[atlas] probe congestion?

> On 11 May 2016, at 12:25, Paul Vlaar <pvlaar at afilias.info> wrote:
> 
> while running a DNS UDM on a fixed set of (reused from a previous UDM) probes, I noticed the following. When I start 6 UDMs against the same set using the web UI, as a one-off measurement, starting "now", the RTTs for all of the measurements shoots up on all of the probes (500-1000+ ms). When I start the 6 tests individually, the RTTs are much lower, and close to what I'd expect them to be.
> 
> It appears to me that when multiple UDMs are scheduled on the same probe, these should be run in serial, not parallel, in order to not run into congestion issues. Or am I simply expecting the wrong behaviour, and should I not schedule one-off measurements in parallel in the first place?

This is known [a, b]. RTT timestamps are applied in user-space.
As such, if a probe is loaded with multiple measurements, the
user-space time stamping will be delayed. These delays will
be more pronounced on v1/v2 probes.

v3 probes reduce the impact of user-space timestamping. As such, v3
probes are more suitable for latency measurements that require
high precision accuracy. RIPE atlas system tags can be used to
separate probes by h/w version.

[a] http://www.sigcomm.org/sites/default/files/ccr/papers/2015/July/0000000-0000005.pdf
[b] http://conferences.sigcomm.org/imc/2015/papers/p437.pdf

> 	~paul

Best, Vaibhav

===================================
Vaibhav Bajpai
www.vaibhavbajpai.com

Room 91, Research I
School of Engineering and Sciences
Jacobs University Bremen, Germany
===================================

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