[igraph] FW: Calculating betweennes centrality in large graphs

[Visser, D.E.]

Hi all!

First of all, let me introduce myself: I am Daniel, a business student from the 
Netherlands.

For my thesis I aim to predict "online knowledge sharing" in open source 
development communities from users' centrality in a given forum. I plan to ask 
stackoverflow.com users to fill in a survey and then calculate their centrality 
with igraph.
I'm in possession of a GML graph of the stackoverflow forum that looks like 
this:

graph [
  directed 1
  node [
    id 0
    label "1"
  ]
  node [
    id 1
    label "2"
  ]

Etc.

The GML file is, obviously, quite huge: 650k nodes and millions of edges. 
Taking a a cutoff point of 2, calculating betweenness centrality for a single 
vertex takes just 15 minutes (I learned that all vertices shouldn't take any 
longer), but a cutoff point of 3 already takes an hour to process.

Secondly, I am not much of a programmer, but after some trial and error I was 
able to read the graph file in igraph (in Python) and got to 
"filename.betweenness(vertices=1, directed=True, cutoff=2)" which yielded the 
centrality index for, I think, the vertex with ID 1 (see above).

So here are my questions:

1) I believe when I use "vertices=None" centrality for all nodes is calculated. 
How can I get igraph to write a CSV file that contains either (or both) "ID" or 
"Label" in the one column, and the corresponding centrality index in the other?

2) Is there anyway to determine what a reliable cutoff point would be? I don't 
mind letting my computer run for a couple of days, but preferably not more than 
a day or three.


Thanks in advance!

Daniel