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

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Date:	Wed, 7 Dec 2011 17:48:17 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Avi Kivity <avi@...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 Wed, Dec 07, 2011 at 04:02:45PM +0200, Sasha Levin wrote:
> On Sun, 2011-12-04 at 20:23 +0200, Sasha Levin wrote:
> 
> [snip]
> 
> Rusty, Michael, does the below looks a reasonable optimization for you?

OK overall but a bit hard to say for sure as it looks pretty incomplete ...

> should I send it as a patch?

What's the performance gain?

> > 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);

0 will have no effect?

> > +
> >  /* 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[],

A global parameter is OK for testing but likely not what we want
in real life. This needs to be different per device.



> > @@ -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;
> >  

free unaffected?

> > @@ -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);

and need to cleanup too?

> >  	list_add_tail(&vq->vq.list, &vdev->vqs);
> >  #ifdef DEBUG
> >  	vq->in_use = false;
> > 
> 
> -- 
> 
> Sasha.
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