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

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Date:	Wed, 07 Dec 2011 16:02:45 +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 20:23 +0200, Sasha Levin wrote:

[snip]

Rusty, Michael, does the below looks a reasonable optimization for you?
should I send it as a patch?

> 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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