[Networkit] Problem computing ApproxBetweeness

Christian Staudt christian.staudt at kit.edu
Tue May 6 15:30:56 CEST 2014

Hi Kevin,
running the program with the log level set to INFO reveals the problem: 

[INFO ][“src/cpp/centrality/ApproxBetweenness.cpp”, 38: virtual void NetworKit::ApproxBetweenness::run()]: estimating vertex diameter
[INFO ][“src/cpp/centrality/ApproxBetweenness.cpp”, 41: virtual void NetworKit::ApproxBetweenness::run()]: estimated diameter: 2
[INFO ][“src/cpp/centrality/ApproxBetweenness.cpp”, 47: virtual void NetworKit::ApproxBetweenness::run()]: taking 9223372036854775808 path samples

A ridiculously high number of samples is taken. Where does it come from? The number of samples is calculated as:

	count r = ceil((c / (epsilon * epsilon)) * (floor(log(vd - 2)) + 1 + log(1 / delta)));

So if the estimated diameter (vd) is 2, then the formula contains the logarithm of 0, which looks like trouble. I am currently not sure how to fix this, because this is how the formula was published. We will investigate.

At the same time I discovered and fixed an error in the formula, but I believe this was unrelated to the problem you describe. Please also update to the latest Dev-Revision for your experiments.

Kind regards

Am 06.05.2014 um 14:57 schrieb Kevin Tierney <kevin.tierney at uni-paderborn.de>:

> Dear NetworKit Gurus,
> We are trying to use the approximate betweenness function and notice an 
> odd thing happening on small graphs. I have attached a graph where calling:
> NetworKit.centrality.ApproxBetweenness(G)
> seems to never finish. At least the CPU time required seems to be way 
> more than should be for such a small graph.
> You might say "Why would you compute approxbetweenness for such a small 
> graph?" -- the reason is this was just mixed in with a bunch of larger 
> graphs where betweenness would take too long.
> Do you have any ideas on what is happening?
> By the way, could this perhaps be related to the sample size of 42 in 
> the .cpp file for approxbetweenness? We aren't getting any issues on 
> large graphs; in fact they are being solved relatively quickly.
> Thanks for your help!
> Sincerely,
> Kevin
> -- 
> Jun.-Prof Dr. Kevin Tierney
> University of Paderborn
> Faculty of Business Administration and Economics
> Department 3: Business Information Systems
> Room: Q2.472
> Tel.: +49 (0)5251 60-5244
> <markshare_5_0.mps.gz.constraint.graph><ATT00001.c>

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