netdev - Re: [RFC] New driver API to speed up small packets xmits

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Date:	Fri, 11 May 2007 07:27:07 -0400
From:	jamal <hadi@...erus.ca>
To:	Krishna Kumar2 <krkumar2@...ibm.com>
Cc:	Gagan Arneja <gaagaan@...il.com>,
	Evgeniy Polyakov <johnpol@....mipt.ru>, netdev@...r.kernel.org,
	Rick Jones <rick.jones2@...com>,
	Sridhar Samudrala <sri@...IBM.COM>
Subject: Re: [RFC] New driver API to speed up small packets xmits

On Fri, 2007-11-05 at 10:52 +0530, Krishna Kumar2 wrote:

> I didn't try to optimize the driver to take any real advantage, I coded it
> as simply as :
> 
> top:
>       next = skb->skb_flink;
>       Original driver code here, or another option is to remove the locking
>             and put it before the "top:" above, no other changes.
>       skb = next;
>       if (skb)
>             goto top;
> This way, the diffs are minimal. However, driver experts might know of some
> tricks to optimize this, since most drivers have a hundred checks before
> sending one skb, and whether some optimization to avoid that for each skb
> is possible.
> 

You will need to at some point, no big deal in order to get things in
place first- the simple example of DMA i talked about yesterday is a
simple example. i.e you avoid kicking the DMA until you have queued all
in the ring (as Gagan pointed out, kicking the DMA is a PCI IO write)
I am sure there are gazillion others.
 

> This is a good idea, I will look at your slides to see what this exactly
> is.

I will try to post my patches updated to the latest net-2.6 kernel
before you come back on monday.

> However, even with this check, drivers will return error, eg trylock failed
> or, like the e1000 has - fifo_workaround failures. But I still think it
> will significantly reduce the failures. 

Yes, this is what i refered to as the "many tunables" and led to the
claim that you may have to re-write the whole tx path in my posting.

> Right now, I get a lot of requeue's.

requeues are a good sign ;->

cheers,
jamal

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