;
 	vq->num_added = 0;
+	if (ind_alloc_thresh)
+		vq->indirect_cache = KMEM_CACHE(vring_desc[ind_alloc_thresh], 0);
 	list_add_tail(&vq->vq.list, &vdev->vqs);
 #ifdef DEBUG
 	vq->in_use = false;

-- 

Sasha.

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