netdev - Re: [PATCH v3 net-next 2/2] netem: add cell concept to simulate special MAC behavior

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Date:	Thu, 01 Dec 2011 09:25:20 +0100
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	<netdev@...r.kernel.org>, Stephen Hemminger <shemminger@...tta.com>
Subject: Re: [PATCH v3 net-next 2/2] netem: add cell concept to simulate  special MAC behavior

On Thu, 01 Dec 2011 04:30:25 +0100, Eric Dumazet wrote:

> Thats a multiply instead of a divide. On many cpus thats a lot faster.
>
> Think about a super packet (TSO) of 65000 bytes and cell_size=64

I've never imagined that I am going to say the following: you are wrong,
Eric! (ok, maybe you are right ;-)

TSO and Netem is a no-go. With netem you are strongly advised to disable
offloading. I mean TSO will result in _one_ delay of several minutes,
followed by a burst of packets. Instead of packets spaced by several
seconds (with the rate of 1000byte/s) - which is what you wan't.

To sum up: skb->len is _never_ larger as the MTU for (normal, correct)
network emulation setups with netem. This was the assumption why I
preferred the iterative solution over the div/mod solution.

Did I miss something?

Hagen

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