Elizabeth Mattijsen > Thread-Conveyor > Thread::Conveyor

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Module Version: 0.19   Source  

NAME ^

Thread::Conveyor - transport of any data-structure between threads

VERSION ^

This documentation describes version 0.19.

SYNOPSIS ^

    use Thread::Conveyor;
    my $belt = Thread::Conveyor->new(
     {
      maxboxes => 50,
      minboxes => 25,
      optimize => 'memory', # or 'cpu'
     }
    );

    $belt->put( "foo", ["bar"], {"zoo"} );
    my ($foo,$bar,$zoo) = $belt->take;
    my ($foo,$bar,$zoo) = $belt->take_dontwait;
    my ($foo,$bar,$zoo) = $belt->peek;
    my ($foo,$bar,$zoo) = $belt->peek_dontwait;
    my $onbelt = $belt->onbelt;

    my @box = $belt->clean;
    my @box = $belt->clean_dontwait;
    my ($foo,$bar,$zoo) = @{$box[0]};

    $belt->maxboxes( 100 );
    $belt->minboxes( 50 );

    $belt->shutdown;
    $belt->thread;
    $belt->tid;

DESCRIPTION ^

                  *** A note of CAUTION ***

 This module only functions on Perl versions 5.8.0 and later.
 And then only when threads are enabled with -Dusethreads.  It
 is of no use with any version of Perl before 5.8.0 or without
 threads enabled.

                  *************************

The Thread::Conveyor object is a thread-safe data structure that mimics the behaviour of a conveyor belt. One or more worker threads can put boxes with frozen values and references on one end of the belt to be taken off by one or more worker threads on the other end of the belt to be thawed and returned.

A box may consist of any combination of scalars and references to scalars, arrays (lists) and hashes. Freezing and thawing is currently done with the Thread::Serialize module, but that may change in the future. Objects and code references are currently not allowed.

By default, the maximum number of boxes on the belt is limited to 50. Putting of boxes on the belt is halted if the maximum number of boxes is exceeded. This throttling feature was added because it was found that excessive memory usage could be caused by having the belt growing too large. Throttling can be disabled if so desired.

CLASS METHODS ^

new

 $belt = Thread::Conveyor->new(
  {
   maxboxes => 50,
   minboxes => 25,
   optimize => 'memory', # or 'cpu'
  }
 );

The "new" function creates a new empty belt. It returns the instantiated Thread::Conveyor object.

The input parameter is a reference to a hash. The following fields are optional in the hash reference:

maxboxes
 maxboxes => 50,

 maxboxes => undef,  # disable throttling

The "maxboxes" field specifies the maximum number of boxes that can be sitting on the belt to be handled (throttling). If a new put would exceed this amount, putting of boxes will be halted until the number of boxes waiting to be handled has become at least as low as the amount specified with the "minboxes" field.

Fifty boxes will be assumed for the "maxboxes" field if it is not specified. If you do not want to have any throttling, you can specify the value "undef" for the field. But beware! If you do not have throttling active, you may wind up using excessive amounts of memory used for storing all of the boxes that have not been handled yet.

The maxboxes method can be called to change the throttling settings during the lifetime of the object.

minboxes
 minboxes => 25, # default: maxboxes / 2

The "minboxes" field specifies the minimum number of boxes that can be waiting on the belt to be handled before the putting of boxes is allowed again (throttling).

If throttling is active and the "minboxes" field is not specified, then half of the "maxboxes" value will be assumed.

The minboxes method can be called to change the throttling settings during the lifetime of the object.

optimize
 optimize => 'cpu', # default: depends on Perl version

The "optimize" field specifies which implementation of the belt will be selected. Currently there are two choices: 'cpu' and 'memory'. For Perl 5.8.0 the default is "memory". For higher versions of perl, the default optimization is "cpu". The reason for this was that Perl 5.8.0 has a severe memory leak with shared arrays, which is what is being used with the "cpu" optimization.

You can call the class method optimize to change the default optimization.

optimize

 Thread::Conveyor->optimize( 'cpu' );

 $optimize = Thread::Conveyor->optimize;

The "optimize" class method allows you to specify the default optimization type that will be used if no "optimize" field has been explicitely specified with a call to new. It returns the current default type of optimization.

Currently two types of optimization can be selected:

memory

Attempt to use as little memory as possible. Currently, this is achieved by starting a seperate thread which hosts an unshared array. This uses the "Thread::Conveyor::Thread" sub-class.

cpu

Attempt to use as little CPU as possible. Currently, this is achieved by using a shared array (using the "Thread::Conveyor::Array" sub-class), encapsulated in a hash reference if throttling is activated (then also using the "Thread::Conveyor::Throttled" sub-class).

OBJECT METHODS ^

The following methods operate on the instantiated Thread::Conveyor object.

put

 $belt->put( 'string',$scalar,[],{} );

The "put" method freezes all the specified parameters together in a box and puts the box on the beginning of the belt.

take

 ($string,$scalar,$listref,$hashref) = $belt->take;

The "take" method waits for a box to become available at the end of the belt, removes that box from the belt, thaws the contents of the box and returns the resulting values and references.

take_dontwait

 ($string,$scalar,$listref,$hashref) = $belt->take_dontwait;

The "take_dontwait" method, like the take method, removes a box from the end of the belt if there is a box waiting at the end of the belt. If there is no box available, then the "take_dontwait" method will return immediately with an empty list. Otherwise the contents of the box will be thawed and the resulting values and references will be returned.

clean

 @box = $belt->clean;
 ($string,$scalar,$listref,$hashref) = @{$box[0]};

The "clean" method waits for one or more boxes to become available at the end of the belt, removes all boxes from the belt, thaws the contents of the boxes and returns the resulting values and references as an array where each element is a reference to the original contents of each box.

clean_dontwait

 @box = $belt->clean_dontwait;
 ($string,$scalar,$listref,$hashref) = @{$box[0]};

The "clean_dontwait" method, like the clean method, removes all boxes from the end of the belt if there are any boxes waiting at the end of the belt. If there are no boxes available, then the "clean_dontwait" method will return immediately with an empty list. Otherwise the contents of the boxes will be thawed and the resulting values and references will be returned an an array where each element is a reference to the original contents of each box.

peek

 ($string,$scalar,$listref,$hashref) = $belt->peek;

 @lookahead = $belt->peek( $index );

The "peek" method waits for a box to become availabe at the end of the belt, but does not remove it from the belt like the take method does. It does however thaw the contents and returns the resulting values and references.

For advanced, and mostly internal, usages, it is possible to specify the ordinal number of the box in which to peek.

Please note that there is no guarantee that "take" will give you the same data as which is returned with this method, as any other thread can have taken the boxes off of the belt in the meantime.

peek_dontwait

 ($string,$scalar,$listref,$hashref) = $belt->peek_dontwait;

 @lookahead = $belt->peek_dontwait( $index );

The "peek_dontwait" method is like the take_dontwait method, but does not remove the box from the belt if there is one available. If there is a box available, then the contents of the box will be thawed and the resulting values and references are returned. An empty list will be returned if there was no box available at the end of the belt.

For advanced, and mostly internal, usages, it is possible to specify the ordinal number of the box in which to peek.

Please note that there is no guarantee that "take" will give you the same data as which is returned with this method, as any other thread can have taken the boxes off of the belt in the meantime.

onbelt

 $onbelt = $belt->onbelt;

The "onbelt" method returns the number of boxes that are still in the belt.

maxboxes

 $belt->maxboxes( 100 );
 $maxboxes = $belt->maxboxes;

The "maxboxes" method returns the maximum number of boxes that can be on the belt before throttling sets in. The input value, if specified, specifies the new maximum number of boxes that may be on the belt. Throttling will be switched off if the value undef is specified.

Specifying the "maxboxes" field when creating the object with new is equivalent to calling this method.

The minboxes method can be called to specify the minimum number of boxes that must be on the belt before the putting of boxes is allowed again after reaching the maximum number of boxes. By default, half of the "maxboxes" value is assumed.

minboxes

 $belt->minboxes( 50 );
 $minboxes = $belt->minboxes;

The "minboxes" method returns the minimum number of boxes that must be on the belt before the putting of boxes is allowed again after reaching the maximum number of boxes. The input value, if specified, specifies the new minimum number of boxes that must be on the belt.

Specifying the "minboxes" field when creating the object with new is equivalent to calling this method.

The maxboxes method can be called to set the maximum number of boxes that may be on the belt before the putting of boxes will be halted.

shutdown

 $belt->shutdown;

The "shutdown" method performs an orderly shutdown of the belt. It waits until all of the boxes on the belt have been removed before it returns.

thread

 $thread = $belt->thread;

The "thread" method returns the thread object that is being used for the belt. It returns undef if no seperate thread is being used.

tid

 $tid = $belt->tid;

The "tid" method returns the thread id of the thread object that is being used for the belt. It returns undef if no seperate thread is being used.

REQUIRED MODULES ^

 load (any)
 Thread::Serialize (any)
 Thread::Tie (0.09)

OPTIMIZATIONS ^

This module uses load to reduce memory and CPU usage. This causes subroutines only to be compiled in a thread when they are actually needed at the expense of more CPU when they need to be compiled. Simple benchmarks however revealed that the overhead of the compiling single routines is not much more (and sometimes a lot less) than the overhead of cloning a Perl interpreter with a lot of subroutines pre-loaded.

CAVEATS ^

Passing unshared values between threads is accomplished by serializing the specified values using Thread::Serialize. Please see the CAVEATS section there for an up-to-date status of what can be passed around between threads.

AUTHOR ^

Elizabeth Mattijsen, <liz@dijkmat.nl>.

Please report bugs to <perlbugs@dijkmat.nl>.

HISTORY ^

This module started life as Thread::Queue::Any and as a sub-class of Thread::Queue. Using the conveyor belt metaphore seemed more appropriate and therefore the name was changed. To cut the cord with Thread::Queue completely, the belt mechanism was implemented from scratch.

Why would you use Thread::Conveyor over Thread::Queue::Any? Well, Thread::Conveyor has the following extra features:

It works with Perl 5.8.0

Shared arrays leak memory very badly in Perl 5.8.0. Therefore, you cannot really use Thread::Queue in Perl 5.8.0, and consequently cannot use Thread::Queue::Any in any type of production environment.

It provides throttling

A thread that enqueues very many values quickly, can cause a large amount of memory to be used. With throttling, any thread that enqueues will have to wait until there is "room" on the belt again before continuing. See methods "minboxes" and "maxboxes".

You can check for a new value without removing it from the belt

Sometimes it can be nice to check whether there is a new value on the belt without actually removing it from the belt. See the "peek" and "peek_dontwait" methods.

You can reset the entire belt

Sometimes you want to be able to reset the contents of the belt. See the "clean" and "clean_dontwait" methods for that.

You can get everything from the belt in one go

Sometimes you want everything that's on the belt in one go. That can also ba accomplished with the "clean" and "clean_dontwait" methods.

COPYRIGHT ^

Copyright (c) 2002, 2003, 2004, 2007, 2010 Elizabeth Mattijsen <liz@dijkmat.nl>. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SEE ALSO ^

threads, threads::shared, Thread::Queue, Thread::Queue::Any, Thread::Serialize.

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