Perl Multithread Program - multithreading

Question

I am new in Perl. I want to write a Perl script using thread.I have few files say 20 files and want to process those files using 5 threads in 4 batches. I am printing the thread no. After completing one batch ,the thread no must start with 1 for the next batch. But instead of that its creating 20 threads.please help. my code is as follows:
#!/usr/bin/perl -w
use strict;
use warnings;
use threads;
use threads::shared;
my $INPUT_DIR="/home/Documents/myscript/IMPORTLDIF/";
opendir(DIR, $INPUT_DIR) ;
my #files = grep { /^InputFile/ } readdir DIR;
my $count = #files;
#print "Total Files: $count \n";
my #threads;
my $noofthread = 5;
my $nooffiles = $count;
my $noofbatch = $nooffiles / $noofthread;
#print "No of batch: $noofbatch \n";
my $fileIndex = 0;
my $batch = 1;
while ($fileIndex < $nooffiles) {
print "Batch: $batch \n";
for (my $i=0; $i < $noofthread && $fileIndex < $nooffiles ; $i++) {
my $t = threads->new(\&doOperation, $files[$fileIndex], $i)->join;
push(#threads, $t);
$fileIndex++;
print "FileIndex: $fileIndex \n";
}
$batch++;
}
sub doOperation () {
my $ithread = threads->tid() ;
print "Thread Index : [id=$ithread]\n" ;
foreach my $item (#_){
my $filename = $item;
print "Filename name: $filename \n";
}
Edited program using thread queue:
#!/usr/bin/perl -w
# This is compiled with threading support
use strict;
use warnings;
use threads;
use Thread::Queue;
my $q = Thread::Queue->new(); # A new empty queue
# Worker thread
my $INPUT_DIR="/home/Documents/myscript/IMPORTLDIF/";
opendir(DIR, $INPUT_DIR) or die "Cannot opendir: $!";
my #thrs = threads->create(\&doOperation ) for 1..5;#for 5 threads
#my #files = `ls -1 /home/Documents/myscript/IMPORTLDIF/`;
my #files = grep { /^Input/ } readdir DIR or die "File not present present. \n";
chomp(#files);
#add files to queue
foreach my $f (#files){
# Send work to the thread
$q->enqueue($f);
print "Pending items: " + $q->pending()."\n";
}
$q->enqueue('_DONE_') for #thrs;
$_->join() for #thrs;
sub doOperation () {
my $ithread = threads->tid() ;
while (my $filename = $q->dequeue()) {
# Do work on $item
return 1 if $filename eq '_DONE_';
print "[id=$ithread]\t$filename\n";
}
return 1;
}

Answer 1

You are spawning a thread and then waiting for it to complete before spawning the next, each thread handling one file. That is why you see as many threads as you have files.
my $t = threads->new(\&doOperation, $files[$fileIndex], $i)->join;
^^^^--- This will block
Instead try something like this:
....
# split the workload into N batches
#
while (my #batch = splice(#files, 0, $batch_size)) {
push #threads, threads->new(\&doOperation, #batch);
}
# now wait for all workers to finish
#
for my $thr (#threads) {
$thr->join;
}
As an aside, Thread::Queue and Thread-Pool might imply better designs for the work you want to do.

Answer 2

You could use Paralel:Queue and create 4 thread and pass them items that they could work on.
To fork or not to fork?
use strict;
use warnings;
use threads;
use Thread::Queue;
my $q = Thread::Queue->new(); # A new empty queue
# Worker thread
my #thrs;
push #thrs, threads->create(\&doOperation ) for 1..5;#for 5 threads
my #files = `ls -1 /tmp/`;chomp(#files);
#add files to queue
foreach my $f (#files){
# Send work to the thread
$q->enqueue($f);
print "Pending items: "$q->pending()."\n";
}
$q->enqueue('_DONE_') for #thrs;
$_->join() for threads->list();
sub doOperation () {
my $ithread = threads->tid() ;
while (my $filename = $q->dequeue()) {
# Do work on $item
return 1 if $filename eq '_DONE_';
print "[id=$ithread]\t$filename\n";
}
return 1;
}