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

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Date:	Wed, 20 Jun 2012 14:26:52 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	shemminger@...tta.com, bhutchings@...arflare.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH v2] ipv4: Early TCP socket demux.

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 20 Jun 2012 23:17:56 +0200

> In most routers setups I used, I had to disable GRO, because 64Kbytes
> packets on output path broke the tc setups (SFQ)

Then you speak of bugs and mis-features, rather than real fundamental
disadvantages of using GRO on a router :-)

> netfilter cost was hardly a problem, once correctly done.

But cost is not zero, and if you can divide it by N then you do it.
And GRO is what allows this.

Every demux, lookup, etc. is transaction cost.

Even routing cache lookup with no dst reference, which is _very_
cheap, takes up a serious amount of cpu cycles.  Enough that we think
early demux is worth it, right?

And such a routing cache lookup is significantly cheaper than a trip
down into netfilter.
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