/ 
MCE-1.898
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/ MCE::Channel

NAME

MCE::Channel - Queue-like and two-way communication capability

VERSION

This document describes MCE::Channel version 1.898

SYNOPSIS

use MCE::Channel;

########################
# Construction
########################

# A single producer and many consumers supporting processes and threads

my $c1 = MCE::Channel->new( impl => 'Mutex' );    # default implementation
my $c2 = MCE::Channel->new( impl => 'Threads' );  # threads::shared locking

# Set the mp flag if two or more workers (many producers) will be calling
# enqueue/send or recv2/recv2_nb on the left end of the channel

my $c3 = MCE::Channel->new( impl => 'Mutex', mp => 1 );
my $c4 = MCE::Channel->new( impl => 'Threads', mp => 1 );

# Tuned for one producer and one consumer, no locking

my $c5 = MCE::Channel->new( impl => 'Simple' );

########################
# Queue-like behavior
########################

# Send data to consumers
$c1->enqueue('item');
$c1->enqueue(qw/item1 item2 item3 itemN/);

# Receive data
my $item  = $c1->dequeue();      # item
my @items = $c1->dequeue(2);     # (item1, item2)

# Receive, non-blocking
my $item  = $c1->dequeue_nb();   # item
my @items = $c1->dequeue_nb(2);  # (item1, item2)

# Signal that there is no more work to be sent
$c1->end();

########################
# Two-way communication
########################

# Producer(s) sending data
$c3->send('message');
$c3->send(qw/arg1 arg2 arg3/);

# Consumer(s) receiving data
my $mesg = $c3->recv();          # message
my @args = $c3->recv();          # (arg1, arg2, arg3)

# Alternatively, non-blocking
my $mesg = $c3->recv_nb();       # message
my @args = $c3->recv_nb();       # (arg1, arg2, arg3)

# A producer signaling no more work to be sent
$c3->end();

# Consumers(s) sending data
$c3->send2('message');
$c3->send2(qw/arg1 arg2 arg3/);

# Producer(s) receiving data
my $mesg = $c3->recv2();         # message
my @args = $c3->recv2();         # (arg1, arg2, arg3)

# Alternatively, non-blocking
my $mesg = $c3->recv2_nb();      # message
my @args = $c3->recv2_nb();      # (arg1, arg2, arg3)

DESCRIPTION

A MCE::Channel object is a container for sending and receiving data using socketpair handles. Serialization is provided by Sereal if available. Defaults to Storable otherwise. Excluding the Simple implementation, both ends of the channel support many workers concurrently (with mp => 1).

new ( impl => STRING, mp => BOOLEAN )

This creates a new channel. Three implementations are provided Mutex, Threads, and Simple indicating the locking mechanism to use MCE::Mutex, threads::shared, and no locking respectively.

$chnl = MCE::Channel->new();     # default: impl => 'Mutex', mp => 0
                                 # default: impl => 'Threads' on Windows

The Mutex implementation supports processes and threads whereas the Threads implementation is suited for Windows and threads only.

$chnl = MCE::Channel->new( impl => 'Mutex' );    # MCE::Mutex locking
$chnl = MCE::Channel->new( impl => 'Threads' );  # threads::shared locking

# on Windows, silently becomes impl => 'Threads' when specifying 'Mutex'

Set the mp (m)any (p)roducers option to a true value if there will be two or more workers calling enqueue, <send>, recv2, or recv2_nb on the left end of the channel. This is important to not incur a race condition.

$chnl = MCE::Channel->new( impl => 'Mutex', mp => 1 );
$chnl = MCE::Channel->new( impl => 'Threads', mp => 1 );

# on Windows, silently becomes impl => 'Threads' when specifying 'Mutex'

The Simple implementation is optimized for one producer and one consumer max. It omits locking for maximum performance. This implementation is preferred for parent to child communication not shared by another worker.

$chnl = MCE::Channel->new( impl => 'Simple' );

QUEUE-LIKE BEHAVIOR

enqueue ( ITEM1 [, ITEM2, ... ] )

Appends a list of items onto the left end of the channel. This will block once the internal socket buffer becomes full (i.e. awaiting workers to dequeue on the other end). This prevents producer(s) from running faster than consumer(s).

Object (de)serialization is handled automatically using Sereal if available or defaults to Storable otherwise.

$chnl->enqueue('item1');
$chnl->enqueue(qw/item2 item3 .../);

$chnl->enqueue([ array_ref1 ]);
$chnl->enqueue([ array_ref2 ], [ array_ref3 ], ...);

$chnl->enqueue({ hash_ref1 });
$chnl->enqueue({ hash_ref2 }, { hash_ref3 }, ...);

dequeue

dequeue ( COUNT )

Removes the requested number of items (default 1) from the right end of the channel. If the channel contains fewer than the requested number of items, the method will block (i.e. until other producer(s) enqueue more items).

$item  = $chnl->dequeue();       # item1
@items = $chnl->dequeue(2);      # ( item2, item3 )

dequeue_nb

dequeue_nb ( COUNT )

Removes the requested number of items (default 1) from the right end of the channel. If the channel contains fewer than the requested number of items, the method will return what it was able to retrieve and return immediately. If the channel is empty, then returns an empty list in list context or undef in scalar context.

$item  = $chnl->dequeue_nb();    # array_ref1
@items = $chnl->dequeue_nb(2);   # ( array_ref2, array_ref3 )

end

This is called by a producer to signal that there is no more work to be sent. Once ended, no more items may be sent by the producer. Calling end by multiple producers is not supported.

$chnl->end;

TWO-WAY IPC - PRODUCER TO CONSUMER

send ( ARG1 [, ARG2, ... ] )

Append data onto the left end of the channel. Unlike enqueue, the values are kept together for the receiving consumer, similarly to calling a method. Object (de)serialization is handled automatically.

$chnl->send('item');
$chnl->send([ list_ref ]);
$chnl->send([ hash_ref ]);

$chnl->send(qw/item1 item2 .../);
$chnl->send($id, [ list_ref ]);
$chnl->send($id, { hash_ref });

The fast channel implementations, introduced in MCE 1.877, support one item for send. If you want to pass multiple arguments, simply join the arguments into a string. That means the receiver will need to split the string.

$chnl = MCE::Channel->new(impl => "SimpleFast");

$chnl->send(join(" ", qw/item1 item2 item3/);
my ($item1, $item2, $item3) = split " ", $chnl->recv();

recv

recv_nb

Blocking and non-blocking fetch methods from the right end of the channel. For the latter and when the channel is empty, returns an empty list in list context or undef in scalar context.

$item      = $chnl->recv();
$array_ref = $chnl->recv();
$hash_ref  = $chnl->recv();

($item1, $item2)  = $chnl->recv_nb();
($id, $array_ref) = $chnl->recv_nb();
($id, $hash_ref)  = $chnl->recv_nb();

TWO-WAY IPC - CONSUMER TO PRODUCER

send2 ( ARG1 [, ARG2, ... ] )

Append data onto the right end of the channel. Unlike enqueue, the values are kept together for the receiving producer, similarly to calling a method. Object (de)serialization is handled automatically.

$chnl->send2('item');
$chnl->send2([ list_ref ]);
$chnl->send2([ hash_ref ]);

$chnl->send2(qw/item1 item2 .../);
$chnl->send2($id, [ list_ref ]);
$chnl->send2($id, { hash_ref });

The fast channel implementations, introduced in MCE 1.877, support one item for send2. If you want to pass multiple arguments, simply join the arguments into a string. Not to forget, the receiver must split the string as well.

$chnl = MCE::Channel->new(impl => "MutexFast");

$chnl->send2(join(" ", qw/item1 item2 item3/);
my ($item1, $item2, $item3) = split " ", $chnl->recv();

recv2

recv2_nb

Blocking and non-blocking fetch methods from the left end of the channel. For the latter and when the channel is empty, returns an empty list in list context or undef in scalar context.

$item      = $chnl->recv2();
$array_ref = $chnl->recv2();
$hash_ref  = $chnl->recv2();

($item1, $item2)  = $chnl->recv2_nb();
($id, $array_ref) = $chnl->recv2_nb();
($id, $hash_ref)  = $chnl->recv2_nb();

DEMONSTRATIONS

Example 1 - threads

MCE::Channel was made to work efficiently with threads. The reason comes from using threads::shared for locking versus MCE::Mutex.

use strict;
use warnings;

use threads;
use MCE::Channel;

my $queue = MCE::Channel->new( impl => 'Threads' );
my $num_consumers = 10;

sub consumer {
   my $count = 0;

   # receive items
   while ( my ($item1, $item2) = $queue->dequeue(2) ) {
      $count += 2;
   }

   # send result
   $queue->send2( threads->tid => $count );
}

threads->create('consumer') for 1 .. $num_consumers;

## producer

$queue->enqueue($_, $_ * 2) for 1 .. 40000;
$queue->end;

my %results;
my $total = 0;

for ( 1 .. $num_consumers ) {
   my ($id, $count) = $queue->recv2;
   $results{$id} = $count;
   $total += $count;
}

$_->join for threads->list;

print $results{$_}, "\n" for keys %results;
print "$total total\n\n";

__END__

# output

8034
8008
8036
8058
7990
7948
8068
7966
7960
7932
80000 total

Example 2 - MCE::Child

The following is similarly threads-like for Perl lacking threads support. It spawns processes instead, thus requires the Mutex channel implementation which is the default if omitted.

use strict;
use warnings;

use MCE::Child;
use MCE::Channel;

my $queue = MCE::Channel->new( impl => 'Mutex' );
my $num_consumers = 10;

sub consumer {
   my $count = 0;

   # receive items
   while ( my ($item1, $item2) = $queue->dequeue(2) ) {
      $count += 2;
   }

   # send result
   $queue->send2( MCE::Child->pid => $count );
}

MCE::Child->create('consumer') for 1 .. $num_consumers;

## producer

$queue->enqueue($_, $_ * 2) for 1 .. 40000;
$queue->end;

my %results;
my $total = 0;

for ( 1 .. $num_consumers ) {
   my ($id, $count) = $queue->recv2;
   $results{$id} = $count;
   $total += $count;
}

$_->join for MCE::Child->list;

print $results{$_}, "\n" for keys %results;
print "$total total\n\n";

Example 3 - Consumer requests item

Like the previous example, but have the manager process await a notification from the consumer before inserting into the queue. This allows the producer to end the channel early (i.e. exit loop).

use strict;
use warnings;

use MCE::Child;
use MCE::Channel;

my $queue = MCE::Channel->new( impl => 'Mutex' );
my $num_consumers = 10;

sub consumer {
   # receive items
   my $count = 0;

   while () {
      # Notify the manager process to send items. This allows the
      # manager process to enqueue only when requested. The benefit
      # is being able to end the channel immediately.

      $queue->send2( MCE::Child->pid ); # channel is bi-directional

      my ($item1, $item2) = $queue->dequeue(2);
      last unless ( defined $item1 );   # channel ended

      $count += 2;
   }

   # result
   return ( MCE::Child->pid => $count );
}

MCE::Child->create('consumer') for 1 .. $num_consumers;

## producer

for my $num (1 .. 40000) {
   # Await worker notification before inserting (blocking).
   my $consumer_pid = $queue->recv2;
   $queue->enqueue($num, $num * 2);
}

$queue->end;

my %results;
my $total = 0;

for my $child ( MCE::Child->list ) {
   my ($id, $count) = $child->join;
   $results{$id} = $count;
   $total += $count;
}

print $results{$_}, "\n" for keys %results;
print "$total total\n\n";

Example 4 - Many producers

Running with 2 or more producers requires setting the mp option. Internally, this enables locking support for the left end of the channel. The mp option applies to Mutex and Threads channel implementations only.

Here, using the MCE facility for gathering the final count.

use strict;
use warnings;

use MCE::Flow;
use MCE::Channel;

my $queue = MCE::Channel->new( impl => 'Mutex', mp => 1 );
my $num_consumers = 10;

sub consumer {
   # receive items
   my $count = 0;
   while ( my ( $item1, $item2 ) = $queue->dequeue(2) ) {
      $count += 2;
   }
   # send result
   MCE->gather( MCE->wid => $count );
}

sub producer {
   $queue->enqueue($_, $_ * 2) for 1 .. 20000;
}

## run 2 producers and many consumers

MCE::Flow->init(
   max_workers => [ 2, $num_consumers ],
   task_name   => [ 'producer', 'consumer' ],
   task_end    => sub {
      my ($mce, $task_id, $task_name) = @_;
      if ( $task_name eq 'producer' ) {
         $queue->end;
      }
   }
);

# consumers call gather above (i.e. send a key-value pair),
# have MCE append to a hash

my %results = mce_flow \&producer, \&consumer;

MCE::Flow->finish;

my $total = 0;

for ( keys %results ) {
   $total += $results{$_};
   print $results{$_}, "\n";
}

print "$total total\n\n";

Example 5 - Many channels

This demonstration configures a channel per consumer. Plus, a common channel for consumers to request the next input item. The Simple implementation is specified for the individual channels whereas locking may be necessary for the $ready channel. However, consumers do not incur reading and what is written is very small (i.e. atomic write is guaranteed by the OS). Thus, am safely choosing the Simple implementation versus Mutex.

use strict;
use warnings;

use MCE::Flow;
use MCE::Channel;

my $prog_name  = $0; $prog_name =~ s{^.*[\\/]}{}g;
my $input_size = shift || 3000;

unless ($input_size =~ /\A\d+\z/) {
   print {*STDERR} "usage: $prog_name [ size ]\n";
   exit 1;
}

my $consumers = 4;

my @chnls = map { MCE::Channel->new( impl => 'Simple' ) } 1 .. $consumers;

my $ready =       MCE::Channel->new( impl => 'Simple' );

sub producer {
   my $id = 0;

   # send the next input item upon request
   for ( 0 .. $input_size - 1 ) {
      my $chnl_num = $ready->recv2;
      $chnls[ $chnl_num ]->send( ++$id, $_ );
   }

   # signal no more work
   $_->send( 0, undef ) for @chnls;
}

sub consumer {
   my $chnl_num = MCE->task_wid - 1;

   while () {
      # notify the producer ready for input
      $ready->send2( $chnl_num );

      # retrieve input data
      my ( $id, $item ) = $chnls[ $chnl_num ]->recv;

      # leave loop if no more work
      last unless $id;

      # compute and send the result to the manager process
      # ordered output requires an id (must be 1st argument)
      MCE->gather( $id, [ $item, sqrt($item) ] );
   }
}

# A custom 'ordered' output iterator for MCE's gather facility.
# It returns a closure block, expecting an ID for 1st argument.

sub output_iterator {
   my %tmp; my $order_id = 1;

   return sub {
      my ( $id, $result ) = @_;
      $tmp{ $id } = $result;

      while () {
         last unless exists $tmp{ $order_id };
         $result = delete $tmp{ $order_id };
         printf "n: %d sqrt(n): %f\n", $result->[0], $result->[1];
         $order_id++;
      }
   };
}

# Run one producer and many consumers.
# Output to be sent orderly to STDOUT.

MCE::Flow->init(
   gather => output_iterator(),
   max_workers => [ 1, $consumers ],
);

MCE::Flow->run( \&producer, \&consumer );
MCE::Flow->finish;

__END__

# Output

n: 0 sqrt(n): 0.000000
n: 1 sqrt(n): 1.000000
n: 2 sqrt(n): 1.414214
n: 3 sqrt(n): 1.732051
n: 4 sqrt(n): 2.000000
n: 5 sqrt(n): 2.236068
n: 6 sqrt(n): 2.449490
n: 7 sqrt(n): 2.645751
n: 8 sqrt(n): 2.828427
n: 9 sqrt(n): 3.000000
...

SEE ALSO

AUTHOR

Mario E. Roy, <marioeroy AT gmail DOT com>

COPYRIGHT AND LICENSE

Copyright (C) 2019-2024 by Mario E. Roy

MCE::Channel is released under the same license as Perl.

See https://dev.perl.org/licenses/ for more information.