Re: [LARTC] HTB and PRIO qdiscs introducing extra latency when output

[Andy Furniss <andy.furniss@dsl.pipex.com>]

Jonathan Lynch wrote:
> I did the same tests that I outlined earlier, but this time by setting
> hysteresis to 0. The config for the core router is included at the
> bottom. The graphs for the delay of the voip stream and the traffic
> going through the core router can be found at the following addresses.
> 
> http://140.203.56.30/~jlynch/htb/core_router_hysteresis.png
> http://140.203.56.30/~jlynch/htb/voip_stream_24761_hysteresis.png
> 
> 
> The max delay of the stream has dropped to 1.8ms. Again the jitter seems
> to be around 1ms. There seems to be a pattern going whereby the delay
> reaches about 1.6ms then drops back to 0.4 ms, jumps back to 1.6ms and
> then back to 0.4ms repeatedly and then it rises from 0.5ms gradually and
> repeats this behaviour. Is there any explanation to this pattern ?
> 
> Would it have anything go to do with burst being 1ms ?

Yes I suppose if you could sample truly randomly you would get a proper 
distribution - I guess the pattern arises because your timers are 
synchronised for the test.

> 
> When the ceil is specified as being 90mbit, is this at IP level ? 
> What does this correspond to when a Mbit = 1,000,000 bits. Im a bit
> confused with the way tc interprets this rate.

Yes htb uses ip level length (but you can specify overhead & min size) , 
the rate calculations use a lookup table which is likely to have a 
granularity of 8 bytes (you can see this with tc -s -d class ls .. look 
for /8 after the burst/cburst).

There is a choice in 2.6 configs about using CPU/jiffies/gettimeofday - 
I use CPU and now I've got a ping that does < 1 sec I get the same 
results as you.

> 
> If the ceil is based at IP level then the max ceil is going to be a
> value between 54 Mbit and 97 Mbit (not the tc values) for a 100 Mbit
> interface depending on the size of the packets passing through, right ?
> 
> Minimum Ethernet frame
> 148,809 * (46 * 8) =   148,809 * 368 = 54,761,712 Mbps
> 
> Maximum Ethernet frame
> 8,127 * (1500 * 8) =   8,127 * 12,000 =  97,524,000 Mbps

If you use the overhead option I think you will be to overcome this 
limitation and push the rates closer to 100mbit.


> About the red settings, I dont understand properly how to configure the
> settings. I was using the configuration that came with the examples.

I don't use red it was just something I noticed - maybe making it longer 
would help, maybe my test wasn't rerpresentative.

FWIW I had a play around with HFSC (not that I know what I am doing 
really) and at 92mbit managed to get -

rtt min/avg/max/mdev = 0.330/0.414/0.493/0.051 ms loaded
from
rtt min/avg/max/mdev = 0.114/0.133/0.187/0.028 ms idle

and that was through a really cheap switch.

Andy.

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