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Revision 1.23 by root, Sat Apr 6 08:29:43 2013 UTC vs.
Revision 1.29 by root, Sat Apr 6 09:15:49 2013 UTC

… …
4 4
5=head1 SYNOPSIS 5=head1 SYNOPSIS
6 6
7 use AnyEvent::Fork; 7 use AnyEvent::Fork;
8 8
9 ################################################################## 9 AnyEvent::Fork
10 ->new
11 ->require ("MyModule")
12 ->run ("MyModule::server", my $cv = AE::cv);
13
14 my $fh = $cv->recv;
15
16=head1 DESCRIPTION
17
18This module allows you to create new processes, without actually forking
19them from your current process (avoiding the problems of forking), but
20preserving most of the advantages of fork.
21
22It can be used to create new worker processes or new independent
23subprocesses for short- and long-running jobs, process pools (e.g. for use
24in pre-forked servers) but also to spawn new external processes (such as
25CGI scripts from a web server), which can be faster (and more well behaved)
26than using fork+exec in big processes.
27
28Special care has been taken to make this module useful from other modules,
29while still supporting specialised environments such as L<App::Staticperl>
30or L<PAR::Packer>.
31
32=head1 WHAT THIS MODULE IS NOT
33
34This module only creates processes and lets you pass file handles and
35strings to it, and run perl code. It does not implement any kind of RPC -
36there is no back channel from the process back to you, and there is no RPC
37or message passing going on.
38
39If you need some form of RPC, you can either implement it yourself
40in whatever way you like, use some message-passing module such
41as L<AnyEvent::MP>, some pipe such as L<AnyEvent::ZeroMQ>, use
42L<AnyEvent::Handle> on both sides to send e.g. JSON or Storable messages,
43and so on.
44
45=head1 PROBLEM STATEMENT
46
47There are two traditional ways to implement parallel processing on UNIX
48like operating systems - fork and process, and fork+exec and process. They
49have different advantages and disadvantages that I describe below,
50together with how this module tries to mitigate the disadvantages.
51
52=over 4
53
54=item Forking from a big process can be very slow.
55
56A 5GB process needs 0.05s to fork on my 3.6GHz amd64 GNU/Linux box. This
57overhead is often shared with exec (because you have to fork first), but
58in some circumstances (e.g. when vfork is used), fork+exec can be much
59faster.
60
61This module can help here by telling a small(er) helper process to fork,
62which is faster then forking the main process, and also uses vfork where
63possible. This gives the speed of vfork, with the flexibility of fork.
64
65=item Forking usually creates a copy-on-write copy of the parent
66process.
67
68For example, modules or data files that are loaded will not use additional
69memory after a fork. When exec'ing a new process, modules and data files
70might need to be loaded again, at extra CPU and memory cost. But when
71forking, literally all data structures are copied - if the program frees
72them and replaces them by new data, the child processes will retain the
73old version even if it isn't used, which can suddenly and unexpectedly
74increase memory usage when freeing memory.
75
76The trade-off is between more sharing with fork (which can be good or
77bad), and no sharing with exec.
78
79This module allows the main program to do a controlled fork, and allows
80modules to exec processes safely at any time. When creating a custom
81process pool you can take advantage of data sharing via fork without
82risking to share large dynamic data structures that will blow up child
83memory usage.
84
85In other words, this module puts you into control over what is being
86shared and what isn't, at all times.
87
88=item Exec'ing a new perl process might be difficult.
89
90For example, it is not easy to find the correct path to the perl
91interpreter - C<$^X> might not be a perl interpreter at all.
92
93This module tries hard to identify the correct path to the perl
94interpreter. With a cooperative main program, exec'ing the interpreter
95might not even be necessary, but even without help from the main program,
96it will still work when used from a module.
97
98=item Exec'ing a new perl process might be slow, as all necessary modules
99have to be loaded from disk again, with no guarantees of success.
100
101Long running processes might run into problems when perl is upgraded
102and modules are no longer loadable because they refer to a different
103perl version, or parts of a distribution are newer than the ones already
104loaded.
105
106This module supports creating pre-initialised perl processes to be used as
107a template for new processes.
108
109=item Forking might be impossible when a program is running.
110
111For example, POSIX makes it almost impossible to fork from a
112multi-threaded program while doing anything useful in the child - in
113fact, if your perl program uses POSIX threads (even indirectly via
114e.g. L<IO::AIO> or L<threads>), you cannot call fork on the perl level
115anymore without risking corruption issues on a number of operating
116systems.
117
118This module can safely fork helper processes at any time, by calling
119fork+exec in C, in a POSIX-compatible way (via L<Proc::FastSpawn>).
120
121=item Parallel processing with fork might be inconvenient or difficult
122to implement. Modules might not work in both parent and child.
123
124For example, when a program uses an event loop and creates watchers it
125becomes very hard to use the event loop from a child program, as the
126watchers already exist but are only meaningful in the parent. Worse, a
127module might want to use such a module, not knowing whether another module
128or the main program also does, leading to problems.
129
130Apart from event loops, graphical toolkits also commonly fall into the
131"unsafe module" category, or just about anything that communicates with
132the external world, such as network libraries and file I/O modules, which
133usually don't like being copied and then allowed to continue in two
134processes.
135
136With this module only the main program is allowed to create new processes
137by forking (because only the main program can know when it is still safe
138to do so) - all other processes are created via fork+exec, which makes it
139possible to use modules such as event loops or window interfaces safely.
140
141=back
142
143=head1 EXAMPLES
144
10 # create a single new process, tell it to run your worker function 145=head2 Create a single new process, tell it to run your worker function.
11 146
12 AnyEvent::Fork 147 AnyEvent::Fork
13 ->new 148 ->new
14 ->require ("MyModule") 149 ->require ("MyModule")
15 ->run ("MyModule::worker, sub { 150 ->run ("MyModule::worker, sub {
… …
25 160
26 # now $slave_filehandle is connected to the $master_filehandle 161 # now $slave_filehandle is connected to the $master_filehandle
27 # in the original prorcess. have fun! 162 # in the original prorcess. have fun!
28 } 163 }
29 164
30 ##################################################################
31 # create a pool of server processes all accepting on the same socket 165=head2 Create a pool of server processes all accepting on the same socket.
32 166
33 # create listener socket 167 # create listener socket
34 my $listener = ...; 168 my $listener = ...;
35 169
36 # create a pool template, initialise it and give it the socket 170 # create a pool template, initialise it and give it the socket
… …
61 while (my $socket = $listener->accept) { 195 while (my $socket = $listener->accept) {
62 # do sth. with new socket 196 # do sth. with new socket
63 } 197 }
64 } 198 }
65 199
66 ##################################################################
67 # use AnyEvent::Fork as a faster fork+exec 200=head2 use AnyEvent::Fork as a faster fork+exec
68 201
69 # this runs /bin/echo hi, with stdout redirected to /tmp/log 202This runs /bin/echo hi, with stdout redirected to /tmp/log and stderr to
70 # and stderr to the communications socket. it is usually faster 203the communications socket. It is usually faster than fork+exec, but still
71 # than fork+exec, but still let's you prepare the environment. 204let's you prepare the environment.
72 205
73 open my $output, ">/tmp/log" or die "$!"; 206 open my $output, ">/tmp/log" or die "$!";
74 207
75 AnyEvent::Fork 208 AnyEvent::Fork
76 ->new 209 ->new
… …
88 ->send_fh ($output) 221 ->send_fh ($output)
89 ->send_arg ("/bin/echo", "hi") 222 ->send_arg ("/bin/echo", "hi")
90 ->run ("run", my $cv = AE::cv); 223 ->run ("run", my $cv = AE::cv);
91 224
92 my $stderr = $cv->recv; 225 my $stderr = $cv->recv;
93
94=head1 DESCRIPTION
95
96This module allows you to create new processes, without actually forking
97them from your current process (avoiding the problems of forking), but
98preserving most of the advantages of fork.
99
100It can be used to create new worker processes or new independent
101subprocesses for short- and long-running jobs, process pools (e.g. for use
102in pre-forked servers) but also to spawn new external processes (such as
103CGI scripts from a web server), which can be faster (and more well behaved)
104than using fork+exec in big processes.
105
106Special care has been taken to make this module useful from other modules,
107while still supporting specialised environments such as L<App::Staticperl>
108or L<PAR::Packer>.
109
110=head1 WHAT THIS MODULE IS NOT
111
112This module only creates processes and lets you pass file handles and
113strings to it, and run perl code. It does not implement any kind of RPC -
114there is no back channel from the process back to you, and there is no RPC
115or message passing going on.
116
117If you need some form of RPC, you can either implement it yourself
118in whatever way you like, use some message-passing module such
119as L<AnyEvent::MP>, some pipe such as L<AnyEvent::ZeroMQ>, use
120L<AnyEvent::Handle> on both sides to send e.g. JSON or Storable messages,
121and so on.
122
123=head1 PROBLEM STATEMENT
124
125There are two ways to implement parallel processing on UNIX like operating
126systems - fork and process, and fork+exec and process. They have different
127advantages and disadvantages that I describe below, together with how this
128module tries to mitigate the disadvantages.
129
130=over 4
131
132=item Forking from a big process can be very slow (a 5GB process needs
1330.05s to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead
134is often shared with exec (because you have to fork first), but in some
135circumstances (e.g. when vfork is used), fork+exec can be much faster.
136
137This module can help here by telling a small(er) helper process to fork,
138or fork+exec instead.
139
140=item Forking usually creates a copy-on-write copy of the parent
141process. Memory (for example, modules or data files that have been
142will not take additional memory). When exec'ing a new process, modules
143and data files might need to be loaded again, at extra CPU and memory
144cost. Likewise when forking, all data structures are copied as well - if
145the program frees them and replaces them by new data, the child processes
146will retain the memory even if it isn't used.
147
148This module allows the main program to do a controlled fork, and allows
149modules to exec processes safely at any time. When creating a custom
150process pool you can take advantage of data sharing via fork without
151risking to share large dynamic data structures that will blow up child
152memory usage.
153
154=item Exec'ing a new perl process might be difficult and slow. For
155example, it is not easy to find the correct path to the perl interpreter,
156and all modules have to be loaded from disk again. Long running processes
157might run into problems when perl is upgraded for example.
158
159This module supports creating pre-initialised perl processes to be used
160as template, and also tries hard to identify the correct path to the perl
161interpreter. With a cooperative main program, exec'ing the interpreter
162might not even be necessary.
163
164=item Forking might be impossible when a program is running. For example,
165POSIX makes it almost impossible to fork from a multi-threaded program and
166do anything useful in the child - strictly speaking, if your perl program
167uses posix threads (even indirectly via e.g. L<IO::AIO> or L<threads>),
168you cannot call fork on the perl level anymore, at all.
169
170This module can safely fork helper processes at any time, by calling
171fork+exec in C, in a POSIX-compatible way.
172
173=item Parallel processing with fork might be inconvenient or difficult
174to implement. For example, when a program uses an event loop and creates
175watchers it becomes very hard to use the event loop from a child
176program, as the watchers already exist but are only meaningful in the
177parent. Worse, a module might want to use such a system, not knowing
178whether another module or the main program also does, leading to problems.
179
180This module only lets the main program create pools by forking (because
181only the main program can know when it is still safe to do so) - all other
182pools are created by fork+exec, after which such modules can again be
183loaded.
184
185=back
186 226
187=head1 CONCEPTS 227=head1 CONCEPTS
188 228
189This module can create new processes either by executing a new perl 229This module can create new processes either by executing a new perl
190process, or by forking from an existing "template" process. 230process, or by forking from an existing "template" process.
… …
269 my ($fork_fh) = @_; 309 my ($fork_fh) = @_;
270 }); 310 });
271 311
272=back 312=back
273 313
274=head1 FUNCTIONS 314=head1 THE C<AnyEvent::Fork> CLASS
315
316This module exports nothing, and only implements a single class -
317C<AnyEvent::Fork>.
318
319There are two class constructors that both create new processes - C<new>
320and C<new_exec>. The C<fork> method creates a new process by forking an
321existing one and could be considered a third constructor.
322
323Most of the remaining methods deal with preparing the new process, by
324loading code, evaluating code and sending data to the new process. They
325usually return the process object, so you can chain method calls.
326
327If a process object is destroyed before calling its C<run> method, then
328the process simply exits. After C<run> is called, all responsibility is
329passed to the specified function.
330
331As long as there is any outstanding work to be done, process objects
332resist being destroyed, so there is no reason to store them unless you
333need them later - configure and forget works just fine.
275 334
276=over 4 335=over 4
277 336
278=cut 337=cut
279 338
… …
289use IO::FDPass; 348use IO::FDPass;
290 349
291our $VERSION = 0.5; 350our $VERSION = 0.5;
292 351
293our $PERL; # the path to the perl interpreter, deduces with various forms of magic 352our $PERL; # the path to the perl interpreter, deduces with various forms of magic
294
295=item my $pool = new AnyEvent::Fork key => value...
296
297Create a new process pool. The following named parameters are supported:
298 353
299=over 4 354=over 4
300 355
301=back 356=back
302 357
… …
398Create a new "empty" perl interpreter process and returns its process 453Create a new "empty" perl interpreter process and returns its process
399object for further manipulation. 454object for further manipulation.
400 455
401The new process is forked from a template process that is kept around 456The new process is forked from a template process that is kept around
402for this purpose. When it doesn't exist yet, it is created by a call to 457for this purpose. When it doesn't exist yet, it is created by a call to
403C<new_exec> and kept around for future calls. 458C<new_exec> first and then stays around for future calls.
404
405When the process object is destroyed, it will release the file handle
406that connects it with the new process. When the new process has not yet
407called C<run>, then the process will exit. Otherwise, what happens depends
408entirely on the code that is executed.
409 459
410=cut 460=cut
411 461
412sub new { 462sub new {
413 my $class = shift; 463 my $class = shift;

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