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

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Message-ID: <61abe6ff1aa870d1b885ce88814920d4@localhost>
Date:	Thu, 01 Dec 2011 10:36:56 +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 10:01:48 +0100, Eric Dumazet wrote:

> Yes :)

damn!

> I want to be able to use netem on a 10Gigabit link, and simulate a 5ms
> delay. I already will hit the shared qdisc bottleneck, dont force me to
> use small packets !

No I don't want that. But with 10Gb/s links you will have packet
scheduling problems anyway - if you focus on an _accurate_ delay. A static
delay differs from rate shaping in use case. In the later we (and probably
you) want a exact/realistic spacing between packets.

Due to timer and scheduling granularity somewhere in between 1bit/s and
10Gb/s netem rate (and tbf) will not scale anymore. You will see burst and
inaccurate spacings, far away from what you want to emulate. For us we want
a realistic and clean behavior, if the result of the emulation is not
identical to the emulated link/device we cannot use it (some background
information). 

Anyway: I was not sure what solution you prefer - for us both are
identical. That's why I presented two solutions, so you can pick up the
favorite one. I will re-code the calculation using a reciprocal divide.
Thanks Eric!

Hagen
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