I am interested in "focused crawling" (crawling web pages of some specific topic and ignoring all the others) and have written a "focused crawler" recently. Perl is a reasonable alternative to writing web crawler for its LWP module and CPAN. However, when I planed to implement multithread strategy in crawling, I was confused by perl 5.8.4's "threads" module, especially threads::shared. How is a object reference shared by multiple threads?
I want to utilize "Cache::File::Heap" module to sort the urls in "crawling frontier" by heuristic prediction of its "harvest outcome". Below is the relevant code part:
#!/usr/bin/perl -w
use strict;
use threads;
use threads::shared;
use Cache::File::Heap;
my $heap = Cache::File::Heap->new('frontier');
my $heap_lock : shared = 0;
...
sub go {#crawling thread's control flow
....
#xtracted best promising url
{
lock $heap_lock;
my($value, $url) = $heap->extract_minimum;
}
...
#after downloading and extract hyperlinks
{
lock $heap_lock;
$heap->add($value, $url) for
}
...
}
my @threads;
for(1..10) {
push @threads, threads->new(\&go);
}
for(@threads) {
$_->join;
}
All is fine, just untill all the threads joined by main thread and main thread exists. Following error message appears:
Scalar leaks : -1
Segmentation fault.
My question is :
How to share object reference (such as Cache::File::Heap) ?? Cache::File::Heap is the wrapper of BerkeleyDB's BTREE, is BerkeleyDB thread-safe?
