netdev - Re: [PATCH 1/4] net: skb_orphan on dev_hard_start

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Message-Id: <200906041324.59118.rusty@rustcorp.com.au>
Date:	Thu, 4 Jun 2009 13:24:57 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	Divy Le Ray <divy@...lsio.com>,
	Roland Dreier <rolandd@...co.com>,
	Pavel Emelianov <xemul@...nvz.org>,
	Dan Williams <dcbw@...hat.com>,
	libertas-dev@...ts.infradead.org
Subject: Re: [PATCH 1/4] net: skb_orphan on dev_hard_start_xmit

On Thu, 4 Jun 2009 06:32:53 am Eric Dumazet wrote:
> Rusty Russell a écrit :
> > On Sat, 30 May 2009 12:41:00 am Eric Dumazet wrote:
> >> For TCP, I agree your patch is a huge benefit, since its paced by remote
> >> ACKS and window control
> >
> > I doubt that.  There'll be some cache friendliness, but I'm not sure
> > it'll be measurable, let alone "huge".
...
> We can see sock_wfree() being number 2 on the profile, because it touches
> three cache lines per socket and transmited packet in TX completion
> handler.

Interesting, I take it back: got some "after" stats as well?

> Also, taking a reference on socket for each xmit packet in flight is very
> expensive, since it slows down receiver in __udp4_lib_lookup(). Several
> cpus are fighting for sk->refcnt cache line.

Now we have decent dynamic per-cpu, we can finally implement bigrefs.  More 
obvious for device counts than sockets, but perhaps applicable here as well?

Rusty.
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