Re: High latency on HTB class with small packets/high packet rates

[Andy Furniss <adf.lists@gmail.com>]

Matthew Fox wrote:
> Thanks.
>
> I didn't mention that the rates were already conservative. Our product has an overhead option and for this customer, it was already set to over 20% (ie if we think we have 10 Mbps on our lines, then the qdisc rate would actually only be 8 Mbps).
>
> After some further testing, I was able to get consistently good latency under high UDP packet rates if I just increased that overhead to 40%, which I've never had to do before. It seems like this might be caused by the connections we're using being backed by ATM and requiring to support high rates of small packets, which is an unusual scenario for us- more frequently we'd need to support ATM OR high rates of small packets, but not both.
>
> I'm satisfied with blaming ATM. If anyone else has a better idea, I'd be happy to hear.

In theory you can do atm overheads perfectly see

man tc-stab

In practice you need to also know what fixed overheads there are in 
addition to atm and the nature of the device that you are actually 
shaping on affects the fixed overhead.

For example, when I had adsl I connected using pppoa/vc mux which gave a 
fixed overhead of 10 bytes over ip.

When shaping on ppp directly (PCI dsl card) this was the number to use - 
but when shaping on eth (connected to external dsl modem/router) tc 
already sees packets as IP +14 so -4 was the overhead to use.