MCE::Queue(3)

NAME
MCE::Queue - Hybrid (normal and priority) queues

VERSION
This document describes MCE::Queue version 1.703

SYNOPSIS
   use MCE;
   use MCE::Queue;

   my $q = MCE::Queue->new;

   $q->enqueue( qw/ wherefore art thou romeo / );

   my $item = $q->dequeue;

   if ( $q->pending ) {
      ;
   }

DESCRIPTION
This module provides a queue interface supporting normal and priority queues and utilizing the IPC engine behind MCE. Data resides under the manager process. MCE::Queue also allows for a worker to create any number of queues locally not available to other workers including the manager process. Think of a CPU having L3 (shared) and L1 (local) cache.

Three options are available for overriding the default value for new queues. The porder option applies to priority queues only.

   use MCE::Queue porder => $MCE::Queue::HIGHEST,
                  type   => $MCE::Queue::FIFO,
                  fast   => 0;

   use MCE::Queue;                # Same as above

   ## Possible values

   porder => $MCE::Queue::HIGHEST # Highest priority items dequeue first
             $MCE::Queue::LOWEST  # Lowest priority items dequeue first

   type   => $MCE::Queue::FIFO    # First in, first out
             $MCE::Queue::LIFO    # Last in, first out
             $MCE::Queue::LILO    # (Synonym for FIFO)
             $MCE::Queue::FILO    # (Synonym for LIFO)

THREE RUN MODES
MCE::Queue can be utilized under the following conditions:

   (A) use MCE;          (B) use MCE::Queue;   (C) use MCE::Queue;
       use MCE::Queue;       use MCE;

(A) MCE is included prior to inclusion of MCE::Queue
    The dequeue method blocks for the manager process including workers. All data resides under the manager process. Workers send/request data via IPC.

    Creating a queue from within the worker process will cause the queue to run in local mode (C). The data resides under the worker process and not available to other workers including the manager process.

(B) MCE::Queue is included prior to inclusion of MCE
    Queues behave as if running in local mode for the manager and worker processes for the duration of the script. I cannot think of a use-case for this, but mentioning the behavior in the event MCE::Queue is included before MCE.

(C) MCE::Queue without inclusion of MCE
    The dequeue method is non-blocking. Queues behave similarly to local queuing. This mode is efficient due to minimum overhead and zero IPC behind the scene. Hence, MCE is not required to use MCE::Queue.

DEMONSTRATION
   use MCE;
   use MCE::Queue;

   my $F = MCE::Queue->new(fast => 1);
   my $consumers = 8;

   my $mce = MCE->new(

      task_end => sub {
         my ($mce, $task_id, $task_name) = @_;

         $F->enqueue((undef) x $consumers)
            if $task_name eq 'dir';
      },

      user_tasks => [{
         max_workers => 1, task_name => 'dir',

         user_func => sub {
            ## Create a "standalone queue" only accessible to this worker.
            ## See included examples for running with multiple workers.
            my $D = MCE::Queue->new(queue => [ MCE->user_args->[0] ]);

            while (defined (my $dir = $D->dequeue_nb)) {
               my (@files, @dirs); foreach (glob("$dir/*")) {
                  if (-d $_) { push @dirs, $_; next; }
                  push @files, $_;
               }
               $D->enqueue(@dirs ) if scalar @dirs;
               $F->enqueue(@files) if scalar @files;
            }
         }
      },{
         max_workers => $consumers, task_name => 'file',

         user_func => sub {
            while (defined (my $file = $F->dequeue)) {
               MCE->say($file);
            }
         }
      }]

   )->run({ user_args => [ $ARGV[0] || '.' ] });

   __END__

   Results from files_mce.pl and files_thr.pl; included with MCE.
   Usage:
      time ./files_mce.pl /usr 0 | wc -l
      time ./files_mce.pl /usr 1 | wc -l
      time ./files_thr.pl /usr   | wc -l

   Darwin (OS)    /usr:    216,271 files
      MCE::Queue, fast => 0 :    4.17s
      MCE::Queue, fast => 1 :    2.62s
      Thread::Queue         :    4.14s

   Linux (VM)     /usr:    186,154 files
      MCE::Queue, fast => 0 :   12.57s
      MCE::Queue, fast => 1 :    3.36s
      Thread::Queue         :    5.91s

   Solaris (VM)   /usr:    603,051 files
      MCE::Queue, fast => 0 :   39.04s
      MCE::Queue, fast => 1 :   18.08s
      Thread::Queue      * Perl not built to support threads

API DOCUMENTATION

  MCE::Queue->new ( [ queue => \@array, await => 1, fast => 1 ] )
This creates a new queue. Available options are queue, porder, type, await, fast, and gather.

   use MCE;
   use MCE::Queue;

   my $q1 = MCE::Queue->new();
   my $q2 = MCE::Queue->new( queue  => [ 0, 1, 2 ] );

   my $q3 = MCE::Queue->new( porder => $MCE::Queue::HIGHEST );
   my $q4 = MCE::Queue->new( porder => $MCE::Queue::LOWEST  );

   my $q5 = MCE::Queue->new( type   => $MCE::Queue::FIFO );
   my $q6 = MCE::Queue->new( type   => $MCE::Queue::LIFO );

   my $q7 = MCE::Queue->new( await  => 1 );
   my $q8 = MCE::Queue->new( fast   => 1 );

The 'await' option, when enabled, allows workers to block (semaphore-like) until the number of items pending is equal or less than a threshold value. The $q->await method is described below.

The 'fast' option speeds up dequeues and is not enabled by default. It is beneficial for queues not calling (->clear or ->dequeue_nb) and not altering the optional count value while running; e.g. ->dequeue($count). Basically, do not enable 'fast' if varying the count dynamically.

The 'gather' option is mainly for running with MCE and wanting to pass item(s) to a callback function for appending to the queue. Multiple queues may point to the same callback function. The callback receives the queue object as the first argument and items after it.

   sub _append {
      my ($q, @items) = @_;
      $q->enqueue(@items);
   }

   my $q7 = MCE::Queue->new( gather => \&_append );
   my $q8 = MCE::Queue->new( gather => \&_append );

   ## Items are diverted to the callback function, not the queue.
   $q7->enqueue( 'apple', 'orange' );

Specifying the 'gather' option allows one to store items temporarily while ensuring output order. Although a queue object is not required, this is simply a demonstration of the gather option in the context of a queue.

   use MCE;
   use MCE::Queue;

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

      return sub {
         my ($q, $chunk_id, $data) = @_;
         $tmp{$chunk_id} = $data;

         while (1) {
            last unless exists $tmp{$order_id};
            $q->enqueue( delete $tmp{$order_id++} );
         }

         return;
      };
   }

   my @squares; my $q = MCE::Queue->new(
      queue => \@squares, gather => preserve_order
   );

   my $mce = MCE->new(
      chunk_size => 1, input_data => [ 1 .. 100 ],
      user_func => sub {
         $q->enqueue( MCE->chunk_id, $_ * $_ );
      }
   );

   $mce->run;

   print "@squares\n";

  $q->await ( $pending_threshold )
The await method is beneficial when wanting to throttle worker(s) appending to the queue. Perhaps, consumers are running a bit behind and wanting to keep tabs on memory consumption. Below, the number of items pending will never go above 20.

   use Time::HiRes qw( sleep );

   use MCE::Flow;
   use MCE::Queue;

   my $q = MCE::Queue->new( await => 1, fast => 1 );
   my ( $producers, $consumers ) = ( 1, 8 );

   mce_flow {
      task_name   => [ 'producer', 'consumer' ],
      max_workers => [ $producers, $consumers ],
   },
   sub {
      ## producer
      for my $item ( 1 .. 100 ) {
         $q->enqueue($item);

         ## blocks until the # of items pending reaches <= 10
         if ($item % 10 == 0) {
            MCE->say( 'pending: '.$q->pending() );
            $q->await(10);
         }
      }

      ## notify consumers no more work
      $q->enqueue( (undef) x $consumers );

   },
   sub {
      ## consumers
      while (defined (my $next = $q->dequeue())) {
         MCE->say( MCE->task_wid().': '.$next );
         sleep 0.100;
      }
   };

  $q->clear ( void )
Clears the queue of any items. This has the effect of nulling the queue and the socket used for blocking.

   my @a; my $q = MCE::Queue->new( queue => \@a );

   @a = ();     ## bad, the blocking socket may become out of sync
   $q->clear;   ## ok

  $q->enqueue ( $item [, $item, ... ] )
Appends a list of items onto the end of the normal queue.

   $q->enqueue( 'foo' );
   $q->enqueue( 'bar', 'baz' );

  $q->enqueuep ( $p, $item [, $item, ... ] )
Appends a list of items onto the end of the priority queue with priority.

   $q->enqueue( $priority, 'foo' );
   $q->enqueue( $priority, 'bar', 'baz' );

  $q->dequeue ( [ $count ] )
Returns the requested number of items (default 1) from the queue. Priority data will always dequeue first before any data from the normal queue.

   $q->dequeue( 2 );
   $q->dequeue; # default 1

The method will block if the queue contains zero items. If the queue contains fewer than the requested number of items, the method will not block, but return the remaining items and "undef" for up to the count requested.

The $count, used for requesting the number of items, is beneficial when workers are passing parameters through the queue. For this reason, always remember to dequeue using the same multiple for the count. This is unlike Thread::Queue which will block until the requested number of items are available.

  $q->dequeue_nb ( [ $count ] )
Returns the requested number of items (default 1) from the queue. Like with dequeue, priority data will always dequeue first. This method is non-blocking and will return "undef" in the absence of data from the queue.

   $q->dequeue_nb( 2 );
   $q->dequeue_nb; # default 1

  $q->insert ( $index, $item [, $item, ... ] )
Adds the list of items to the queue at the specified index position (0 is the head of the list). The head of the queue is that item which would be removed by a call to dequeue.

   $q = MCE::Queue->new( type => $MCE::Queue::FIFO );
   $q->enqueue(1, 2, 3, 4);
   $q->insert(1, 'foo', 'bar');
   # Queue now contains: 1, foo, bar, 2, 3, 4

   $q = MCE::Queue->new( type => $MCE::Queue::LIFO );
   $q->enqueue(1, 2, 3, 4);
   $q->insert(1, 'foo', 'bar');
   # Queue now contains: 1, 2, 3, 'foo', 'bar', 4

  $q->insertp ( $p, $index, $item [, $item, ... ] )
Adds the list of items to the queue at the specified index position with priority. The behavior is similarly to "$q-"insert> otherwise.

  $q->pending ( void )
Returns the number of items in the queue. The count includes both normal and priority data.

   $q = MCE::Queue->new();
   $q->enqueuep(5, 'foo', 'bar');
   $q->enqueue('sunny', 'day');

   print $q->pending(), "\n";
   # Output: 4

  $q->peek ( [ $index ] )
Returns an item from the normal queue, at the specified index, without dequeuing anything. It defaults to the head of the queue if index is not specified. The head of the queue is that item which would be removed by a call to dequeue. Negative index values are supported, similarly to arrays.

   $q = MCE::Queue->new( type => $MCE::Queue::FIFO );
   $q->enqueue(1, 2, 3, 4, 5);

   print $q->peek(1), ' ', $q->peek(-2), "\n";
   # Output: 2 4

   $q = MCE::Queue->new( type => $MCE::Queue::LIFO );
   $q->enqueue(1, 2, 3, 4, 5);

   print $q->peek(1), ' ', $q->peek(-2), "\n";
   # Output: 4 2

  $q->peekp ( $p [, $index ] )
Returns an item from the queue with priority, at the specified index, without dequeuing anything. It defaults to the head of the queue if index is not specified. The behavior is similarly to "$q-"peek> otherwise.

  $q->peekh ( [ $index ] )
Returns an item from the head of the heap or at the specified index.

   $q = MCE::Queue->new( porder => $MCE::Queue::HIGHEST );
   $q->enqueuep(5, 'foo');
   $q->enqueuep(6, 'bar');
   $q->enqueuep(4, 'sun');

   print $q->peekh(0), "\n";
   # Output: 6

   $q = MCE::Queue->new( porder => $MCE::Queue::LOWEST );
   $q->enqueuep(5, 'foo');
   $q->enqueuep(6, 'bar');
   $q->enqueuep(4, 'sun');

   print $q->peekh(0), "\n";
   # Output: 4

  $q->heap ( void )
Returns an array containing the heap data. Heap data consists of priority numbers, not the data.

   @h = $q->heap;   # $MCE::Queue::HIGHEST
   # Heap contains: 6, 5, 4

   @h = $q->heap;   # $MCE::Queue::LOWEST
   # Heap contains: 4, 5, 6

ACKNOWLEDGEMENTS

List::BinarySearch
    The bsearch_num_pos method was helpful for accommodating the highest and lowest order in MCE::Queue.

POE::Queue::Array
    For extra optimization, two if statements were adopted for checking if the item belongs at the end or head of the queue.

List::Priority
    MCE::Queue supports both normal and priority queues.

Thread::Queue
    Thread::Queue is used as a template for identifying and documenting the methods.

    MCE::Queue is not fully compatible due to supporting normal and priority queues simultaneously; e.g.

       $q->enqueue( $item [, $item, ... ] );         # normal queue
       $q->enqueuep( $p, $item [, $item, ... ] );    # priority queue

       $q->dequeue( [ $count ] );      # priority data dequeues first
       $q->dequeue_nb( [ $count ] );

       $q->pending();                  # counts both normal/priority queues

Parallel::DataPipe
    The recursion example, in the sysopsis above, was largely adopted from this module.

INDEX
MCE, MCE::Core

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