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

From: Andy Furniss <andy.furniss@dsl.pipex.com>
To: lartc@vger.kernel.org
Subject: Re: [LARTC] HTB and PRIO qdiscs introducing	extra	latency	when	output
Date: Thu, 11 Aug 2005 16:36:10 +0000	[thread overview]
Message-ID: <42FB7E7A.5040301@dsl.pipex.com> (raw)
In-Reply-To: <1123699068.4768.67.camel@pgala.it.nuigalway.ie>

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\x100, 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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