/ 
MCE-1.897
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/ MCE::Grep

NAME

MCE::Grep - Parallel grep model similar to the native grep function

VERSION

This document describes MCE::Grep version 1.897

SYNOPSIS

## Exports mce_grep, mce_grep_f, and mce_grep_s
use MCE::Grep;

## Array or array_ref
my @a = mce_grep { $_ % 5 == 0 } 1..10000;
my @b = mce_grep { $_ % 5 == 0 } \@list;

## Important; pass an array_ref for deeply input data
my @c = mce_grep { $_->[1] % 2 == 0 } [ [ 0, 1 ], [ 0, 2 ], ... ];
my @d = mce_grep { $_->[1] % 2 == 0 } \@deeply_list;

## File path, glob ref, IO::All::{ File, Pipe, STDIO } obj, or scalar ref
## Workers read directly and not involve the manager process
my @e = mce_grep_f { /pattern/ } "/path/to/file"; # efficient

## Involves the manager process, therefore slower
my @f = mce_grep_f { /pattern/ } $file_handle;
my @g = mce_grep_f { /pattern/ } $io;
my @h = mce_grep_f { /pattern/ } \$scalar;

## Sequence of numbers (begin, end [, step, format])
my @i = mce_grep_s { %_ * 3 == 0 } 1, 10000, 5;
my @j = mce_grep_s { %_ * 3 == 0 } [ 1, 10000, 5 ];

my @k = mce_grep_s { %_ * 3 == 0 } {
   begin => 1, end => 10000, step => 5, format => undef
};

DESCRIPTION

This module provides a parallel grep implementation via Many-Core Engine. MCE incurs a small overhead due to passing of data. A fast code block will run faster natively. However, the overhead will likely diminish as the complexity increases for the code.

my @m1 =     grep { $_ % 5 == 0 } 1..1000000;          ## 0.065 secs
my @m2 = mce_grep { $_ % 5 == 0 } 1..1000000;          ## 0.194 secs

Chunking, enabled by default, greatly reduces the overhead behind the scene. The time for mce_grep below also includes the time for data exchanges between the manager and worker processes. More parallelization will be seen when the code incurs additional CPU time.

my @m1 =     grep { /[2357][1468][9]/ } 1..1000000;    ## 0.353 secs
my @m2 = mce_grep { /[2357][1468][9]/ } 1..1000000;    ## 0.218 secs

Even faster is mce_grep_s; useful when input data is a range of numbers. Workers generate sequences mathematically among themselves without any interaction from the manager process. Two arguments are required for mce_grep_s (begin, end). Step defaults to 1 if begin is smaller than end, otherwise -1.

my @m3 = mce_grep_s { /[2357][1468][9]/ } 1, 1000000;  ## 0.165 secs

Although this document is about MCE::Grep, the MCE::Stream module can write results immediately without waiting for all chunks to complete. This is made possible by passing the reference to an array (in this case @m4 and @m5).

use MCE::Stream default_mode => 'grep';

my @m4; mce_stream \@m4, sub { /[2357][1468][9]/ }, 1..1000000;

   ## Completed in 0.203 secs. This is amazing considering the
   ## overhead for passing data between the manager and workers.

my @m5; mce_stream_s \@m5, sub { /[2357][1468][9]/ }, 1, 1000000;

   ## Completed in 0.120 secs. Like with mce_grep_s, specifying a
   ## sequence specification turns out to be faster due to lesser
   ## overhead for the manager process.

A common scenario is grepping for pattern(s) inside a massive log file. Notice how parallelism increases as complexity increases for the pattern. Testing was done against a 300 MB file containing 250k lines.

use MCE::Grep;

my @m; open my $LOG, "<", "/path/to/log/file" or die "$!\n";

@m = grep { /pattern/ } <$LOG>;                      ##  0.756 secs
@m = grep { /foobar|[2357][1468][9]/ } <$LOG>;       ## 24.681 secs

## Parallelism with mce_grep. This involves the manager process
## due to processing a file handle.

@m = mce_grep { /pattern/ } <$LOG>;                  ##  0.997 secs
@m = mce_grep { /foobar|[2357][1468][9]/ } <$LOG>;   ##  7.439 secs

## Even faster with mce_grep_f. Workers access the file directly
## with zero interaction from the manager process.

my $LOG = "/path/to/file";
@m = mce_grep_f { /pattern/ } $LOG;                  ##  0.112 secs
@m = mce_grep_f { /foobar|[2357][1468][9]/ } $LOG;   ##  6.840 secs

PARSING HUGE FILES

The MCE::Grep module lacks an optimization for quickly determining if a match is found from not knowing the pattern inside the code block. Use the following snippet as a template to achieve better performance. Also, take a look at examples/egrep.pl, included with the distribution.

use MCE::Loop;

MCE::Loop->init(
   max_workers => 8, use_slurpio => 1
);

my $pattern  = 'karl';
my $hugefile = 'very_huge.file';

my @result = mce_loop_f {
   my ($mce, $slurp_ref, $chunk_id) = @_;

   ## Quickly determine if a match is found.
   ## Process slurped chunk only if true.

   if ($$slurp_ref =~ /$pattern/m) {
      my @matches;

      ## The following is fast on Unix. Performance degrades
      ## drastically on Windows beyond 4 workers.

      open my $MEM_FH, '<', $slurp_ref;
      binmode $MEM_FH, ':raw';
      while (<$MEM_FH>) { push @matches, $_ if (/$pattern/); }
      close   $MEM_FH;

      ## Therefore, use the following construct on Windows.

      while ( $$slurp_ref =~ /([^\n]+\n)/mg ) {
         my $line = $1; # save $1 to not lose the value
         push @matches, $line if ($line =~ /$pattern/);
      }

      ## Gather matched lines.

      MCE->gather(@matches);
   }

} $hugefile;

print join('', @result);

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::Grep
    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::Grep Sereal => 0;

CUSTOMIZING MCE

MCE::Grep->init ( options )
MCE::Grep::init { options }

The init function accepts a hash of MCE options. The gather option, if specified, is ignored due to being used internally by the module.

In scalar context (API available since 1.897), call MCE::Grep-finish> automatically upon leaving the scope or program.

use MCE::Grep;

my $guard = MCE::Grep->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_grep { $_ % 5 == 0 } 1..100;

print "\n", "@a", "\n";

-- Output

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

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

API DOCUMENTATION

MCE::Grep->run ( sub { code }, list )
mce_grep { code } list

Input data may be defined using a list or an array reference. Unlike MCE::Loop, Flow, and Step, specifying a hash reference as input data isn't allowed.

## Array or array_ref
my @a = mce_grep { /[2357]/ } 1..1000;
my @b = mce_grep { /[2357]/ } \@list;

## Important; pass an array_ref for deeply input data
my @c = mce_grep { $_->[1] =~ /[2357]/ } [ [ 0, 1 ], [ 0, 2 ], ... ];
my @d = mce_grep { $_->[1] =~ /[2357]/ } \@deeply_list;

## Not supported
my @z = mce_grep { ... } \%hash;
MCE::Grep->run_file ( sub { code }, file )
mce_grep_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.

my @c = mce_grep_f { /pattern/ } "/path/to/file";  # faster
my @d = mce_grep_f { /pattern/ } $file_handle;
my @e = mce_grep_f { /pattern/ } $io;              # IO::All
my @f = mce_grep_f { /pattern/ } \$scalar;
MCE::Grep->run_seq ( sub { code }, $beg, $end [, $step, $fmt ] )
mce_grep_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");

my @f = mce_grep_s { /[1234]\.[5678]/ } $beg, $end, $step, $fmt;
my @g = mce_grep_s { /[1234]\.[5678]/ } [ $beg, $end, $step, $fmt ];

my @h = mce_grep_s { /[1234]\.[5678]/ } {
   begin => $beg, end => $end,
   step => $step, format => $fmt
};
MCE::Grep->run ( sub { code }, iterator )
mce_grep { code } iterator

An iterator reference may be specified for input_data. Iterators are described under section "SYNTAX for INPUT_DATA" at MCE::Core.

my @a = mce_grep { $_ % 3 == 0 } make_iterator(10, 30, 2);

MANUAL SHUTDOWN

MCE::Grep->finish
MCE::Grep::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::Grep;

MCE::Grep->init(
   chunk_size => 20, max_workers => 'auto'
);

my @a = mce_grep { ... } 1..100;

MCE::Grep->finish;

INDEX

MCE, MCE::Core

AUTHOR

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