Ubuntu Manpage: load - control when subroutines will be loaded

NAME

       load - control when subroutines will be loaded

SYNOPSIS

         use load;            # default, same as 'autoload'

         use load 'autoload'; # export AUTOLOAD handler to this namespace

         use load 'ondemand'; # load subroutines after __END__ when requested, default

         use load 'now';      # load subroutines after __END__ now

         use load ();         # same as qw(dontscan inherit)

         use load 'dontscan'; # don't scan module until it is really needed

         use load 'inherit';  # do NOT export AUTOLOAD handler to this namespace

VERSION

       This documentation describes version 0.23.

DESCRIPTION

The "load" pragma allows a module developer to give the application developer more options with regards
to optimize for memory or CPU usage. The "load" pragma gives more control on the moment when subroutines
are loaded and start taking up memory. This allows the application developer to optimize for CPU usage
(by loading all of a module at compile time and thus reducing the amount of CPU used during the execution
of an application). Or allow the application developer to optimize for memory usage, by loading
subroutines only when they are actually needed, thereby however increasing the amount of CPU needed
during execution.

The "load" pragma combines the best of both worlds from AutoLoader and SelfLoader. And adds some more
features.

In a situation where you want to use as little memory as possible, the "load" pragma (in the context of a
module) is a drop-in replacement for AutoLoader. But for situations where you want to have a module load
everything it could ever possibly need (e.g. when starting a mod_perl server in pre-fork mode), the
"load" pragma can be used (in the context of an application) to have all subroutines of a module loaded
without having to make any change to the source of the module in question.

So the typical use inside a module is to have:

package Your::Module;
use load;

in the source. And to place all subroutines that you want to be loadable on demand after the (first)
__END__.

If an application developer decides that all subroutines should be loaded at compile time, (s)he can say
in the application:

use load 'now';
use Your::Module;

This will cause the subroutines of Your::Module to all be loaded at compile time.

MODES OF OPERATION

There are basically two places where you can call the "load" pragma:

inside a module
When you call the "load" pragma inside a module, you're basically enabling that module for having an
external control when certain subroutines will be loaded. As with AutoLoader, any subroutines that
should be loaded on demand, should be located after an __END__ line.

If no parameters are specified with the "use load", then the "autoload" parameter is assumed. Whether
the module's subroutines are loaded at compile time or on demand, is determined by the calling
application. If the application doesn't specify anything specific, the "ondemand" keyword will also be
assumed.

inside an application
When you call the "load" pragma inside an application, you're basically specifying when subroutines will
be loaded by "load" enhanced modules. As an application developer, you can basically use two keywords:
"ondemand" and "now".

If an application does not call the "load" pragma, the "ondemand" keyword will be assumed. With
"ondemand", subroutines will only be loaded when they are actually executed. This saves memory at the
expense of extra CPU the first time the subroutine is called.

The "now" keyword indicates that all subroutines of all modules that are enhanced with the "load" pragma,
will be loaded at compile time (thus using more memory, but not having an extra CPU overhead the first
time the subroutine is executed).

KEYWORDS

The following keywords are recognized with the "use" command:

ondemand
The "ondemand" keyword indicates that subroutines, of modules that are enhanced with the "load" pragma,
will only be loaded when they are actually called.

If the "ondemand" keyword is used in the context of an application, all modules that are subsequently
"use"d, will be forced to load subroutines only when they are actually called (unless the module itself
forces a specific setting).

If the "ondemand" keyword is used in the context of a module, it indicates that the subroutines of that
module, should always be loaded when they are actually needed. Since this takes away the choice from the
application developer, the use of the "ondemand" keyword in module context is not encouraged. See also
the now and dontscan keywords.

now
The "now" keyword indicates that subroutines, of modules that are enhanced with the "load" pragma, will
be loaded at compile time.

If the "now" keyword is used in the context of an application, all modules that are subsequently "use"d,
will be forced to load all subroutines at compile time (unless the module forces a specific setting
itself).

If the "now" keyword is used in the context of a module, it indicates that the subroutines of that
module, should always be loaded at compile time. Since this takes away the choice from the application
developer, the use of the "now" keyword in module context is not encouraged. See also the ondemand
keyword.

dontscan
The "dontscan" keyword only makes sense when used in the context of a module. Normally, when a module
that is enhanced with the "load" pragma is compiled, the source after the __END__ is scanned for the
locations of the subroutines. This makes the compiling of modules a little slower, but allows for a
faster (initial) lookup of (yet) unloaded subroutines during execution.

If the "dontscan" keyword is specified, this scanning of the source is skipped at compile time. However,
as soon as an attempt is made to ececute a subroutine from this module, then first the scanning of the
source is performed, before the subroutine in question is loaded.

So, you should use the "dontscan" keyword if you are reasonably sure that you will only need subroutines
from the module in special cases. In all other cases it will make more sense to have the source scanned
at compile time.

The "dontscan" keyword will be ignored if an application developer forces subroutines to be loaded at
compile time with the now keyword.

autoload
The "autoload" keyword only makes sense when used in the context of a module. It indicates that a
generic AUTOLOAD subroutine will be exported to the module's namespace. It is selected by default if you
use the "load" pragma without parameters in the source of a module. See also the inherit keyword to not
export the generic AUTOLOAD subroutine.

inherit
The "inherit" keyword only makes sense when used in the context of a module. It indicates that no
AUTOLOAD subroutine will be exported to the module's namespace. This can e.g. be used when you need to
have your own AUTOLOAD routine. That AUTOLOAD routine should then contain:

$load::AUTOLOAD = $sub;
goto &load::AUTOLOAD;

to access the "load" pragma functionality. Another case to use the "inherit" keyword would be in a sub-
class of a module which also is "load" enhanced. In that case, the inheritance will cause the AUTOLOAD
subroutine of the base class to be used, thereby accessing the "load" pragma automagically (and hence the
naming of the keyword of course). See also the autoload keyword to have the module use the generic
AUTOLOAD subroutine.

AutoLoader
The "AutoLoader" keyword enables AutoLoader emulation mode. It basically takes over the functionality of
the AutoLoader module (which is part of Perl's core, and which is used by many of Perl's core modules).

Use of AutoLoader emulation mode usually only makes sense in a mod_perl prefork environment (in
combination with the "now" keyword), or a threaded Perl environment.

It basically adds the flexibility of subroutine loading options of the "load" pragma to the existing
codebase of Perl's core and CPAN modules. It is typically invoked from the command line:

perl -Mload=AutoLoader

or in a mod_perl configuration:

<Perl>
use load qw(AutoLoader now); # as early as possible
# rest of modules to be loaded
</Perl>

The AutoLoader emulation mode has the further advantage for modules being developed with AutoLoader, as
it is possible to run the module before having to have installed the module (which is normally a
requirement with using AutoLoader).

Please note that AutoLoader emulation will only work properly for any modules loaded after the "load"
module is loaded. It is therefore important to activate the AutoLoader as soon as possible, before any
other modules have been loaded. Of particular interest in this respect are the threads and the ifdef
modules.

REQUIRED MODULES

        (none)

DIFFERENCES WITH SIMILAR MODULES

There are a number of (core) modules that more or less do the same thing as the "load" pragma.

AutoSplit / AutoLoader
The "load" pragma is very similar to the AutoSplit / AutoLoader combination. The main difference is that
the splitting takes place when the "load" import is called in a module and that there are no external
files created. Instead, just the offsets and lengths are recorded in a hash (when "ondemand" is active)
or all the source after __END__ is eval'led (when "now" is active).

From a module developer point of view, the advantage is that you do not need to install a module before
you can test it. From an application developer point of view, you have the flexibility of having
everything loaded now or later (on demand).

From a memory usage point of view, the "load" offset/length hash takes up more memory than the equivalent
AutoLoader setup. On the other hand, accessing the source of a subroutine may generally be faster
because the file is more likely to reside in the operating system's buffers already.

As an extra feature, the "load" pragma allows an application to force all subroutines to be loaded at
compile time, which is not possible with AutoLoader.

The "AutoLoader emulation" mode causes AutoLoader to be replaced by "load", increasing further
flexibility in loading options (which can be particularly important in the "mod_perl prefork" situation)
and ease of use during development of modules using AutoLoader (as you don't need to install the module
before you can test it).

SelfLoader
The "load" pragma also has some functionality in common with the SelfLoader module. But it gives more
granularity: with SelfLoader, all subroutines that are not loaded directly, will be loaded if any not yet
loaded subroutine is requested. It also adds complexities if your module needs to use the <DATA> handle.
So the "load" pragma gives more flexibility and fewer development complexities. And of course, an
application can force all subroutines to be loaded at compile time when needed with the "load" pragma.

UNIVERSAL::can

       To ensure the functioning of the ->can class method and &UNIVERSAL::can, the "load" pragma hijacks the
       standard UNIVERSAL::can routine so that it can check whether the subroutine/method that you want to check
       for, actually exists and have a code reference to it returned.  This has a side effect that you the
       subroutine checked for, is loaded.  You can use this side effect to load subroutines without calling
       them.

        Your::Module->can( 'loadthisnow' );

       will load the subroutine "loadthisnow" of the Your::Module module without actually calling it.

CAVEATS

       Currently you may not have multiple packages in the same file, nor can you have fully qualified
       subroutine names.

       The parser that looks for package names and subroutines, is not very smart.  This is intentionally so, as
       making it smarter will make it a lot slower, but probably still not smart enough.  Therefore, the
       "package" and "sub"'s must be at the start of a line.  And the name of the "sub" must be on the same line
       as the "sub".

EXAMPLES

       Some code examples.  Please note that these are just a part of an actual situation.

   base class
        package Your::Module;
        use load;

       Exports the generic AUTOLOAD subroutine and adheres to whatever the application developer specifies as
       mode of operation.

   sub class
        package Your::Module::Adapted;
        @ISA = qw(Your::Module);
        use load ();

       Does not export the generic AUTOLOAD subroutine, but inherits it from its base class.  Also implicitely
       specifies the "dontscan" keyword, causing the source of the module to be scanned only when the first not
       yet loaded subroutine is about to be executed.  If you only want to have the "inherit" keyword
       functionality, then you must specify that explicitly:

        package Your::Module::Adapted;
        @ISA = qw(Your::Module);
        use load 'inherit';

   custom AUTOLOAD
        package Your::Module;
        use load 'inherit';

        sub AUTOLOAD {
          if (some condition) {
            $load::AUTOLOAD = $Your::Module::AUTOLOAD;
            goto &load::AUTOLOAD;
          }
          # do your own stuff
        }

       If you want to use your own AUTOLOAD subroutine, but still want to use the functionality offered by the
       "load" pragma, you can use the above construct.

   mod_perl prefork
        use load qw(AutoLoader now);
        use Your::Module;

       In pre-fork mod_perl applications (the default mod_perl applications before mod_perl 2.0), it is
       advantageous to load all possible subroutines when the Apache process is started.  This is because the
       operating system will share memory using a process called "Copy On Write".  So even though it will take
       more memory initially, that memory loss is easily evened out by the gains of having everything shared.
       Loading a not yet loaded subroutine in that situation, will cause otherwise shared memory to become
       unshared.  Thereby increasing the overall memory usage, because the amount that becomes unshared is
       typically a lot more than the extra memory used by the subroutine (which is caused by fragmentation of
       allocated memory).

       The AutoLoader emulation mode causes all modules that use "AutoLoader" to be handled by "load".  In
       combination with the "now" mode, this means that many system modules will also be loaded completely at
       server startup (causing a grow in initial use of memory, but sharing more memory means that overall
       memory usage is significantly reduced.

   threaded applications and mod_perl worker
        use Your::Module;

       Threaded Perl applications, of which mod_perl applications using the "worker" module are a special case,
       function best when subroutines are only loaded when they are actually needed.  This is caused by the
       nature of the threading model of Perl, in which all data-structures are copied to each thread
       (essentially forcing them to become unshared as far as the operating system is concerned).

       Benchmarks have shown that the overhead of the extra CPU is easily offset by the reduction of the amount
       of data that needs to be copied (and processed) when a thread is created.

       A little additional memory reduction can be achieved with the AutoLoader emulation mode: this will
       prevent the AutoLoader module to be loaded (but have its functionality handled by the "load" pragma).

SOURCE FILTERS

       If your module wants to use "load" to load subroutines on demand and that module needs a source filter
       (which is usually activated with a "use" statement), then those modules need to be used when the source
       of the subroutine is compiled.  The class method "register" is intended to be used from such a module,
       typicallly like this:

        sub import {
          my $package = caller();
          load->register( $package,__PACKAGE__ )  # register caller's package
           if defined( $load::VERSION )           # if load.pm loaded
            and $load::VERSION > 0.11;            # and recent enough
        }

       The first parameter is the name of the package in which subroutines need extra modules "use"d.  The
       second parameter is the name of the module that needs to be "use"d.

TODO

       The coordinates of a subroutine in a module (start,number of bytes) are stored in a hash in the load
       namespace.  Ideally, this information should be stored in the stash of the module to which they apply.
       Then the internals that check for the existence of a subroutine, would see that the subroutine doesn't
       exist (yet), but that there is an offset and length (and implicitely, a file from %INC) from which the
       source could be read and evalled.

       Loading all of the subroutines should maybe be handled inside the Perl parser, having it skip __END__
       when the global "now" flag is set.

       Possibly we should use the <DATA> handle from a module if there is one, or dup it and use that, rather
       than opening the file again.

       Add SelfLoader emulation mode.

MODULE RATING

       If you want to find out how this module is appreciated by other people, please check out this module's
       rating at <http://cpanratings.perl.org/l/load> (if there are any ratings for this module).  If you like
       this module, or otherwise would like to have your opinion known, you can add your rating of this module
       at <http://cpanratings.perl.org/rate/?distribution=load>.

ACKNOWLEDGEMENTS

       Frank Tolstrup for helping ironing out all of the Windows related issues.

AUTHOR

       Elizabeth Mattijsen, <liz@dijkmat.nl>.

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

COPYRIGHT

       Copyright (c) 2002, 2003, 2004, 2005, 2006, 2007, 2010, 2012 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.