1prefork(3) User Contributed Perl Documentation prefork(3)
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6 prefork - Optimized module loading for forking or non-forking processes
7
9 In a module that normally delays module loading with require
10 # Module Foo::Bar only uses This::That 25% of the time.
12 # We want to preload in in forking scenarios (like mod_perl), but
13 # we want to delay loading in non-forking scenarios (like CGI)
14 use prefork 'This::That';
15 sub do_something {
17 my $arg = shift;
18 # Load the module at run-time as normal
20 if ( $special_case ) {
21 require This::That;
22 This::That::blah(@_);
23 }
24 }
25 # Register a module to be loaded before forking directly
27 prefork::prefork('Module::Name');
28 In a script or module that is going to be forking.
30 package Module::Forker;
32 # Enable forking mode
34 use prefork ':enable';
35 # Or call it directly
37 prefork::enable();
38 In a third-party run-time loader
40 package Runtime::Loader;
42 use prefork ();
44 prefork::notify( \&load_everything );
45 ...
47 sub load_everything { ... }
49 1;
51
53 The task of optimizing module loading in Perl tends to move in two
54 different directions, depending on the context.
55 In a procedural context, such as scripts and CGI-type situations, you
57 can improve the load times and memory usage by loading a module at run-
58 time, only once you are sure you will need it.
59 In the other common load profile for perl applications, the application
61 will start up and then fork off various worker processes. To take full
62 advantage of memory copy-on-write features, the application should load
63 as many modules as possible before forking to prevent them consuming
64 memory in multiple worker processes.
65 Unfortunately, the strategies used to optimise for these two load
67 profiles are diametrically opposed. What improves a situation for one
68 tends to make life worse for the other.
69
71 The "prefork" pragma is intended to allow module writers to optimise
72 module loading for both scenarios with as little additional code as
73 possible.
74 prefork.pm is intended to serve as a central and optional marshalling
76 point for state detection (are we running in compile-time or run-time
77 mode) and to act as a relatively light-weight module loader.
78 Loaders and Forkers
80 "prefork" is intended to be used in two different ways.
81 The first is by a module that wants to indicate that another module
83 should be loaded before forking. This is known as a "Loader".
84 The other is a script or module that will be initiating the forking. It
86 will tell prefork.pm that it is either going to fork, or is about to
87 fork, or for some other reason all modules previously mentioned by the
88 Loaders should be loaded immediately.
89 Usage as a Pragma
91 A Loader can register a module to be loaded using the following
92 use prefork 'My::Module';
94 The same thing can be done in such a way as to not require prefork
96 being installed, but taking advantage of it if it is.
97 eval "use prefork 'My::Module';";
99 A Forker can indicate that it will be forking with the following
101 use prefork ':enable';
103 In any use of "prefork" as a pragma, you can only pass a single value
105 as argument. Any additional arguments will be ignored. (This may throw
106 an error in future versions).
107 Compatibility with mod_perl and others
109 Part of the design of "prefork", and its minimalistic nature, is that
110 it is intended to work easily with existing modules, needing only small
111 changes.
112 For example, "prefork" itself will detect the $ENV{MOD_PERL}
114 environment variable and automatically start in forking mode.
115 prefork has support for integrating with third-party modules, such as
117 Class::Autouse. The "notify" function allows these run-time loaders to
118 register callbacks, to be called once prefork enters forking mode.
119 The synopsis entry above describes adding support for prefork.pm as a
121 dependency. To allow your third-party module loader without a
122 dependency and only if it is installed use the following:
123 eval { require prefork; }
125 prefork::notify( \&function ) unless $@;
126 Using prefork.pm
128 From the Loader side, it is fairly simple. prefork becomes a dependency
129 for your module, and you use it as a pragma as documented above.
130 For the Forker, you have two options. Use as a dependency or optional
132 use.
133 In the dependency case, you add prefork as a dependency and use it as a
135 pragma with the ':enable' option.
136 To add only optional support for prefork, without requiring it to be
138 installed, you should wait until the moment just before you fork and
139 then call "prefork::enable" directly ONLY if it is loaded.
140 # Load modules if any use the prefork pragma.
142 prefork::enable() if $INC{prefork.pm};
143 This will cause the modules to be loaded ONLY if there are any modules
145 that need to be loaded. The main advantage of the dependency version is
146 that you only need to enable the module once, and not before each fork.
147 If you wish to have your own module leverage off the forking-detection
149 that prefork provides, you can also do the following.
150 use prefork;
152 if ( $prefork::FORKING ) {
153 # Complete some preparation task
154 }
155 Modules that are prefork-aware
157 mod_perl/mod_perl2
158 Class::Autouse
159
161 prefork $module
162 The 'prefork' function indicates that a module should be loaded before
163 the process will fork. If already in forking mode the module will be
164 loaded immediately.
165 Otherwise it will be added to a queue to be loaded later if it receives
167 instructions that it is going to be forking.
168 Returns true on success, or dies on error.
170 enable
172 The "enable" function indicates to the prefork module that the process
173 is going to fork, possibly immediately.
174 When called, prefork.pm will immediately load all outstanding modules,
176 and will set a flag so that any further 'prefork' calls will load the
177 module at that time.
178 Returns true, dying as normal is there is a problem loading a module.
180 notify &function
182 The "notify" function is used to integrate support for modules other
183 than prefork.pm itself.
184 A module loader calls the notify function, passing it a reference to a
186 "CODE" reference (either anon or a function reference). "prefork" will
187 store this CODE reference, and execute it immediately as soon as it
188 knows it is in forking-mode, but after it loads its own modules.
189 Callbacks are called in the order they are registered.
191 Normally, this will happen as soon as the "enable" function is called.
193 However, you should be aware that if prefork is already in preforking
195 mode at the time that the notify function is called, prefork.pm will
196 execute the function immediately.
197 This means that any third party module loader should be fully loaded
199 and initialised before the callback is provided to "notify".
200 Returns true if the function is stored, or dies if not passed a "CODE"
202 reference, or the callback is already set in the notify queue.
203
205 - Add checks for more pre-forking situations
206
208 Bugs should be always submitted via the CPAN bug tracker, located at
209 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=prefork>
211 For other issues, or commercial enhancement or support, contact the
213 author.
214
216 Adam Kennedy <adamk@cpan.org>
217
219 Thank you to Phase N Australia (<http://phase-n.com/>) for permitting
220 the open sourcing and release of this distribution.
221 Copyright 2004 - 2009 Adam Kennedy.
223 This program is free software; you can redistribute it and/or modify it
225 under the same terms as Perl itself.
226 The full text of the license can be found in the LICENSE file included
228 with this module.
229perl v5.38.0 2023-07-21 prefork(3)