Hey. I'm trying to get the feel for how multithreading works with perl in an effort to try to speed up my scripts. My test case is that I want to parse two relatively large files and place all the lines from each file into separate hashes. Normally I would just go over them one by one with a while loop and populate my hashes that way. Like so.

open IN, '<', $ARGV[0];
my %hash1;
while (<IN>) {
    next unless ( $_ =~ m/^\@HWI/ );
    my ($header) = split(/ /, $_);
    $hash1{$header} = 1;   
}
close IN;

open IN2, '<', $ARGV[1];
my %hash2;
while (<IN2>) {
    next unless ( $_ =~ m/^\@HWI/ );
    my ($header) = split(/ /, $_);
    $hash2{$header} = 1;   
}
close IN;
[download]

Instead I thought I'd try doing both files at once. Like so:

my @threads = ("1","2");

# Loop through the array:
foreach(@threads){
    # Tell each thread to perform our 'parseLines()' subroutine.
    $_ = threads->create(\&parseLines, shift(@ARGV));
}

#Tries to join the running threads
#Some check implemented to avoid quitting the loop, before everything 
+is joined
my @running = threads->list(threads::running); #Array of running threa
+ds
my @joinable = threads->list(threads::joinable); #Array of joinable th
+reads
my @catcher;
while (scalar(@running) != 0 || scalar(@joinable) > 0) { #While as lon
+g as there are running or joinable threads
    @running = threads->list(threads::running); #Repopulate running, n
+ot sure if needed.
    foreach(@threads){
    if ($_->is_joinable()) {
        push(@catcher, $_->join()); #Put's parsed file as hash-ref int
+o array
    }
    }
    @joinable = threads->list(threads::joinable);
    @running = threads->list(threads::running);
}

sub parseLines{
    open IN, '<', $_[0];
    my %hash;
    while (<IN>) {
    next unless ( $_ =~ m/^\@HWI/ );
    my ($header) = split(/ /, $_);
    $hash{$header} = 1;   
    }
    close IN;
    return \%hash;
}
[download]

While the multithreaded code works, it's about 50% slower than the first one, so nothing is really accomplished there. It seems to me that maybe the problem is that the threads take a long time passing the created hashes back to the main script. So it's shuffling things around in memory, but I'm on really thin ice here I must admit. Any input is appreciated.

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