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 10:01:48 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hagen Paul Pfeifer <hagen@...u.net>
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

Le jeudi 01 décembre 2011 à 09:25 +0100, Hagen Paul Pfeifer a écrit :
> 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?
> 

Yes :)

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 !

We did cleanups in net/sched to properly handle large packets as well.
(SFQ for example is OK)

Really, reciprocal divide is the way to go, its faster anyway on modern
cpus than your loop.



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