Provided by: libmce-perl_1.889-1_all bug

NAME

       MCE::Loop - MCE model for building parallel loops

VERSION

       This document describes MCE::Loop version 1.889

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

        ## Important; pass an array_ref for deeply input data
        mce_loop { do_work($_) } [ [ 0, 1 ], [ 0, 2 ], ... ];
        mce_loop { do_work($_) } \@deeply_list;

        ## File path, glob ref, IO::All::{ File, Pipe, STDIO } obj, or scalar ref
        ## Workers read directly and not involve the manager process
        mce_loop_f { chomp; do_work($_) } "/path/to/file"; # efficient

        ## Involves the manager process, therefore slower
        mce_loop_f { chomp; do_work($_) } $file_handle;
        mce_loop_f { chomp; do_work($_) } $io;
        mce_loop_f { chomp; do_work($_) } \$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.

        ## Array or array_ref
        mce_loop { do_work($_) for (@{ $_ }) } 1..10000;
        mce_loop { do_work($_) for (@{ $_ }) } \@list;

        ## Important; pass an array_ref for deeply input data
        mce_loop { do_work($_) for (@{ $_ }) } [ [ 0, 1 ], [ 0, 2 ], ... ];
        mce_loop { do_work($_) for (@{ $_ }) } \@deeply_list;

        ## Resembles code using the core MCE 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
            init_relay => 0,                 # Default undef; MCE 1.882+
            use_threads => 0,                # Default undef; MCE 1.882+
        ;

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

        # Important; pass an array_ref for deeply input data

        mce_loop { do_work($_) } [ [ 0, 1 ], [ 0, 2 ], ... ];
        mce_loop { do_work($_) } \@deeply_list;

        # Chunking; any chunk_size => 1 or greater

        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.

       "IO::All" { File, Pipe, STDIO } is supported since MCE 1.845.

        # $_ contains the line when chunk_size => 1

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

        # chunking, any chunk_size => 1 or greater

        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 greater

        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>