netdev - Re: Optimizing instruction-cache, more packets at each stage

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Date:	Thu, 21 Jan 2016 13:23:04 +0100
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Tom Herbert <tom@...bertland.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Or Gerlitz <gerlitz.or@...il.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Alexander Duyck <alexander.duyck@...il.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Daniel Borkmann <borkmann@...earbox.net>,
	Marek Majkowski <marek@...udflare.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Florian Westphal <fw@...len.de>,
	Paolo Abeni <pabeni@...hat.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Amir Vadai <amirva@...il.com>, brouer@...hat.com
Subject: Re: Optimizing instruction-cache, more packets at each stage

On Wed, 20 Jan 2016 15:27:38 -0800
Tom Herbert <tom@...bertland.com> wrote:

> weaknesses of Toeplitz we talked about recently and that fact that
> Jenkins is really fast to compute, I am starting to think maybe we
> should always do a software hash and not rely on HW for it...

Please don't enforce a software hash.  You are proposing a hash
computation per packet which cost in the area 50-100 nanosec (?). And
on data which is cache cold (even with DDIO, you take the L3 cache
cost/hit).

Consider the increase in network hardware speeds.

Worst-case (pkt size 64 bytes) time between packets:
 *  10 Gbit/s -> 67.2 nanosec
 *  40 Gbit/s -> 16.8 nanosec
 * 100 Gbit/s ->  6.7 nanosec

Adding such a per packet cost is not going to fly.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer