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## Re: [igraph] using igraph parallelly

 From: Gábor Csárdi Subject: Re: [igraph] using igraph parallelly Date: Wed, 24 Apr 2013 21:15:48 -0400

Hi, AFAIK you need to compute all shortest paths to compute the betweenness of a single vertex, at least igraph does it this way and I am not aware of a better algorithm.

So it is not worth running betweenness in parallel, you gain nothing.

Gabor

On Wed, Apr 24, 2013 at 8:19 PM, 四正（红砖） wrote:
Hi everyone,

I want to calculate the betweenness of each node for a network with 13498 nodes and 101565 edges.

by simply call betweenness(nodes,directed=False), it takes about 32 seconds on my 8-core computer. That seems good.

However, I just considering use full of my CPU to accelerate it. So I use PPL, which is a parallel library for C++ in VS2012, and I made following code:

igraph_vector_t test;
igraph_vector_init(&test, 0);
parallel_for(0,vcount-1,[&g,&test](igraph_integer_t i){
clock_t t1=clock();
igraph_betweenness(&g,&test,igraph_vss_1(i),IGRAPH_UNDIRECTED,0,1);
clock_t t2=clock();
cout<<i<<"\t"<<igraph_vector_e(&test,0)<<"\t"<<t2-t1<<endl;
});
finish=clock();
cout<<finish-start<<endl;

BUT! It is very slow! It takes about 2 minutes to calculate betweenesses of 7 nodes in parallel!

It is faster if I use:

parallel_for(0,vcount-1,10000,[&g,&test](igraph_integer_t i){
clock_t t1=clock();
igraph_betweenness(&g,&test,igraph_vss_seq(i,i+9999),IGRAPH_UNDIRECTED,0,1);
clock_t t2=clock();
cout<<i<<"\t"<<igraph_vector_e(&test,0)<<"\t"<<t2-t1<<endl;
});

It takes about 1 minute...

So why is it?

In my opinion, if we put many nodes into a same thread, they can share the information of visited paths by other nodes. Am I right?

Has anybody tried parallel computing of igraph?

Gang

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