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

On 6/20/12 13:01 , Randy Bush wrote:
>
>> They are fine for measuring multi millisecond delays on WAN links but
>> not for sub-millisecond delays on local links.
> some experiments care about jitter.  we are seeing variance noticeably
> greater than bsd boxen.
Obviously the current Atlas probes will be worse than just about 
anything running on older Intel boxes.

At the moment we don't have engineering targets for jitter. But it is 
certainly worth putting on the wish list. That requires some kind of 
project description, i.e. what kind of variance are you trying to 
measure and what kind of jitter is the probe allowed to have before the 
measurement is spoiled. Keeping jitter low conflicts with doing many 
experiments on a single underpowered probe. So it won't be easy.
>
> @stelios: yes icmp goes the slow path.  but atlas has a very constrained
> measurment model, and it seems to be pretty much based on icmp.
>
>
I could be wrong, but as far as I know, routers don't have any kind of 
'ping' feature in the fast path. But if you have suggestions for 
protocols that work better than ICMP ECHO then please let us know.

As far as I know, a dedicated (x86) box that just acts as a ping target 
is best for round trip latency measurements.