[Tmrg] Tmrg-interest Digest, Vol 11, Issue 4

[Douglas Leith]

On 22 Aug 2007, at 09:31, Lars Eggert wrote:

> On 2007-8-22, at 10:26, ext Douglas Leith wrote:
>> Re dummynet, my experience these days is that it can run up to 1Gb
>> using modern hardware.  In my experience more of an issue is that end
>> hosts can still have difficulty at high bandwidth-delay products due
>> to sack processing overhead etc. and it is this that has placed the
>> upper limit on test speeds rather than anything else.  I'm not sure
>> where things break these days but it would be easy enough to check.
>
> It may be interesting to compare the results in a dummynet setup  
> with one that uses real hardwarde (Lachlan's setup, for example.)
Sounds like a good idea.

> What I saw a few years back was that dummynet bunched together  
> packets in bursts, due to the way device interrupts were handled by  
> BSD at the time. Essentially, device driver processing interrupts  
> IP-layer processing, and so dummynet only got cycles  
> intermittently. The result was that although you'd get a simulated  
> path that had the desired bandwidth/delay properties over longer  
> timescales (>> CPU quantum), if you looked at the packet-level  
> trace, there were some oddities there. For my stuff (queueing),  
> that caused some issues, but I'm not sure if it'd matter much for  
> TCP evaluation.
Makes sense.

Doug