Date: Sat, 31 Mar 2001 22:55:22 +0200 (CEST) From: Luigi Rizzo <luigi@info.iet.unipi.it> To: Jochen Kaiser <Jochen.Kaiser@rrze.uni-erlangen.de> Cc: FreeBSD-hackers@FreeBSD.ORG Subject: Re: unexpected peaks in dummynet;scheduling oddity? Message-ID: <200103312055.WAA33314@info.iet.unipi.it> In-Reply-To: <20010331111427.A3209@devil.rrze.uni-erlangen.de> from Jochen Kaiser at "Mar 31, 2001 11:14:27 am"
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Hi, the packet scheduler is invoked by dummynet at every packet arrival and then at multiple of 1/HZ intervals. If you use the default kernel setting, HZ=100 so you have most times rounded to multiples of 10ms. I have been running my kernels with HZ=1000 for the past 4-5 years so the resolution is much better -- 1ms, which is appropriate for links up to a few megabits/s I think the above should explain the results you see. cheers luigi > Hello, > > I did some measurements using Dummynet. It was just to see how > it works. Tests are made with 0,7MBit stream (each 429Bytes Packets), > 5,10,15,25,50 and 75 MBit. > > I tried a delay of 10ms for the testing connection. > The tests were done with a Smartbits6000 with the capability to > record the difference between sending and receiving a packet. > Each test had a 8192 packets. > > > The result show a strange behaviour, depending on the data rate. > > _ > | | vs. /\ > > 0.7MBit - 15MBit: diffuse, fairly hig variance from 9.5 - 10.3ms > > 25 MBit: between > > 50MBit - 75MBit: very sharp, extreme precise, delivers a nearly 10ms > > (variance not calculated yet ... to be done ..) > > I am now a bit confused. Is it a matter of scheduling? > Maybe too much fairness for such tasks :) > > Or is it some kind of prediction in dummynet which favours > high data rates. I think I've seen something like that for packet > loss in dummynet. > > > Any opionions appreciated. Maybe I did something wrong > and forget to set some sysctl properly. > > with kind regards, > Jochen Kaiser > -- > Jochen Kaiser kind@IRCNET, phone +49 9131 85-28134 > Network Administration mailto:jochen.kaiser@rrze.uni-erlangen.de > Regionales Rechenzentrum Universitaet Erlangen-Nuernberg, Germany > GPG public key: http://www.uni-erlangen.de/~unrza2/public_key.txt > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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