traffic bandwidth limit with dummynet

[Luigi Rizzo]

On Fri, Jun 04, 2010 at 01:19:32AM -0700, bored to death wrote:
> 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?
> 

HZ=1000 is the default, for the records.
Setting the burst size should have no practical effects,
whereas setting the queue size e.g.
o
        ipfw pipe 10 config bw 800Mbit/s queue 200kbytes

should help a lot, but check your configuration with 'ipfw pipe show'
because if you supply an invalid parameter ipfw silently uses
a default or something different. 
As an example, you said you used 80-90 Mbytes but the max queue
size is 100 packets or 1023Kbytes and larger values do not produce
the desired effect.

As a rule of thumb, to make sure that drops are not caused
by short queues, you should set the queue size to 1/HZ seconds
worth of data -- at HZ=1000 and 1Gbit/s this means 128Kbytes.
Note that after the dummynet queue, there might be some other
queue that saturates. As an example, when using the box as a router,
packets go in bursts to the output interface, and the burst can
be as large as 1500 packets per tick on a fully saturated Gig-E
(the interface's queue ranges normally between 128 and 1024 slots).
The only fix for this is probably using higher values of HZ.

chers
luigi