NAME
Algorithm::PageRank::XS  A Fast PageRank implementation
DESCRIPTION
This module implements a simple PageRank algorithm in C. The goal is to quickly get a vector that is closed to the eigenvector of the stochastic matrix of a graph.
Algorithm::PageRank does some pagerank calculations, but it's slow and memory intensive. This module was developed to compute pagerank on graphs with millions of arcs. This module will not, however, scale up to quadrillions of arcs (see the TODO).
SYNOPSYS
use Algorithm::PageRank::XS;
my $pr = Algorithm::PageRank::XS>new();
$pr>graph([
'John' => 'Joey',
'John' => 'James',
'Joey' => 'John',
'James' => 'Joey',
]
);
$pr>result();
# {
# 'James' => '0.569840431213379',
# 'Joey' => '1',
# 'John' => '0.754877686500549'
# }
#
#
# The following simple program takes up arcs and prints the ranks.
use Algorithm::PageRank::XS;
my $pr = Algorithm::PageRank::XS>new();
while (<>) {
chomp;
my ($from, to) = split(/\t/, $_);
$pr>add_arc($from, $to);
}
my $r = $pr>results();
while (my ($name, $rank) = each(%{$r})) {
print "$name,$rank\n";
}
METHODS
new %PARAMS
Create a new PageRank object. Possible parameters:
 alpha

This is (1  how much people can move from one node to another unconnected one randomly). Decreasing this number makes convergence more likely, but brings us further from the true eigenvector.
 max_tries

The maximum number of tries until we give up trying to achieve convergence.
 convergence

The maximum number the difference between two subsequent vectors must be before we say we are "convergent enough". The convergence rate is the rate at which
alpha^t
goes to 0. Thus, if you setalpha
to0.85
, andconvergence
to0.000001
, then you will need85
tries.
add_arc
Add an arc to the pagerank object before running the computation. The actual values don't matter. So you can run:
$pr>add_arc("Apple", "Orange");
and you mean that "Apple"
links to "Orange"
.
graph
Add a graph, which is just an array of from, to combinations. This is equivalent to calling add_arc
a bunch of times, but may be more convenient.
from_file FILE
This will load arcs from a file, whose lines contain:
from,to\n
It's designed to be fast, and doesn't handle quoting or even commas in the from string. This will just allow you to load a bit faster and maybe save a few megabytes of ram if you wanted to.
iterate
Doesn't do anything, but provided so that you can substitute this module in for Algorithm::PageRank.
result
Compute the pagerank vector, and return it as a hash.
Whatever you called the nodes when specifying the arcs will be the keys of this hash, where the values will be the vector.
The result vector is normalized such that the sum is 1
(the L1 norm). You can normalize it any other way you like if you don't like this.
BUGS
None known.
TODO
Support for "Personalized PageRank" (see http://ilpubs.stanford.edu:8090/596/)
We may want to support
double
values rather than single floatsWe may or may not want to adjust the weighting of individual arcs, as you cannot do now.
At present the indexes are
unsigned int
, rather thansize_t
. Thus this will not scale with 64bit architectures.It'd be nice to be able to use
mmap(2)
to efficiently use the hard drive to scale to places where memory can't take us.
SPEED
This module is pretty fast. I ran this on a 1 million node set with 4.5 million arcs in 57 seconds on my 32bit 1.8GHz laptop. Let me know if you have any performance tips.
Below are the tables for the current iteration in trials per second and arcs per second. Keep in mind that for some of these there are large numbers of arcs (.2%
load with 100,000
nodes means 20,000,000
arcs!
++++++
 test  XS trials / sec  PL trials / sec  XS arcs / sec  PL arcs / sec 
++++++
 10 nodes @50%  4533.207  53.741  6890.474  81.687 
 10 nodes @100%  3822.595  46.084  13761.342  165.901 
 1000 @10%  4.542  0.120  18109.287  2390.898 
 1000 @50%  1.055  0.031  21082.599  15720.595 
 1000 @100%  0.562  0.016  56121.722  16301.088 
 100000 @.0001%*  1.348   141855.819  
 100000 @.01%*  0.217   23174.341  
 100000 @.1%*  0.034   344796.415  
 100000 @.2%*  0.017   348070.697  
++++++
* For some of these tests I cheated a little bit and used from_file() since there were so many arcs.
SEE ALSO
AUTHOR
Michael Axiak <mike@axiak.net>
COPYRIGHT
Copyright (C) 2008 by Michael Axiak <mike@axiak.net>
This package is free software; you can redistribute it and/or modify it under the same terms as Perl itself