Parallel::ForkManager(User Contributed Perl DocumentParallel::ForkManager(3pm)NAMEParallel::ForkManager - A simple parallel processing fork manager
VERSION
version 1.19
SYNOPSIS
use Parallel::ForkManager;
my $pm = Parallel::ForkManager->new($MAX_PROCESSES);
DATA_LOOP:
foreach my $data (@all_data) {
# Forks and returns the pid for the child:
my $pid = $pm->start and next DATA_LOOP;
... 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;
...
my @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 = Parallel::ForkManager->new(30);
LINKS:
foreach my $linkarray (@links) {
$pm->start and next LINKS; # do the fork
my ($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
The comment letter indicates where the method should be run. P for
parent, C for child.
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.
The optional second parameter, $tempdir, is only used if you want
the children to send back a reference to some data (see RETRIEVING
DATASTRUCTURES below). If not provided, it is set via a call to
File::Temp::tempdir().
The new method will die if the temporary directory does not exist
or it is not a directory.
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.
start_child [ $process_identifier, ] \&callback
Like "start", but will run the &callback as the child. If the
callback returns anything, it'll be passed as the data to transmit
back to the parent process via "finish()".
finish [ $exit_code [, $data_structure_reference] ]
Closes the child process by exiting and accepts an optional exit
code (default exit code is 0) which can be retrieved in the parent
via callback. If the second optional parameter is provided, the
child attempts to send its contents back to the parent. If you use
the program in debug mode ($processes == 0), this method just
calls the callback.
If the $data_structure_reference is provided, then it is
serialized and passed to the parent process. See RETRIEVING
DATASTRUCTURES for more info.
set_max_procs $processes
Allows you to set a new maximum number of children to maintain.
wait_all_children
You can call this method to wait for all the processes which have
been forked. This is a blocking wait.
reap_finished_children
This is a non-blocking call to reap children and execute callbacks
independent of calls to "start" or "wait_all_children". Use this
in scenarios where "start" is called infrequently but you would
like the callbacks executed quickly.
is_parent
Returns "true" if within the parent or "false" if within the
child.
is_child
Returns "true" if within the child or "false" if within the
parent.
max_procs
Returns the maximal number of processes the object will fork.
running_procs
Returns the pids of the forked processes currently monitored by
the "Parallel::ForkManager". Note that children are still reported
as running until the fork manager harvest them, via the next call
to "start" or "wait_all_children".
my @pids = $pm->running_procs;
my $nbr_children =- $pm->running_procs;
wait_for_available_procs( $n )
Wait until $n available process slots are available. If $n is not
given, defaults to 1.
waitpid_blocking_sleep
Returns the sleep period, in seconds, of the pseudo-blocking
calls. The sleep period can be a fraction of second.
Returns 0 if disabled.
Defaults to 1 second.
See BLOCKING CALLS for more details.
set_waitpid_blocking_sleep $seconds
Sets the the sleep period, in seconds, of the pseudo-blocking
calls. Set to 0 to disable.
See BLOCKING CALLS for more details.
CALLBACKS
You can define callbacks in the code, which are called on events like
starting a process or upon finish. Declare these before the first call
to start().
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 parameters 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)
- datastructure reference or undef (see RETRIEVING DATASTRUCTURES)
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 between the
two calls. Note, $period can be fractional number also. The exact
"$period seconds" is not guaranteed, 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.
BLOCKING CALLS
When it comes to waiting for child processes to terminate,
"Parallel::ForkManager" is between a fork and a hard place (if you
excuse the terrible pun). The underlying Perl "waitpid" function that
the module relies on can block until either one specific or any child
process terminate, but not for a process part of a given group.
This means that the module can do one of two things when it waits for
one of its child processes to terminate:
Only wait for its own child processes
This is done via a loop using a "waitpid" non-blocking call and a
sleep statement. The code does something along the lines of
while(1) {
if ( any of the P::FM child process terminated ) {
return its pid
}
sleep $sleep_period
}
This is the default behavior that the module will use. This is not
the most efficient way to wait for child processes, but it's the
safest way to ensure that "Parallel::ForkManager" won't interfere
with any other part of the codebase.
The sleep period is set via the method
"set_waitpid_blocking_sleep".
Block until any process terminate
Alternatively, "Parallel::ForkManager" can call "waitpid" such that
it will block until any child process terminate. If the child
process was not one of the monitored subprocesses, the wait will
resume. This is more efficient, but mean that "P::FM" can captures
(and discards) the termination notification that a different part
of the code might be waiting for.
If this is a race condition that doesn't apply to your codebase,
you can set the waitpid_blocking_sleep period to 0, which will
enable "waitpid" call blocking.
my $pm = Parallel::ForkManager->new( 4 );
$pm->set_waitpid_blocking_sleep(0); # true blocking calls enabled
for ( 1..100 ) {
$pm->start and next;
...; # do work
$pm->finish;
}
RETRIEVING DATASTRUCTURES from child processes
The ability for the parent to retrieve data structures is new as of
version 0.7.6.
Each child process may optionally send 1 data structure back to the
parent. By data structure, we mean a reference to a string, hash or
array. The contents of the data structure are written out to temporary
files on disc using the Storable modules' store() method. The reference
is then retrieved from within the code you send to the run_on_finish
callback.
The data structure can be any scalar perl data structure which makes
sense: string, numeric value or a reference to an array, hash or
object.
There are 2 steps involved in retrieving data structures:
1) A reference to the data structure the child wishes to send back to
the parent is provided as the second argument to the finish() call. It
is up to the child to decide whether or not to send anything back to
the parent.
2) The data structure reference is retrieved using the callback
provided in the run_on_finish() method.
Keep in mind that data structure retrieval is not the same as returning
a data structure from a method call. That is not what actually occurs.
The data structure referenced in a given child process is serialized
and written out to a file by Storable. The file is subsequently read
back into memory and a new data structure belonging to the parent
process is created. Please consider the performance penalty it can
imply, so try to keep the returned structure small.
EXAMPLES
Parallel get
This small example can be used to get URLs in parallel.
use Parallel::ForkManager;
use LWP::Simple;
my $pm = Parallel::ForkManager->new(10);
LINKS:
for my $link (@ARGV) {
$pm->start and next LINKS;
my ($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 $pm = Parallel::ForkManager->new($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
);
NAMES:
foreach my $child ( 0 .. $#names ) {
my $pid = $pm->start($names[$child]) and next NAMES;
# 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";
Data structure retrieval
In this simple example, each child sends back a string reference.
use Parallel::ForkManager 0.7.6;
use strict;
my $pm = Parallel::ForkManager->new(2, '/server/path/to/temp/dir/');
# data structure retrieval and handling
$pm -> run_on_finish ( # called BEFORE the first call to start()
sub {
my ($pid, $exit_code, $ident, $exit_signal, $core_dump, $data_structure_reference) = @_;
# retrieve data structure from child
if (defined($data_structure_reference)) { # children are not forced to send anything
my $string = ${$data_structure_reference}; # child passed a string reference
print "$string\n";
}
else { # problems occurring during storage or retrieval will throw a warning
print qq|No message received from child process $pid!\n|;
}
}
);
# prep random statement components
my @foods = ('chocolate', 'ice cream', 'peanut butter', 'pickles', 'pizza', 'bacon', 'pancakes', 'spaghetti', 'cookies');
my @preferences = ('loves', q|can't stand|, 'always wants more', 'will walk 100 miles for', 'only eats', 'would starve rather than eat');
# run the parallel processes
PERSONS:
foreach my $person (qw(Fred Wilma Ernie Bert Lucy Ethel Curly Moe Larry)) {
$pm->start() and next PERSONS;
# generate a random statement about food preferences
my $statement = $person . ' ' . $preferences[int(rand @preferences)] . ' ' . $foods[int(rand @foods)];
# send it back to the parent process
$pm->finish(0, \$statement); # note that it's a scalar REFERENCE, not the scalar itself
}
$pm->wait_all_children;
A second datastructure retrieval example demonstrates how children
decide whether or not to send anything back, what to send and how the
parent should process whatever is retrieved.
use Parallel::ForkManager 0.7.6;
use Data::Dumper; # to display the data structures retrieved.
use strict;
my $pm = Parallel::ForkManager->new(20); # using the system temp dir $L<File::Temp::tempdir()
# data structure retrieval and handling
my %retrieved_responses = (); # for collecting responses
$pm -> run_on_finish (
sub {
my ($pid, $exit_code, $ident, $exit_signal, $core_dump, $data_structure_reference) = @_;
# see what the child sent us, if anything
if (defined($data_structure_reference)) { # test rather than assume child sent anything
my $reftype = ref($data_structure_reference);
print qq|ident "$ident" returned a "$reftype" reference.\n\n|;
if (1) { # simple on/off switch to display the contents
print &Dumper($data_structure_reference) . qq|end of "$ident" sent structure\n\n|;
}
# we can also collect retrieved data structures for processing after all children have exited
$retrieved_responses{$ident} = $data_structure_reference;
} else {
print qq|ident "$ident" did not send anything.\n\n|;
}
}
);
# generate a list of instructions
my @instructions = ( # a unique identifier and what the child process should send
{'name' => '%ENV keys as a string', 'send' => 'keys'},
{'name' => 'Send Nothing'}, # not instructing the child to send anything back to the parent
{'name' => 'Childs %ENV', 'send' => 'all'},
{'name' => 'Child chooses randomly', 'send' => 'random'},
{'name' => 'Invalid send instructions', 'send' => 'Na Na Nana Na'},
{'name' => 'ENV values in an array', 'send' => 'values'},
);
INSTRUCTS:
foreach my $instruction (@instructions) {
$pm->start($instruction->{'name'}) and next INSTRUCTS; # this time we are using an explicit, unique child process identifier
# last step in child processing
$pm->finish(0) unless $instruction->{'send'}; # no data structure is sent unless this child is told what to send.
if ($instruction->{'send'} eq 'keys') {
$pm->finish(0, \join(', ', keys %ENV));
} elsif ($instruction->{'send'} eq 'values') {
$pm->finish(0, [values %ENV]); # kinda useless without knowing which keys they belong to...
} elsif ($instruction->{'send'} eq 'all') {
$pm->finish(0, \%ENV); # remember, we are not "returning" anything, just copying the hash to disc
# demonstrate clearly that the child determines what type of reference to send
} elsif ($instruction->{'send'} eq 'random') {
my $string = q|I'm just a string.|;
my @array = qw(I am an array);
my %hash = (type => 'associative array', synonym => 'hash', cool => 'very :)');
my $return_choice = ('string', 'array', 'hash')[int(rand 3)]; # randomly choose return data type
$pm->finish(0, \$string) if ($return_choice eq 'string');
$pm->finish(0, \@array) if ($return_choice eq 'array');
$pm->finish(0, \%hash) if ($return_choice eq 'hash');
# as a responsible child, inform parent that their instruction was invalid
} else {
$pm->finish(0, \qq|Invalid instructions: "$instruction->{'send'}".|); # ordinarily I wouldn't include invalid input in a response...
}
}
$pm->wait_all_children; # blocks until all forked processes have exited
# post fork processing of returned data structures
for (sort keys %retrieved_responses) {
print qq|Post processing "$_"...\n|;
}
SECURITYParallel::ForkManager uses temporary files when a child process returns
information to its parent process. The filenames are based on the
process of the parent and child processes, so they are fairly easy to
guess. So if security is a concern in your environment, make sure the
directory used by Parallel::ForkManager is restricted to the current
user only (the default behavior is to create a directory, via
File::Temp's "tempdir", which does that).
TROUBLESHOOTING
PerlIO::gzip and Parallel::ForkManager do not play nice together
If you are using PerlIO::gzip in your child processes, you may end up
with garbled files. This is not really P::FM's fault, but rather a
problem between PerlIO::gzip and "fork()" (see
<https://rt.cpan.org/Public/Bug/Display.html?id=114557>).
Fortunately, it seems there is an easy way to fix the problem by adding
the "unix" layer? I.e.,
open(IN, '<:unix:gzip', ...
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.
CREDITS
Michael Gang (bug report)
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)
Ken Clarke <www.perlprogrammer.net> (datastructure retrieval)
AUTHORS
· dLux (Szabó, Balázs) <dlux@dlux.hu>
· Yanick Champoux <yanick@cpan.org>
· Gabor Szabo <gabor@szabgab.com>
COPYRIGHT AND LICENSE
This software is copyright (c) 2000 by Balázs Szabó.
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.
perl v5.22.2 2016-07-20 Parallel::ForkManager(3pm)
