netdev - Re: Network performance with small packets

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Date:	Wed, 09 Mar 2011 07:45:43 -0800
From:	Shirley Ma <mashirle@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Krishna Kumar2 <krkumar2@...ibm.com>,
	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	netdev@...r.kernel.org, steved@...ibm.com,
	Tom Lendacky <tahm@...ux.vnet.ibm.com>
Subject: Re: Network performance with small packets - continued

On Wed, 2011-03-09 at 09:15 +0200, Michael S. Tsirkin wrote:
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 82dba5a..ebe3337 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -514,11 +514,11 @@ static unsigned int free_old_xmit_skbs(struct
> virtnet_info *vi)
>         struct sk_buff *skb;
>         unsigned int len, tot_sgs = 0;
> 
> -       while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
> +       if ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
>                 pr_debug("Sent skb %p\n", skb);
>                 vi->dev->stats.tx_bytes += skb->len;
>                 vi->dev->stats.tx_packets++;
> -               tot_sgs += skb_vnet_hdr(skb)->num_sg;
> +               tot_sgs = 2+MAX_SKB_FRAGS;
>                 dev_kfree_skb_any(skb);
>         }
>         return tot_sgs;

Return value should be different based on indirect or direct buffers
here?

> @@ -576,9 +576,6 @@ static netdev_tx_t start_xmit(struct sk_buff *skb,
> struct net_device *dev)
>         struct virtnet_info *vi = netdev_priv(dev);
>         int capacity;
> 
> -       /* Free up any pending old buffers before queueing new ones.
> */
> -       free_old_xmit_skbs(vi);
> -
>         /* Try to transmit */
>         capacity = xmit_skb(vi, skb);
> 
> @@ -605,6 +602,10 @@ static netdev_tx_t start_xmit(struct sk_buff
> *skb, struct net_device *dev)
>         skb_orphan(skb);
>         nf_reset(skb);
> 
> +       /* Free up any old buffers so we can queue new ones. */
> +       if (capacity < 2+MAX_SKB_FRAGS)
> +               capacity += free_old_xmit_skbs(vi);
> +
>         /* Apparently nice girls don't return TX_BUSY; stop the queue
>          * before it gets out of hand.  Naturally, this wastes
> entries. */
>         if (capacity < 2+MAX_SKB_FRAGS) { 

I tried a similar patch before, it didn't help much on TCP stream
performance. But I didn't try multiple stream TCP_RR.

Shirley

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