netdev - Re: [PATCH] [sis900] convert to NAPI, WAS Re: pktgen terminating condition

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Date:	Tue, 4 Sep 2007 10:21:08 -0700
From:	"Mandeep Baines" <mandeep.baines@...il.com>
To:	hadi@...erus.ca
Cc:	"Daniele Venzano" <venza@...wnhat.org>, davem@...emloft.net,
	rick.jones2@...com, msb@...gle.com, netdev@...r.kernel.org,
	grundler@...gle.com, robert.olsson@....uu.se, jeff@...zik.org,
	nhorman@...driver.com
Subject: Re: [PATCH] [sis900] convert to NAPI, WAS Re: pktgen terminating condition

On 9/4/07, jamal <hadi@...erus.ca> wrote:
> On Mon, 2007-03-09 at 20:20 -0700, Mandeep Singh Baines wrote:
>
> > I didn't see much saving in interrupts on my machine (too fast, I guess).
>
> You could try the idea suggested by Dave earlier and just turn interupts
> for every nth packet. That should cut down the numbers.
>

I wanted to do that but I couldn't figure out how to free the last skb
if it happened to be a transmit that didn't generate a tx completion
interrupt.

Let's say I interrupt every 4th packet.I've already sent packets 0 to
4. Now I send packets 5 and 6. I can't figure out how to ensure that
the skb's for these packets get freed in a deterministic amount of
time if there is no interrupt? They will get freed when packet 8 gets
transmitted but there is no upper bound on when that will happen.

Maybe I could create a tx skb cleanup timer that I kickoff in
hard_start_xmit(). Every call to hard_start_xmit would reset the
timer. I could try this for a future patch. Not sure if the extra code
would cost me more than I get back in savings.

> > I did see a significant boost to tx performance by optimizing start_xmit: more
> > than double pps in pktgen.
>
> 148Kpps on a slow piece of hardware aint bad - Good Stuff. I wonder how
> much CPU is being abused.
>
> If you wanna go one extra mile (separate future patch): get rid of that
> tx lock and use netif_tx_lock on the interupt path. Look at some sane
> driver like tg3 for reference.
>

Cool. I'll check out the tg3.
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