netdev - Re: [PATCH] ucc_geth: Add support for skb recycling

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Date:	Wed, 8 Jul 2009 00:37:12 +0400
From:	Anton Vorontsov <avorontsov@...mvista.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	David Miller <davem@...emloft.net>,
	Andy Fleming <afleming@...escale.com>,
	Li Yang <leoli@...escale.com>, netdev@...r.kernel.org,
	linuxppc-dev@...abs.org, Lennert Buytenhek <buytenh@...vell.com>
Subject: Re: [PATCH] ucc_geth: Add support for skb recycling

On Tue, Jul 07, 2009 at 11:47:38AM -0700, Rick Jones wrote:
> Anton Vorontsov wrote:
> >We can reclaim transmitted skbs to use in the receive path, so-called
> >skb recycling support.
> >
> >Also reorder ucc_geth_poll() steps, so that we'll clean tx ring firstly,
> >thus maybe reclaim some skbs for rx.
> 
> Admittedly, all the world is not TCP, but a big chunk is, so are you
> likely to have reference counts go to zero on the tx queue for
> anything other than small standalone TCP ACK segments?

That's a generic question wrt skb recycling, right? Whether we can
always recycle transmitted skbs. No, sometimes (or mostly) we can't.

Initially, I was quite puzzled by this support... looking at how
gianfar driver works (it has the same support as of 0fd56bb5be6455d0),
I noticed that skb_recycle_check() always returns 0, and so we
don't recycle the skbs.

Though, things change when the kernel starts packets forwarding,
*then* skb recycling path actually triggers.

Lennert (skb recycling author) hints us that the gain is indeed
in forwarding/routing workload:

http://kerneltrap.org/mailarchive/linux-netdev/2008/9/28/3433514

Hope I understood everything correctly. :-)

Thanks!

-- 
Anton Vorontsov
email: cbouatmailru@...il.com
irc://irc.freenode.net/bd2
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