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

Hello Philip.

   Ping relies heavily on the system's CPU. That is why it is considered
unreliable for measuring when you are pinging CPU loaded machines/devices.
The CPU on the probe is a n ARM7TDMI running at 55MHz, based on information
from this link:
http://www.digi.com/products/wireless-wired-embedded-solutions/solutions-on-module/digi-connect/digiconnectme#specs


Regards,
Stelios Mersinas

On Wed, Jun 20, 2012 at 11:48 AM, Philip Homburg <philip.homburg at ripe.net>wrote:

> 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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