[atlas] probe resolution, overhead, or ...?

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.