NAME¶
Parallel::ForkManager - A simple parallel processing fork manager
SYNOPSIS¶
use Parallel::ForkManager;
$pm = new Parallel::ForkManager($MAX_PROCESSES);
foreach $data (@all_data) {
# Forks and returns the pid for the child:
my $pid = $pm->start and next;
... do some work with $data in the child process ...
$pm->finish; # Terminates the child process
}
DESCRIPTION¶
This module is intended for use in operations that can be done in parallel where
the number of processes to be forked off should be limited. Typical use is a
downloader which will be retrieving hundreds/thousands of files.
The code for a downloader would look something like this:
use LWP::Simple;
use Parallel::ForkManager;
...
@links=(
["http://www.foo.bar/rulez.data","rulez_data.txt"],
["http://new.host/more_data.doc","more_data.doc"],
...
);
...
# Max 30 processes for parallel download
my $pm = new Parallel::ForkManager(30);
foreach my $linkarray (@links) {
$pm->start and next; # do the fork
my ($link,$fn) = @$linkarray;
warn "Cannot get $fn from $link"
if getstore($link,$fn) != RC_OK;
$pm->finish; # do the exit in the child process
}
$pm->wait_all_children;
First you need to instantiate the ForkManager with the "new"
constructor. You must specify the maximum number of processes to be created.
If you specify 0, then NO fork will be done; this is good for debugging
purposes.
Next, use $pm->start to do the fork. $pm returns 0 for the child process, and
child pid for the parent process (see also "
fork()" in
perlfunc(1p)). The "and next" skips the internal loop in the
parent process. NOTE: $pm->start dies if the fork fails.
$pm->finish terminates the child process (assuming a fork was done in the
"start").
NOTE: You cannot use $pm->start if you are already in the child process. If
you want to manage another set of subprocesses in the child process, you must
instantiate another Parallel::ForkManager object!
METHODS¶
- new $processes
- Instantiate a new Parallel::ForkManager object. You must
specify the maximum number of children to fork off. If you specify 0
(zero), then no children will be forked. This is intended for debugging
purposes.
- start [ $process_identifier ]
- This method does the fork. It returns the pid of the child
process for the parent, and 0 for the child process. If the $processes
parameter for the constructor is 0 then, assuming you're in the child
process, $pm->start simply returns 0.
An optional $process_identifier can be provided to this method... It is used
by the "run_on_finish" callback (see CALLBACKS) for identifying
the finished process.
- finish [ $exit_code ]
- Closes the child process by exiting and accepts an optional
exit code (default exit code is 0) which can be retrieved in the parent
via callback. If you use the program in debug mode ($processes == 0), this
method doesn't do anything.
- set_max_procs $processes
- Allows you to set a new maximum number of children to
maintain. Returns the previous setting.
- wait_all_children
- You can call this method to wait for all the processes
which have been forked. This is a blocking wait.
CALLBACKS¶
You can define callbacks in the code, which are called on events like starting a
process or upon finish.
The callbacks can be defined with the following methods:
- run_on_finish $code [, $pid ]
- You can define a subroutine which is called when a child is
terminated. It is called in the parent process.
The paremeters of the $code are the following:
- pid of the process, which is terminated
- exit code of the program
- identification of the process (if provided in the "start" method)
- exit signal (0-127: signal name)
- core dump (1 if there was core dump at exit)
- run_on_start $code
- You can define a subroutine which is called when a child is
started. It called after the successful startup of a child in the parent
process.
The parameters of the $code are the following:
- pid of the process which has been started
- identification of the process (if provided in the "start" method)
- run_on_wait $code, [$period]
- You can define a subroutine which is called when the child
process needs to wait for the startup. If $period is not defined, then one
call is done per child. If $period is defined, then $code is called
periodically and the module waits for $period seconds betwen the two
calls. Note, $period can be fractional number also. The exact
"$period seconds" is not guarranteed, signals can shorten and
the process scheduler can make it longer (on busy systems).
The $code called in the "start" and the
"wait_all_children" method also.
No parameters are passed to the $code on the call.
EXAMPLE¶
Parallel get
This small example can be used to get URLs in parallel.
use Parallel::ForkManager;
use LWP::Simple;
my $pm=new Parallel::ForkManager(10);
for my $link (@ARGV) {
$pm->start and next;
my ($fn)= $link =~ /^.*\/(.*?)$/;
if (!$fn) {
warn "Cannot determine filename from $fn\n";
} else {
$0.=" ".$fn;
print "Getting $fn from $link\n";
my $rc=getstore($link,$fn);
print "$link downloaded. response code: $rc\n";
};
$pm->finish;
};
Callbacks
Example of a program using callbacks to get child exit codes:
use strict;
use Parallel::ForkManager;
my $max_procs = 5;
my @names = qw( Fred Jim Lily Steve Jessica Bob Dave Christine Rico Sara );
# hash to resolve PID's back to child specific information
my $pm = new Parallel::ForkManager($max_procs);
# Setup a callback for when a child finishes up so we can
# get it's exit code
$pm->run_on_finish(
sub { my ($pid, $exit_code, $ident) = @_;
print "** $ident just got out of the pool ".
"with PID $pid and exit code: $exit_code\n";
}
);
$pm->run_on_start(
sub { my ($pid,$ident)=@_;
print "** $ident started, pid: $pid\n";
}
);
$pm->run_on_wait(
sub {
print "** Have to wait for one children ...\n"
},
0.5
);
foreach my $child ( 0 .. $#names ) {
my $pid = $pm->start($names[$child]) and next;
# This code is the child process
print "This is $names[$child], Child number $child\n";
sleep ( 2 * $child );
print "$names[$child], Child $child is about to get out...\n";
sleep 1;
$pm->finish($child); # pass an exit code to finish
}
print "Waiting for Children...\n";
$pm->wait_all_children;
print "Everybody is out of the pool!\n";
BUGS AND LIMITATIONS¶
Do not use Parallel::ForkManager in an environment, where other child processes
can affect the run of the main program, so using this module is not
recommended in an environment where
fork() /
wait() is already
used.
If you want to use more than one copies of the Parallel::ForkManager, then you
have to make sure that all children processes are terminated, before you use
the second object in the main program.
You are free to use a new copy of Parallel::ForkManager in the child processes,
although I don't think it makes sense.
COPYRIGHT¶
Copyright (c) 2000 Szabó, Balázs (dLux)
All right reserved. This program is free software; you can redistribute it
and/or modify it under the same terms as Perl itself.
AUTHOR¶
dLux (Szabó, Balázs) <dlux@kapu.hu>
CREDITS¶
Noah Robin <sitz@onastick.net> (documentation tweaks)
Chuck Hirstius <chirstius@megapathdsl.net> (callback exit status, example)
Grant Hopwood <hopwoodg@valero.com> (win32 port)
Mark Southern <mark_southern@merck.com> (bugfix)