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).

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.

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.

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).