Re: HTB and PRIO qdiscs introducing extra latency when output interface is saturated

Linux Advanced Routing and Traffic Control

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Jonathan Lynch wrote:


I dont understand what you mean when you say "if you could sample truly
randomly you would get a proper distribution".
Also having the timers synchronized will allow for more accurate
measurements of the delay. I cant see how this would have an impact on
the pattern.

I mean it's possibly just to do with the test if a 0ms - 1ms delay is expected then you could see patterns arising depending on how you measure delay/clock drift or something.

Now I have two pings that do intervels < 1 sec - the inetutils GNU ping guys implemented it for me :-), and I also have the iputils one I can simulate a stream better.

While doing this I noticed that iputils ping actually gives lower latency readings when sending many pps. Using tcpdump deltas I can see the network latency is the same however many pps I do - it's just that when measuring <1ms delays and doing many pps it seems that some code gets cached (guess) and the reported delay changes as a result.

I mention that just to illustrate that measuring small delays can be misleading and influenced by the exact nature of your setup.




I have the default setting which is to set it to jiffies. There is a
comment in the kernal config for Packet scheduler clock source that
mentions that Jiffies "its resolution is too low for accurate shaping
except at very low speed". I will recompile the kernel and try the CPU
option tomorrow to see if there is any change.

Maybe not in the case of htb - I use CPU and see similar results, the comment about accurate shaping was probably written when HZ=100, but I suppose it will be better for something :-)




I didnt realise such options existed for htb (mpu + overhead). These parameters are not mentioned in the man pages or in the htb manual. I presume I have to patch tc to get these features ?.


There is mention on the htb page - it was added as a patch so was not designed in, which explains why burst doesn't use it.

You don't need to patch recent iproute2 it's already in there.

Andy.
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