MCE::Loop(3)

NAME
MCE::Loop - MCE model for building parallel loops

VERSION
This document describes MCE::Loop version 1.837

DESCRIPTION
This module provides a parallel loop implementation through Many-Core Engine. MCE::Loop is not MCE::Map but more along the lines of an easy way to spin up a MCE instance and have user_func pointing to your code block. If you want something similar to map, then see MCE::Map.

 ## Construction when chunking is not desired

 use MCE::Loop;

 MCE::Loop::init {
    max_workers => 5, chunk_size => 1
 };

 mce_loop {
    my ($mce, $chunk_ref, $chunk_id) = @_;
    MCE->say("$chunk_id: $_");
 } 40 .. 48;

 -- Output

 3: 42
 1: 40
 2: 41
 4: 43
 5: 44
 6: 45
 7: 46
 8: 47
 9: 48

 ## Construction for 'auto' or greater than 1

 use MCE::Loop;

 MCE::Loop::init {
    max_workers => 5, chunk_size => 'auto'
 };

 mce_loop {
    my ($mce, $chunk_ref, $chunk_id) = @_;
    for (@{ $chunk_ref }) {
       MCE->say("$chunk_id: $_");
    }
 } 40 .. 48;

 -- Output

 1: 40
 2: 42
 1: 41
 4: 46
 2: 43
 5: 48
 3: 44
 4: 47
 3: 45

SYNOPSIS when CHUNK_SIZE EQUALS 1
All models in MCE default to 'auto' for chunk_size. The arguments for the block are the same as writing a user_func block using the Core API.

Beginning with MCE 1.5, the next input item is placed into the input scalar variable $_ when chunk_size equals 1. Otherwise, $_ points to $chunk_ref containing many items. Basically, line 2 below may be omitted from your code when using $_. One can call MCE->chunk_id to obtain the current chunk id.

 line 1:  user_func => sub {
 line 2:     my ($mce, $chunk_ref, $chunk_id) = @_;
 line 3:
 line 4:     $_ points to $chunk_ref->[0]
 line 5:        in MCE 1.5 when chunk_size == 1
 line 6:
 line 7:     $_ points to $chunk_ref
 line 8:        in MCE 1.5 when chunk_size  > 1
 line 9:  }

Follow this synopsis when chunk_size equals one. Looping is not required from inside the block. Hence, the block is called once per each item.

 ## Exports mce_loop, mce_loop_f, and mce_loop_s
 use MCE::Loop;

 MCE::Loop::init {
    chunk_size => 1
 };

 ## Array or array_ref
 mce_loop { do_work($_) } 1..10000;
 mce_loop { do_work($_) } [ 1..10000 ];

 ## File_path, glob_ref, or scalar_ref
 mce_loop_f { chomp; do_work($_) } "/path/to/file";
 mce_loop_f { chomp; do_work($_) } $file_handle;
 mce_loop_f { chomp; do_work($_) } \$scalar;

 ## Sequence of numbers (begin, end [, step, format])
 mce_loop_s { do_work($_) } 1, 10000, 5;
 mce_loop_s { do_work($_) } [ 1, 10000, 5 ];

 mce_loop_s { do_work($_) } {
    begin => 1, end => 10000, step => 5, format => undef
 };

SYNOPSIS when CHUNK_SIZE is GREATER THAN 1
Follow this synopsis when chunk_size equals 'auto' or greater than 1. This means having to loop through the chunk from inside the block.

 use MCE::Loop;

 MCE::Loop::init {          ## Chunk_size defaults to 'auto' when
    chunk_size => 'auto'    ## not specified. Therefore, the init
 };                         ## function may be omitted.

 ## Syntax is shown for mce_loop for demonstration purposes.
 ## Looping inside the block is the same for mce_loop_f and
 ## mce_loop_s.

 mce_loop { do_work($_) for (@{ $_ }) } 1..10000;

 ## Same as above, resembles code using the Core API.

 mce_loop {
    my ($mce, $chunk_ref, $chunk_id) = @_;

    for (@{ $chunk_ref }) {
       do_work($_);
    }

 } 1..10000;

Chunking reduces the number of IPC calls behind the scene. Think in terms of chunks whenever processing a large amount of data. For relatively small data, choosing 1 for chunk_size is fine.

OVERRIDING DEFAULTS
The following list options which may be overridden when loading the module.

 use Sereal qw( encode_sereal decode_sereal );
 use CBOR::XS qw( encode_cbor decode_cbor );
 use JSON::XS qw( encode_json decode_json );

 use MCE::Loop
     max_workers => 4,                # Default 'auto'
     chunk_size => 100,               # Default 'auto'
     tmp_dir => "/path/to/app/tmp",   # $MCE::Signal::tmp_dir
     freeze => \&encode_sereal,       # \&Storable::freeze
     thaw => \&decode_sereal          # \&Storable::thaw
 ;

From MCE 1.8 onwards, Sereal 3.015+ is loaded automatically if available. Specify "Sereal =" 0> to use Storable instead.

 use MCE::Loop Sereal => 0;

CUSTOMIZING MCE

MCE::Loop->init ( options )
MCE::Loop::init { options }
    The init function accepts a hash of MCE options.

     use MCE::Loop;

     MCE::Loop::init {
        chunk_size => 1, max_workers => 4,

        user_begin => sub {
           print "## ", MCE->wid, " started\n";
        },

        user_end => sub {
           print "## ", MCE->wid, " completed\n";
        }
     };

     my %a = mce_loop { MCE->gather($_, $_ * $_) } 1..100;

     print "\n", "@a{1..100}", "\n";

     -- Output

     ## 3 started
     ## 1 started
     ## 2 started
     ## 4 started
     ## 1 completed
     ## 2 completed
     ## 3 completed
     ## 4 completed

     1 4 9 16 25 36 49 64 81 100 121 144 169 196 225 256 289 324 361
     400 441 484 529 576 625 676 729 784 841 900 961 1024 1089 1156
     1225 1296 1369 1444 1521 1600 1681 1764 1849 1936 2025 2116 2209
     2304 2401 2500 2601 2704 2809 2916 3025 3136 3249 3364 3481 3600
     3721 3844 3969 4096 4225 4356 4489 4624 4761 4900 5041 5184 5329
     5476 5625 5776 5929 6084 6241 6400 6561 6724 6889 7056 7225 7396
     7569 7744 7921 8100 8281 8464 8649 8836 9025 9216 9409 9604 9801
     10000

API DOCUMENTATION
The following assumes chunk_size equals 1 in order to demonstrate all the possibilities for providing input data.

MCE::Loop->run ( sub { code }, list )
mce_loop { code } list
    Input data may be defined using a list, an array ref, or a hash ref.

     # $_ contains the item when chunk_size => 1

     mce_loop { $_ } 1..1000;
     mce_loop { $_ } \@list;

     # chunking, any chunk_size => 1 or higher

     my %res = mce_loop {
        my ($mce, $chunk_ref, $chunk_id) = @_;
        my %ret;
        for my $item (@{ $chunk_ref }) {
           $ret{$item} = $item * 2;
        }
        MCE->gather(%ret);
     }
     \@list;

     # input hash, current API available since 1.828

     my %res = mce_loop {
        my ($mce, $chunk_ref, $chunk_id) = @_;
        my %ret;
        for my $key (keys %{ $chunk_ref }) {
           $ret{$key} = $chunk_ref->{$key} * 2;
        }
        MCE->gather(%ret);
     }
     \%hash;

MCE::Loop->run_file ( sub { code }, file )
mce_loop_f { code } file
    The fastest of these is the /path/to/file. Workers communicate the next offset position among themselves with zero interaction by the manager process.

     # $_ contains the line when chunk_size => 1

     mce_loop_f { $_ } "/path/to/file";  # faster
     mce_loop_f { $_ } $file_handle;
     mce_loop_f { $_ } \$scalar;

     # chunking, any chunk_size => 1 or higher

     my %res = mce_loop_f {
        my ($mce, $chunk_ref, $chunk_id) = @_;
        my $buf = '';
        for my $line (@{ $chunk_ref }) {
           $buf .= $line;
        }
        MCE->gather($chunk_id, $buf);
     }
     "/path/to/file";

MCE::Loop->run_seq ( sub { code }, $beg, $end [, $step, $fmt ] )
mce_loop_s { code } $beg, $end [, $step, $fmt ]
    Sequence may be defined as a list, an array reference, or a hash reference. The functions require both begin and end values to run. Step and format are optional. The format is passed to sprintf (% may be omitted below).

     my ($beg, $end, $step, $fmt) = (10, 20, 0.1, "%4.1f");

     # $_ contains the sequence number when chunk_size => 1

     mce_loop_s { $_ } $beg, $end, $step, $fmt;
     mce_loop_s { $_ } [ $beg, $end, $step, $fmt ];

     mce_loop_s { $_ } {
        begin => $beg, end => $end,
        step => $step, format => $fmt
     };

     # chunking, any chunk_size => 1 or higher

     my %res = mce_loop_s {
        my ($mce, $chunk_ref, $chunk_id) = @_;
        my $buf = '';
        for my $seq (@{ $chunk_ref }) {
           $buf .= "$seq\n";
        }
        MCE->gather($chunk_id, $buf);
     }
     [ $beg, $end ];

    The sequence engine can compute 'begin' and 'end' items only, for the chunk, and not the items in between (hence boundaries only). This option applies to sequence only and has no effect when chunk_size equals 1.

    The time to run is 0.006s below. This becomes 0.827s without the bounds_only option due to computing all items in between, thus creating a very large array. Basically, specify bounds_only => 1 when boundaries is all you need for looping inside the block; e.g. Monte Carlo simulations.

    Time was measured using 1 worker to emphasize the difference.

     use MCE::Loop;

     MCE::Loop::init {
        max_workers => 1, chunk_size => 1_250_000,
        bounds_only => 1
     };

     # Typically, the input scalar $_ contains the sequence number
     # when chunk_size => 1, unless the bounds_only option is set
     # which is the case here. Thus, $_ points to $chunk_ref.

     mce_loop_s {
        my ($mce, $chunk_ref, $chunk_id) = @_;

        # $chunk_ref contains 2 items, not 1_250_000
        # my ( $begin, $end ) = ( $_->[0], $_->[1] );

        my $begin = $chunk_ref->[0];
        my $end   = $chunk_ref->[1];

        # for my $seq ( $begin .. $end ) {
        #    ...
        # }

        MCE->printf("%7d .. %8d\n", $begin, $end);
     }
     [ 1, 10_000_000 ];

     -- Output

           1 ..  1250000
     1250001 ..  2500000
     2500001 ..  3750000
     3750001 ..  5000000
     5000001 ..  6250000
     6250001 ..  7500000
     7500001 ..  8750000
     8750001 .. 10000000

MCE::Loop->run ( sub { code }, iterator )
mce_loop { code } iterator
    An iterator reference may be specified for input_data. Iterators are described under section "SYNTAX for INPUT_DATA" at MCE::Core.

     mce_loop { $_ } make_iterator(10, 30, 2);

GATHERING DATA
Unlike MCE::Map where gather and output order are done for you automatically, the gather method is used to have results sent back to the manager process.

 use MCE::Loop chunk_size => 1;

 ## Output order is not guaranteed.
 my @a1 = mce_loop { MCE->gather($_ * 2) } 1..100;
 print "@a1\n\n";

 ## Outputs to a hash instead (key, value).
 my %h1 = mce_loop { MCE->gather($_, $_ * 2) } 1..100;
 print "@h1{1..100}\n\n";

 ## This does the same thing due to chunk_id starting at one.
 my %h2 = mce_loop { MCE->gather(MCE->chunk_id, $_ * 2) } 1..100;
 print "@h2{1..100}\n\n";

The gather method may be called multiple times within the block unlike return which would leave the block. Therefore, think of gather as yielding results immediately to the manager process without actually leaving the block.

 use MCE::Loop chunk_size => 1, max_workers => 3;

 my @hosts = qw(
    hosta hostb hostc hostd hoste
 );

 my %h3 = mce_loop {
    my ($output, $error, $status); my $host = $_;

    ## Do something with $host;
    $output = "Worker ". MCE->wid .": Hello from $host";

    if (MCE->chunk_id % 3 == 0) {
       ## Simulating an error condition
       local $? = 1; $status = $?;
       $error = "Error from $host"
    }
    else {
       $status = 0;
    }

    ## Ensure unique keys (key, value) when gathering to
    ## a hash.
    MCE->gather("$host.out", $output);
    MCE->gather("$host.err", $error) if (defined $error);
    MCE->gather("$host.sta", $status);

 } @hosts;

 foreach my $host (@hosts) {
    print $h3{"$host.out"}, "\n";
    print $h3{"$host.err"}, "\n" if (exists $h3{"$host.err"});
    print "Exit status: ", $h3{"$host.sta"}, "\n\n";
 }

 -- Output

 Worker 2: Hello from hosta
 Exit status: 0

 Worker 1: Hello from hostb
 Exit status: 0

 Worker 3: Hello from hostc
 Error from hostc
 Exit status: 1

 Worker 2: Hello from hostd
 Exit status: 0

 Worker 1: Hello from hoste
 Exit status: 0

The following uses an anonymous array containing 3 elements when gathering data. Serialization is automatic behind the scene.

 my %h3 = mce_loop {
    ...

    MCE->gather($host, [$output, $error, $status]);

 } @hosts;

 foreach my $host (@hosts) {
    print $h3{$host}->[0], "\n";
    print $h3{$host}->[1], "\n" if (defined $h3{$host}->[1]);
    print "Exit status: ", $h3{$host}->[2], "\n\n";
 }

Although MCE::Map comes to mind, one may want additional control when gathering data such as retaining output order.

 use MCE::Loop;

 sub preserve_order {
    my %tmp; my $order_id = 1; my $gather_ref = $_[0];

    return sub {
       $tmp{ (shift) } = \@_;

       while (1) {
          last unless exists $tmp{$order_id};
          push @{ $gather_ref }, @{ delete $tmp{$order_id++} };
       }

       return;
    };
 }

 my @m2;

 MCE::Loop::init {
    chunk_size => 'auto', max_workers => 'auto',
    gather => preserve_order(\@m2)
 };

 mce_loop {
    my @a; my ($mce, $chunk_ref, $chunk_id) = @_;

    ## Compute the entire chunk data at once.
    push @a, map { $_ * 2 } @{ $chunk_ref };

    ## Afterwards, invoke the gather feature, which
    ## will direct the data to the callback function.
    MCE->gather(MCE->chunk_id, @a);

 } 1..100000;

 MCE::Loop::finish;

 print scalar @m2, "\n";

All 6 models support 'auto' for chunk_size unlike the Core API. Think of the models as the basis for providing JIT for MCE. They create the instance, tune max_workers, and tune chunk_size automatically regardless of the hardware.

The following does the same thing using the Core API.

 use MCE;

 sub preserve_order {
    ...
 }

 my $mce = MCE->new(
    max_workers => 'auto', chunk_size => 8000,

    user_func => sub {
       my @a; my ($mce, $chunk_ref, $chunk_id) = @_;

       ## Compute the entire chunk data at once.
       push @a, map { $_ * 2 } @{ $chunk_ref };

       ## Afterwards, invoke the gather feature, which
       ## will direct the data to the callback function.
       MCE->gather(MCE->chunk_id, @a);
    }
 );

 my @m2;

 $mce->process({ gather => preserve_order(\@m2) }, [1..100000]);
 $mce->shutdown;

 print scalar @m2, "\n";

MANUAL SHUTDOWN

MCE::Loop->finish
MCE::Loop::finish
    Workers remain persistent as much as possible after running. Shutdown occurs automatically when the script terminates. Call finish when workers are no longer needed.

     use MCE::Loop;

     MCE::Loop::init {
        chunk_size => 20, max_workers => 'auto'
     };

     mce_loop { ... } 1..100;

     MCE::Loop::finish;

INDEX
MCE, MCE::Core

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