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NAME

       MCE::Step - Parallel step model for building creative steps

VERSION

       This document describes MCE::Step version 1.515

DESCRIPTION

       MCE::Step is similar to MCE::Flow for writing custom apps to maximize
       on all available cores. The main difference comes from the transparent
       inclusion of queues between sub-tasks.

       It's trivial to parallelize with mce_stream as shown below.

          ## Native map function
          my @a = map { $_ * 4 } map { $_ * 3 } map { $_ * 2 } 1..10000;

          ## Same as with MCE::Stream (processing from right to left)
          @a = mce_stream
               sub { $_ * 4 }, sub { $_ * 3 }, sub { $_ * 2 }, 1..10000;

          ## Pass an array reference to have writes occur simultaneously
          mce_stream \@a,
               sub { $_ * 4 }, sub { $_ * 3 }, sub { $_ * 2 }, 1..10000;

       However, let's have MCE::Step compute the same in parallel. Unlike the
       example shown in MCE::Flow, the MCE::Queue objects are created and
       managed for you automatically. The use of MCE::Queue is totally
       transparent.

          use MCE::Step;

       This calls for preserving output order. This time, will call on a
       function to emit a closure, the iterator itself for the gather option.
       The closure saves one from having to re-initialize $order_id prior to
       each run.

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

             @{ $gather_ref } = ();     ## Optional: clear the array

             return sub {
                $tmp{ $_[1] } = $_[0];

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

                return;
             };
          }

       Next are the 3 sub-tasks. Compare these 3 sub-tasks with the same as
       described in MCE::Flow. The call to MCE->step is all that's needed for
       passing data into the next sub-task.

          sub task_a {
             my @ans; my ($mce, $chunk_ref, $chunk_id) = @_;
             push @ans, map { $_ * 2 } @{ $chunk_ref };
             MCE->step(\@ans, $chunk_id);
          }

          sub task_b {
             my @ans; my ($mce, $chunk_ref, $chunk_id) = @_;
             push @ans, map { $_ * 3 } @{ $chunk_ref };
             MCE->step(\@ans, $chunk_id);
          }

          sub task_c {
             my @ans; my ($mce, $chunk_ref, $chunk_id) = @_;
             push @ans, map { $_ * 4 } @{ $chunk_ref };
             MCE->gather(\@ans, $chunk_id);
          }

       In summary, MCE::Step builds out a MCE instance behind the scene and
       starts running. Both task_name (shown below) and max_workers can take
       an anonymous array for specifying the values individually for each sub-
       task.

          my @a;

          mce_step {
             gather => output_iterator(\@a), task_name => [ 'a', 'b', 'c' ]

          }, \&task_a, \&task_b, \&task_c, 1..10000;

          print "@a
";

GATHER AND STEP DEMO

       One may call gather or step any number of times although calling step
       is not allowed from the very last sub-block. There's a lot going on
       below. For one, STDOUT output is serialized back to the main process.
       We see that chunk_size is specified when loading the module and set to
       1 for the demonstration.

       Data is gathered to @arr which may likely be out of order. Gathering
       data is optional and not a requirement to use MCE::Step. Note that all
       sub-tasks receive the $mce instance as the very first argument.

       The tasks below run in parallel, and each with multiple workers as
       well.  One may copy this code snippet and add MCE->task_wid or MCE->wid
       to the output. Remember that max_workers can take an anonymous array
       for specifying max_workers individually per each task block, similarly
       to task_name in the previous section.

          use MCE::Step chunk_size => 1;

          my @arr = mce_step

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

             if ($_ % 2 == 0) {
                MCE->gather($_);
              # MCE->gather($_ * 4);     ## Ok to gather multiple times
             }
             else {
                MCE->print("a step: $_, $_ * $_
");
                MCE->step($_, $_ * $_);
              # MCE->step($_, $_ * 4 );  ## Ok to step multiple times
             }
          },

          sub {
             my ($mce, $arg1, $arg2) = @_;

             MCE->print("b args: $arg1, $arg2
");

             if ($_ % 3 == 0) {          ## $_ is the same as $arg1
                MCE->gather($_);
             }
             else {
                MCE->print("b step: $_ * $_
");
                MCE->step($_ * $_);
             }
          },

          sub {
             my ($mce, $arg1) = @_;

             MCE->print("c: $_
");
             MCE->gather($_);
          },

          1..10;

          @arr = sort { $a <=> $b } @arr;

          print "
@arr

";

          -- Output

          a step: 1, 1 * 1
          a step: 3, 3 * 3
          a step: 5, 5 * 5
          a step: 7, 7 * 7
          a step: 9, 9 * 9
          b args: 1, 1
          b step: 1 * 1
          b args: 3, 9
          b args: 7, 49
          b step: 7 * 7
          b args: 5, 25
          b step: 5 * 5
          b args: 9, 81
          c: 1
          c: 49
          c: 25

          1 2 3 4 6 8 9 10 25 49

SYNOPSIS when CHUNK_SIZE EQUALS 1

       Although MCE::Loop may be preferred for running using a single code
       block, the text below also applies to this module, particularly for the
       first block.

       All models in MCE default to 'auto' for chunk_size. The arguments for
       the block are the same as writing a user_func block for 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 first block. Hence, the block is called once
       per each item.

          ## Exports mce_step, mce_step_f, and mce_step_s
          use MCE::Step;

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

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

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

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

          mce_step_s sub { 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 is greater than
       1.  This means having to loop through the chunk from inside the first
       block.

          use MCE::Step;

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

          ## Syntax is shown for mce_step for demonstration purposes.
          ## Looping inside the block is the same for mce_step_f and
          ## mce_step_s.

          mce_step sub { do_work($_) for (@{ $_ }) }, 1..10000;

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

          mce_step sub {
             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 5 options which may be overridden when loading the
       module.

          use Sereal qw(encode_sereal decode_sereal);

          use MCE::Step
                max_workers => 8,                    ## Default 'auto'
                chunk_size  => 500,                  ## Default 'auto'
                tmp_dir     => "/path/to/app/tmp",   ## $MCE::Signal::tmp_dir
                freeze      => \&encode_sereal,      ## \&Storable::freeze
                thaw        => \&decode_sereal       ## \&Storable::thaw
          ;

       There is a simpler way to enable Sereal with MCE 1.5. The following
       will attempt to use Sereal if available, otherwise will default back to
       using Storable for serialization.

          use MCE::Step Sereal => 1;

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

          ## Serialization is through Sereal if available.
          my %answer = mce_step sub { MCE->gather( $_, sqrt $_ ) }, 1..10000;

CUSTOMIZING MCE

       init
          The init function accepts a hash of MCE options. Unlike with
          MCE::Stream, both the gather and bounds_only options may be
          specified when calling init (not shown below).

             use MCE::Step;

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

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

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

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

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

             -- Output

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

       Like with MCE::Step::init above, MCE options may be specified using an
       anonymous hash for the first argument. Notice how both max_workers and
       task_name can take an anonymous array for setting values individually
       for each code block.

       Unlike MCE::Stream which processes from right-to-left, MCE::Step begins
       with the first code block, thus processing from left-to-right.

       The following script takes 9 seconds to complete. Removing both calls
       to MCE->step will cause the script to complete in just 1 second. The
       reason is due to the 2nd and subsequent sub-tasks awaiting data from
       their queues.  Workers terminate internally when receiving an undef
       from the queue. The 9 seconds is from having only 2 workers assigned
       for the last sub-task and waiting 1 or 2 seconds initially before
       calling MCE->step.

          use MCE::Step;

          my @a = mce_step {
             max_workers => [ 3, 4, 2, ], task_name => [ 'a', 'b', 'c' ],

             user_end => sub {
                my ($task_id, $task_name) = (MCE->task_id, MCE->task_name);
                MCE->print("$task_id - $task_name completed
");
             },

             task_end => sub {
                my ($mce, $task_id, $task_name) = @_;
                MCE->print("$task_id - $task_name ended
");
             }
          },
          sub { sleep 1; MCE->step(""); },   ## 3 workers, named a
          sub { sleep 2; MCE->step(""); },   ## 4 workers, named b
          sub { sleep 3;                };   ## 2 workers, named c

          -- Output

          0 - a completed
          0 - a completed
          0 - a completed
          0 - a ended
          1 - b completed
          1 - b completed
          1 - b completed
          1 - b completed
          1 - b ended
          2 - c completed
          2 - c completed
          2 - c ended

API DOCUMENTATION

       Although input data is optional for MCE::Step, the following assumes
       chunk_size equals 1 in order to demonstrate all the possibilities of
       passing input data.

       mce_step sub { code }, list
          Input data can be defined using a list or passing a reference to an
          array.

             mce_step sub { $_ }, 1..1000;
             mce_step sub { $_ }, [ 1..1000 ];

       mce_step_f sub { code }, file
          The fastest of these is the /path/to/file. Workers communicate the
          next offset position among themselves without any interaction from
          the manager process.

             mce_step_f sub { $_ }, "/path/to/file";
             mce_step_f sub { $_ }, $file_handle;
             mce_step_f sub { $_ }, \$scalar;

       mce_step_s sub { code }, sequence
          Sequence can 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");

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

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

       mce_step { input_data => iterator }, sub { code }
          An iterator reference can by specified for input data. Notice the
          anonymous hash as the first argument to mce_step. The only other way
          is to specify input_data via MCE::Step::init. This prevents
          MCE::Step from configuring the iterator reference as another user
          task which will not work.

          Iterators are described under "SYNTAX for INPUT_DATA" at MCE::Core.

             MCE::Step::init {
                input_data => iterator
             };

             mce_step sub { $_ };

       The sequence engine can compute the begin and end items only, for the
       chunk, leaving out the items in between with the bounds_only option
       (boundaries only).  This option applies to sequence and has no effect
       when chunk_size equals 1.

       The time to run for MCE below is 0.013s. This becomes 0.834s without
       the bounds_only option due to computing all items in between as well,
       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::Step;

          MCE::Step::init {
             max_workers => 1,
           # chunk_size  => 'auto',     ## btw, 'auto' will never drop below 2
             chunk_size  => 1_250_000,
             bounds_only => 1
          };

          ## For sequence, the input scalar $_ points to $chunk_ref
          ## when chunk_size > 1, otherwise equals $chunk_ref->[0].
          ##
          ## mce_step_s sub {
          ##    my $begin = $_->[0]; my $end = $_->[-1];
          ##
          ##    for ($begin .. $end) {
          ##       ... have fun with MCE ...
          ##    }
          ##
          ## }, 1, 10_000_000;

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

             ## $chunk_ref contains just 2 items, not 1_250_000

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

             MCE->printf("%7d .. %8d
", $begin, $end);

          }, 1, 10_000_000;

          -- Output

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

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::Step chunk_size => 1;

          ## Output order is not guaranteed.
          my @a = mce_step sub { MCE->gather($_ * 2) }, 1..100;
          print "@a

";

          ## However, one can store to a hash by gathering 2 items per
          ## each gather call (key, value).
          my %h1 = mce_step sub { MCE->gather($_, $_ * 2) }, 1..100;
          print "@h1{1..100}

";

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

";

       The gather method can 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::Step chunk_size => 1, max_workers => 3;

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

          my %h3 = mce_step sub {
             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"}, "
";
             print $h3{"$host.err"}, "
" if (exists $h3{"$host.err"});
             print "Exit status: ", $h3{"$host.sta"}, "

";
          }

          -- Output

          Worker 3: Hello from hosta
          Exit status: 0

          Worker 2: Hello from hostb
          Exit status: 0

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

          Worker 3: Hello from hostd
          Exit status: 0

          Worker 2: 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_step sub {

             ...

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

          }, @hosts;

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

";
          }

       Perhaps you want more control with gather such as appending to an array
       while retaining output order. Although MCE::Map comes to mind, some
       folks want "full" control. And here we go... but this time around in
       chunking style... :)

       The two options passed to MCE::Step are optional as they default to
       'auto'. The beauty of chunking data is that IPC occurs once per chunk
       versus once per item.  Although IPC is quite fast, chunking becomes
       beneficial the larger the data becomes. Hence the reason for the
       demonstration below.

          use MCE::Step chunk_size => 'auto', max_workers => 'auto';

          my (%_tmp, $_gather_ref, $_order_id);

          sub preserve_order {
             $_tmp{ (shift) } = \@_;

             while (1) {
                last unless exists $_tmp{$_order_id};
                push @{ $_gather_ref }, @{ $_tmp{$_order_id} };
                delete $_tmp{$_order_id++};
             }

             return;
          }

          ## Workers persist after running. Therefore, not recommended to
          ## use a closure for gather unless calling MCE::Step::init each
          ## time inside the loop. Use this demonstration when wanting
          ## MCE::Step to maintain output order.

          MCE::Step::init { gather => \&preserve_order };

          for (1..2) {
             my @m2;

             ## Remember to set $_order_id back to 1 prior to running.
             $_gather_ref = \@m2; $_order_id = 1;

             mce_step 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);

             }, 1..100000;

             print scalar @m2, "
";
          }

       All 6 models support 'auto' for chunk_size whereas the core API
       doesn't. Think of the models as the basis for providing JIT for MCE.
       They create the instance and tune max_workers plus chunk_size
       automatically irregardless of the hardware being run on.

       The following does the same thing using the core API.

          use MCE;

          ...

          my $mce = MCE->new(
             max_workers => 'auto', chunk_size => 8000,
             gather => \&preserve_order,

             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);
             }
          );

          $mce->process([1..100000]);

          ...

MANUAL SHUTDOWN

       finish
          MCE workers remain persistent as much as possible after running.
          Shutdown occurs when the script exits. One can manually shutdown MCE
          by simply calling finish after running. This resets the MCE
          instance.

             use MCE::Step;

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

             mce_step sub { ... }, 1..100;

             MCE::Step::finish;

INDEX

       MCE

AUTHOR

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

LICENSE

       This program is free software; you can redistribute it and/or modify it
       under the terms of either: the GNU General Public License as published
       by the Free Software Foundation; or the Artistic License.

       See <http://dev.perl.org/licenses/> for more information.



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