Subject: Re: [Xen-devel] [PATCHv1] xen-netfront: always keep the Rx ring full of requests

From: annie li <[email-masked]>
Date: Mon, 06 Oct 2014 11:35:46 -0400

On 2014/10/2 9:33, David Vrabel wrote:
> A full Rx ring only requires 1 MiB of memory.  This is not enough
> memory that it is useful to dynamically scale the number of Rx
> requests in the ring based on traffic rates.
>
> Keeping the ring full of Rx requests handles bursty traffic better
> than trying to converges on an optimal number of requests to keep
> filled.
>
> On a 4 core host, an iperf -P 64 -t 60 run from dom0 to a 4 VCPU guest
> improved from 5.1 Gbit/s to 5.6 Gbit/s.  Gains with more bursty
> traffic are expected to be higher.

Although removing sysfs is connected with the code change for full Rx 
ring utilization, I assume it is better to split this patch into two to 
make it simpler?

  ...snip...
>   
> +	queue->rx.req_prod_pvt = req_prod;
> +
> +	/* Not enough requests? Try again later. */
> +	if (req_prod - queue->rx.rsp_cons < NET_RX_SLOTS_MIN) {
> +		mod_timer(&queue->rx_refill_timer, jiffies + (HZ/10));
> +		return;

If the previous for loop breaks because of failure of 
xennet_alloc_one_rx_buffer, then notify_remote_via_irq is missed here if 
the code returns directly.

Thanks
Annie
> +	}
> +