[atlas] probe resolution, overhead, or ...?
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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 [0], number > of pings: 356*3 > > psg.com pinging bbgp.psg.com, average RTT: 0.253424332344 [0], 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 130.37.15.50 PING 130.37.15.50 (130.37.15.50): 56 data bytes 64 bytes from 130.37.15.50: icmp_seq=0 ttl=64 time=2.515 ms 64 bytes from 130.37.15.50: icmp_seq=1 ttl=64 time=0.913 ms 64 bytes from 130.37.15.50: icmp_seq=2 ttl=64 time=0.915 ms 64 bytes from 130.37.15.50: 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 (130.37.15.36): 56 data bytes 64 bytes from 130.37.15.36: icmp_seq=0 ttl=96 time=0.364 ms 64 bytes from 130.37.15.36: icmp_seq=1 ttl=96 time=0.210 ms 64 bytes from 130.37.15.36: icmp_seq=2 ttl=96 time=0.211 ms 64 bytes from 130.37.15.36: 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.
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