[atlas] probe resolution, overhead, or ...?
Philip Homburg philip.homburg at ripe.net
Wed Jun 20 10:48:48 CEST 2012
On 6/20/12 9:11 , Randy Bush wrote: > we want to use atlas probes in an experiment. being prudent (you can > tell it was not i), we decided to try to get some basic calibration. > one run was just on a local LAN. > > three hosts on the same gige switch > o probe 2285 > o psg.com, a not very fancy or fast freebsd 9 box with intel/pro1000 > gige ports > o bbgp.psg.com, a funky older freebsd 9 box with bge gige > > probe 2285 pinging bbgp.psg.com, average RTT: 1.5606994382 , number > of pings: 356*3 > > psg.com pinging bbgp.psg.com, average RTT: 0.253424332344 , number of > pings: 674 > > has anyone done similar probe calibration experiments? does anyone have > any clue as to why the difference? > > Just to confirm my suspicion, I tried to other way around: This is an old AMD64 running FreeBSD pinging an Atlas probe on the same LAN: $ ping 220.127.116.11 PING 18.104.22.168 (22.214.171.124): 56 data bytes 64 bytes from 126.96.36.199: icmp_seq=0 ttl=64 time=2.515 ms 64 bytes from 188.8.131.52: icmp_seq=1 ttl=64 time=0.913 ms 64 bytes from 184.108.40.206: icmp_seq=2 ttl=64 time=0.915 ms 64 bytes from 220.127.116.11: icmp_seq=3 ttl=64 time=0.929 ms And this is the same FreeBSD box pinging a Celeron 766 MHz, running a micro kernel operating system, also on the same LAN: $ ping prism PING prism.hq.phicoh.net (18.104.22.168): 56 data bytes 64 bytes from 22.214.171.124: icmp_seq=0 ttl=96 time=0.364 ms 64 bytes from 126.96.36.199: icmp_seq=1 ttl=96 time=0.210 ms 64 bytes from 188.8.131.52: icmp_seq=2 ttl=96 time=0.211 ms 64 bytes from 184.108.40.206: icmp_seq=3 ttl=96 time=0.214 ms This does not involve any of the Atlas software, just the ucLinux kernel running on the probe. My conclusion is: probes are just very slow. They are fine for measuring multi millisecond delays on WAN links but not for sub-millisecond delays on local links.