GSP
Quick Navigator

Search Site

Unix VPS
A - Starter
B - Basic
C - Preferred
D - Commercial
MPS - Dedicated
Previous VPSs
* Sign Up! *

Support
Contact Us
Online Help
Handbooks
Domain Status
Man Pages

FAQ
Virtual Servers
Pricing
Billing
Technical

Network
Facilities
Connectivity
Topology Map

Miscellaneous
Server Agreement
Year 2038
Credits
 

USA Flag

 

 

Man Pages
MCE::Queue(3) User Contributed Perl Documentation MCE::Queue(3)
 

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

This document describes MCE::Queue version 1.837

 use MCE;
 use MCE::Queue;
 my $q = MCE::Queue->new;
 $q->enqueue( qw/ wherefore art thou romeo / );
 my $item = $q->dequeue;
 if ( $q->pending ) {
    ;
 }

This module provides a queue interface supporting normal and priority queues and utilizing the IPC engine behind MCE. Data resides under the manager process. 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)

MCE::Queue provides two run modes.
(A) The "MCE::Queue" object is constructed before running MCE. The data resides under the manager process. Workers send and request data via IPC.
(B) Workers might want to construct a queue for local access. In this mode, the data resides under the worker process and not available to other workers including the manager process.
 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->end() 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.
          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 taken from files_mce.pl and files_thr.pl on the web.
 https://github.com/marioroy/mce-examples/tree/master/other
 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

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";

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

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

Stops the queue from receiving more items. Any worker blocking on "dequeue" will be unblocked automatically. Subsequent calls to "dequeue" will behave like "dequeue_nb". Current API available since MCE 1.818.
 $q->end();
MCE Models (e.g. MCE::Flow) may persist between runs. In that case, one might want to enqueue "undef"'s versus calling "end". The number of "undef"'s depends on how many items workers dequeue at a time.
 $q->enqueue((undef) x ($N_workers * 1));  # $q->dequeue()   1 item
 $q->enqueue((undef) x ($N_workers * 2));  # $q->dequeue(2)  2 items
 $q->enqueue((undef) x ($N_workers * N));  # $q->dequeue(N)  N items

Appends a list of items onto the end of the normal queue.
 $q->enqueue( 'foo' );
 $q->enqueue( 'bar', 'baz' );

Appends a list of items onto the end of the priority queue with priority.
 $q->enqueue( $priority, 'foo' );
 $q->enqueue( $priority, 'bar', 'baz' );

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 whatever items there are on the queue.
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.
 # MCE::Queue 1.820 and prior releases
 while ( my @items = $q->dequeue(2) ) {
    last unless ( defined $items[0] );
    ...
 }
 # MCE::Queue 1.821 and later
 while ( my @items = $q->dequeue(2) ) {
    ...
 }

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 returns "undef" in the absence of data.
 $q->dequeue_nb( 2 );
 $q->dequeue_nb; # default 1

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

Adds the list of items to the queue at the specified index position with priority. The behavior is similarly to "$q-"insert> otherwise.

Returns the number of items in the queue. The count includes both normal and priority data. Returns "undef" if the queue has been ended, and there are no more items in the queue.
 $q = MCE::Queue->new();
 $q->enqueuep(5, 'foo', 'bar');
 $q->enqueue('sunny', 'day');
 print $q->pending(), "\n";
 # Output: 4

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

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.

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

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

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 synopsis above, was largely adopted from this module.

MCE, MCE::Core

Mario E. Roy, <marioeroy AT gmail DOT com>
2018-08-25 perl v5.28.1

Search for    or go to Top of page |  Section 3 |  Main Index

Powered by GSP Visit the GSP FreeBSD Man Page Interface.
Output converted with ManDoc.