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Mario Roy
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NAME

MCE::Shared::Queue - Hybrid-queue helper class

VERSION

This document describes MCE::Shared::Queue version 1.840

DESCRIPTION

A queue helper class for use as a standalone or managed by MCE::Shared.

This module is mostly compatible with MCE::Queue except for the gather option which is not supported in this context. It provides a queue interface supporting normal and priority queues. Data from shared queues reside under the shared-manager process, otherwise locally.

SYNOPSIS

 # non-shared or local construction for use by a single process

 use MCE::Shared::Queue;

 my $qu = MCE::Shared::Queue->new(
    await => 1, fast => 0, queue => [ "." ]
 );

 # construction for sharing with other threads and processes

 use MCE::Shared;
 use MCE::Shared::Queue;

 my $qu = MCE::Shared->queue(
    porder => $MCE::Shared::Queue::HIGHEST,
    type   => $MCE::Shared::Queue::FIFO,
    fast   => 0
 );

 # possible values for "porder" and "type"

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

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

 # below, [ ... ] denotes optional parameters

 $qu->await( [ $pending_threshold ] );
 $qu->clear();
 $qu->end();

 $qu->enqueue( $item [, $item, ... ] );
 $qu->enqueuep( $priority, $item [, $item, ... ] );

 $item  = $qu->dequeue();
 @items = $qu->dequeue( $count );
 $item  = $qu->dequeue_nb();
 @items = $qu->dequeue_nb( $count );
 
 $qu->insert( $index, $item [, $item, ... ] );
 $qu->insertp( $priority, $index, $item [, $item, ... ] );

 $count = $qu->pending();
 $item  = $qu->peek( [ $index ] );
 $item  = $qu->peekp( $priority [, $index ] );
 @array = $qu->heap();

API DOCUMENTATION

new ( [ options ] )

Constructs a new object. Supported options are queue, porder, type, await, and fast.

 # non-shared or local construction for use by a single process

 use MCE::Shared::Queue;

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

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

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

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

 # construction for sharing with other threads and processes

 use MCE::Shared;
 use MCE::Shared::Queue;

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

 $q3 = MCE::Shared->queue( porder => $MCE::Shared::Queue::HIGHEST );
 $q4 = MCE::Shared->queue( porder => $MCE::Shared::Queue::LOWEST  );

 $q5 = MCE::Shared->queue( type   => $MCE::Shared::Queue::FIFO );
 $q6 = MCE::Shared->queue( type   => $MCE::Shared::Queue::LIFO );

 $q7 = MCE::Shared->queue( await  => 1 );
 $q8 = MCE::Shared->queue( 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 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.

await ( pending_threshold )

Waits until the queue drops down to threshold items. The await method is beneficial when wanting to throttle worker(s) appending to the queue. Perhaps, consumers are running a bit behind and wanting prevent memory consumption from increasing too high. Below, the number of items pending will never go above 20.

 use Time::HiRes qw( sleep );

 use MCE::Flow;
 use MCE::Shared;

 my $q = MCE::Shared->queue( 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;
    }
 };
clear ( )

Clears the queue of any items.

 $q->clear;
end ( )

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::Shared 1.814.

 $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
enqueue ( item [, item, ... ] )

Appends a list of items onto the end of the normal queue.

 $q->enqueue( 'foo' );
 $q->enqueue( 'bar', 'baz' );
enqueuep ( priority, 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' );
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 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::Shared::Queue 1.816 and prior releases
 while ( my @items = $q->dequeue(2) ) {
    last unless ( defined $items[0] );
    ...
 }

 # MCE::Shared::Queue 1.817 and later
 while ( my @items = $q->dequeue(2) ) {
    ...
 }
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 returns undef in the absence of data.

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

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

pending ( )

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::Shared->queue();
 $q->enqueuep(5, 'foo', 'bar');
 $q->enqueue('sunny', 'day');

 print $q->pending(), "\n";
 # Output: 4
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::Shared->queue( type => $MCE::Shared::Queue::FIFO );
 $q->enqueue(1, 2, 3, 4, 5);

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

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

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

peekh ( [ index ] )

Returns an item from the head of the heap or at the specified index.

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

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

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

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

Returns an array containing the heap data. Heap data consists of priority numbers, not the data.

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

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

ACKNOWLEDGMENTS

List::BinarySearch

The bsearch_num_pos method was helpful for accommodating the highest and lowest order in MCE::Shared::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::Shared::Queue supports both normal and priority queues.

Thread::Queue

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

LIMITATIONS

Perl must have IO::FDPass for constructing a shared condvar or queue while the shared-manager process is running. For platforms where IO::FDPass isn't possible, construct condvar and queue before other classes. On systems without IO::FDPass, the manager process is delayed until sharing other classes or started explicitly.

 use MCE::Shared;

 my $has_IO_FDPass = $INC{'IO/FDPass.pm'} ? 1 : 0;

 my $cv  = MCE::Shared->condvar();
 my $que = MCE::Shared->queue();

 MCE::Shared->start() unless $has_IO_FDPass;

Regarding mce_open, IO::FDPass is needed for constructing a shared-handle from a non-shared handle not yet available inside the shared-manager process. The workaround is to have the non-shared handle made before the shared-manager is started. Passing a file by reference is fine for the three STD* handles.

 # The shared-manager knows of \*STDIN, \*STDOUT, \*STDERR.

 mce_open my $shared_in,  "<",  \*STDIN;   # ok
 mce_open my $shared_out, ">>", \*STDOUT;  # ok
 mce_open my $shared_err, ">>", \*STDERR;  # ok
 mce_open my $shared_fh1, "<",  "/path/to/sequence.fasta";  # ok
 mce_open my $shared_fh2, ">>", "/path/to/results.log";     # ok

 mce_open my $shared_fh, ">>", \*NON_SHARED_FH;  # requires IO::FDPass

The IO::FDPass module is known to work reliably on most platforms. Install 1.1 or later to rid of limitations described above.

 perl -MIO::FDPass -le "print 'Cheers! Perl has IO::FDPass.'"

INDEX

MCE, MCE::Hobo, MCE::Shared

AUTHOR

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