traffic bandwidth limit with dummynet

bored to death bored_to_death85 at
Fri Jun 4 08:19:34 UTC 2010

thank you luigi for your reply, it helped.

i changed the hz parameter to 1000 and then 4000 and then 8000 in my /boot/loader.conf. the result got much better.
i configured my system as a router and i send 1GB traffic rate passing by it and set an 800MBytes bandwidth limit on input traffic with dummynet. 
this was the result:
with hz=1 (default) between 200MBytes/s and 300MBytes/s
with hz=1000 between 200MBytes/s and 300MBytes/s
with hz=4000 between 350MBytes/s and 450MBytes/s
with hz=8000 between 250MBytes/s and 550MBytes/s

the maximum traffic rate is got so much better, but 2 problems still remain:
1- the maximum rate is still not high enough.
2- the rate variation range is high (250-550) and it's not a steady enough.

i've also tried setting different "queue" and "burst" values for the pipe. the result is a little better when i set "queue" to a value between 80MBytes and 90MBytes and "burst" to a big number.

any other ideas?

From: Luigi Rizzo <rizzo at>
To: bored to death <bored_to_death85 at>
Cc: freebsd-ipfw <freebsd-ipfw at>
Sent: Thu, June 3, 2010 9:57:33 PM
Subject: Re: traffic bandwidth limit with dummynet

On Thu, Jun 03, 2010 at 09:29:20AM -0700, bored to death wrote:
> hello,
> i'm trying to limit my input traffic bandwidth on freebsd. i used
> ipfw+dummynet. without limitation, i have almost 1Gbit/s input traffic
> on my system. when i try to limit the bandwidth, it works fine on low
> to normal limitation number (up to 450 Mbit/s).
> but when i limit my traffic to an amount bigger than 500MBit/s, my
> input traffic lowers down to average of 430 MBit/s. no matter what
> limitations i set(from 500MBit/s to 1000MBit/s) my input traffic rate
> is getting stucked at 430MBit/s. 

try to run the kernel with higher value of HZ (in /boot/loader.conf


and see if the situation improves.
I think the symptoms you are seeing depend on the fact that
packets are released every 1/HZ seconds probably causing some
queue overflow at high packet rates and [relatively] low HZ


> i suspected low hardware performance, so i tried this on 2 different
> systems with freebsd-8.0 and the result is exactly the same.
> my ipfw rules are:
> Code:
> ipfw -q flush
> ipfw -q add 500 set 1 pipe 1 all from any to any in
> ipfw pipe 1 config bw 700Mbit/s
> ipfw add 1000 allow all from any to anyso my question is:
> 1- is ipfw+dummynet traffic limit control, has performance problem that
> cannot limit traffic correctly if limitation number is bigger than
> 500MBit/s?
> 2- is there any other way (other than ipfw+dummynet) to limit input bandwidth in freebsd?
> thank you. 
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