From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Furniss Date: Thu, 11 Aug 2005 16:36:10 +0000 Subject: Re: [LARTC] HTB and PRIO qdiscs introducing extra latency when output Message-Id: <42FB7E7A.5040301@dsl.pipex.com> List-Id: References: <1123699068.4768.67.camel@pgala.it.nuigalway.ie> In-Reply-To: <1123699068.4768.67.camel@pgala.it.nuigalway.ie> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: lartc@vger.kernel.org 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 HZ0, 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. _______________________________________________ LARTC mailing list LARTC@mailman.ds9a.nl http://mailman.ds9a.nl/cgi-bin/mailman/listinfo/lartc