[Networkit] Problem computing ApproxBetweeness

Henning Meyerhenke meyerhenke at kit.edu
Wed May 7 10:34:53 CEST 2014


Hi Kevin,

What were the approximation and confidence parameters you used? Or did 
you go along with the default epsilon=0.01 and delta=0.1?

Best,
Henning


Am 07.05.14 09:34, schrieb Kevin Tierney:
> Hi,
>
> We have come across another graph where there seems to be an issue with
> ApproxBetweenness. This graph is not trivially small, nor is it
> completely connected, meaning we'd actually be rather interested in
> finding out the ApproxBetweenness value for it. I let it run overnight
> (~14 hours with 8 cores) and did not get an answer, so I assume it was
> never going to finish.
>
> The graph is a bit large, so I put it here:
> https://dl.dropboxusercontent.com/u/16348505/ns894786.mps.gz.variable.graph.bz2
>
> Any help you can provide would be greatly appreciated.
>
> By the way, we tried to compute the number of samples ourselves and it
> seemed like it would only be around ~63000, so perhaps there is another
> issue?
>
> Thanks for your help.
>
> Kevin
>
> On 06.05.2014 16:32, Christian Staudt wrote:
>> The important thing is that you get sensible results on the real networks that are of interest to you. If that’s the case, I give a low priority to this bug. I agree that betweenness is uninteresting on near-complete graphs.
>>
>> Am 06.05.2014 um 16:21 schrieb Kevin Tierney <kevin.tierney at upb.de>:
>>
>>> Hi Christian,
>>>
>>> Thanks for looking into the issue and for your update.
>>>
>>> We've noticed similar behavior on complete or nearly-complete graphs. I
>>> suppose computing a betweenness measure on such graphs is pointless
>>> anyway (at least for what we're doing), but at least that seems like a
>>> possible cause of the weird behavior.
>>>
>>> Kevin
>>>
>>>
>>> On 06.05.2014 15:30, Christian Staudt wrote:
>>>> 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
>>>> Christian
>>>>
>>>>
>>>>
>>>> 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>
>>>>
>>>
>>> --
>>> 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
>>
>

-- 

=======================================================
Karlsruhe Institute of Technology (KIT)
Institute of Theoretical Informatics (ITI)

Juniorprof. Dr. Henning Meyerhenke
Theoret. Informatics / Parallel Computing

Phone: +49-721-608-41876
Web: http://parco.iti.kit.edu/henningm/

KIT - University of the State of Baden-Wuerttemberg and
National Research Center of the Helmholtz Association
=======================================================

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