netdev - Re: [PATCH v2] ipv4: Early TCP socket demux.

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Date:	Wed, 20 Jun 2012 07:49:19 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	shemminger@...tta.com, netdev@...r.kernel.org
Subject: Re: [PATCH v2] ipv4: Early TCP socket demux.

On Tue, 2012-06-19 at 21:46 -0700, David Miller wrote:
> From: Stephen Hemminger <shemminger@...tta.com>
> Date: Tue, 19 Jun 2012 19:35:49 -0700
> 
> > Any benchmark numbers?
> 
> Measuring the path from ip_rcv_finish() to where we lock the socket in
> tcp_v4_rcv(), on a SPARC-T3, with a pre-warmed routing cache:
> 
> Both sk and RT lookup:	~4200 cycles
> Optimized early demux:	~2800 cycles
> 
> These numbers can be decreased further, because since we're already
> looking at the TCP header we can pre-cook the TCP control block in the
> SKB and skip much of the stuff that tcp_v4_rcv() does since we've done
> it already in the early demux code.
> 
> > I think the number of ref count operations per packet is going to be
> > the next line in the sand.
> 
> There is only one, for the socket.  We haven't taken a reference on the
> route for years.

Actually this patch makes things probably slower for :

1) routers :

Each incoming tcp packet has to perform lookups 
(ESTABLISHED and TIMEWAIT), adding one cache miss

2) small lived tcp sessions

   input dst is now dirtied because of the additional
dst_clone()/dst_release()


1) can be solved using a knob as suggested by Changli, possibly using a
JUMP_LABEL shadowing ip_forward ?


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