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

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Date:	Fri, 11 May 2007 10:52:16 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	hadi@...erus.ca
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

J Hadi Salim <j.hadi123@...il.com> wrote on 05/11/2007 01:41:27 AM:

> > It's not just small packets. The cost of calling hard_start_xmit/byte
> > was rather high on your particular device. I've seen  PCI read
> > transaction in hard_start_xmit taking ~10,000 cycles on one particular
> > device. Count the cycles your brand of NIC is taking in it's
> > xmit_routine. The worse it is, the stronger your case for cluster
> > transmits.
>
> You would need to almost re-write the driver to make sure it does IO
> which is taking advantage of the batching.

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 could avoid totaly requeueing by asking the driver how much space it
> has. Once you shove down to it a number of packets, you are then
> guaranteed they will never be requeued (refer to the slides i pointed to
> earlier).

This is a good idea, I will look at your slides to see what this exactly
is.
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. Right now, I get a lot of requeue's.

thanks,

- KK

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