netdev - Re: sunvnet and ->xmit

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Date:	Tue, 7 Oct 2014 15:29:22 -0400
From:	Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, david.stevens@...cle.com,
	Raghuram.Kothakota@...cle.com
Subject: Re: sunvnet and ->xmit_more

On (10/07/14 15:18), David Miller wrote:
> 
> David and others working on sunvnet, I just wanted to point out that
> in the net-next tree there is a new facility that can improve
> performance quite a bit in sunvnet.
> 
> Basically in the ->ndo_start_xmit() handler, if you see skb->xmit_more
> set then the stack is telling you that it guarentees that another
> packet will be given to you immediately when ->ndo_start_xmit()
> returns.
> 
> This means that, unless you have filled up your TX queue, you can
> defer the TX indication to the device.

I'm not sure how this can be useful to sunvnet- in sunvnet's case
we send the TX indication at the *start* of a burst, so if xmit_more
was set, sure- we can send out another packet immediately, and
avoid another START message (which we already do today), but 
nothing else to be gained from xmit_more?

BTW, I have most of the NAPI done, getting it stress-tested etc
(the recent jumbo commit added a few more races between vnet_port_remove
and vnet_start_xmit, thanks to the extra clean_timer) but I figure
I might as well fully test this internally since net-next is closed
for the moment anyway?

> 
> For example, in the virtio_net driver the test is:
> 
> 	if (__netif_subqueue_stopped(dev, qnum) || !skb->xmit_more)
> 		virtqueue_kick(sq->vq);
> 
> The pktgen module also has a new "burst" parameter you can use to test
> out this facility directly, and the qdisc layer has heuristics for
> dequeueing multiple packets at a time for normal traffic.
> 
> Just FYI...
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