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1NAME 1NAME
2 AnyEvent::Fork - everything you wanted to use fork() for, but couldn't 2 AnyEvent::Fork - everything you wanted to use fork() for, but couldn't
3 3
4 ATTENTION, this is a very early release, and very untested. Consider it
5 a technology preview.
6
7SYNOPSIS 4SYNOPSIS
8 use AnyEvent::Fork; 5 use AnyEvent::Fork;
9 6
10 ################################################################## 7 AnyEvent::Fork
8 ->new
9 ->require ("MyModule")
10 ->run ("MyModule::server", my $cv = AE::cv);
11
12 my $fh = $cv->recv;
13
14DESCRIPTION
15 This module allows you to create new processes, without actually forking
16 them from your current process (avoiding the problems of forking), but
17 preserving most of the advantages of fork.
18
19 It can be used to create new worker processes or new independent
20 subprocesses for short- and long-running jobs, process pools (e.g. for
21 use in pre-forked servers) but also to spawn new external processes
22 (such as CGI scripts from a web server), which can be faster (and more
23 well behaved) than using fork+exec in big processes.
24
25 Special care has been taken to make this module useful from other
26 modules, while still supporting specialised environments such as
27 App::Staticperl or PAR::Packer.
28
29 WHAT THIS MODULE IS NOT
30 This module only creates processes and lets you pass file handles and
31 strings to it, and run perl code. It does not implement any kind of RPC
32 - there is no back channel from the process back to you, and there is no
33 RPC or message passing going on.
34
35 If you need some form of RPC, you could use the AnyEvent::Fork::RPC
36 companion module, which adds simple RPC/job queueing to a process
37 created by this module.
38
39 And if you need some automatic process pool management on top of
40 AnyEvent::Fork::RPC, you can look at the AnyEvent::Fork::Pool companion
41 module.
42
43 Or you can implement it yourself in whatever way you like: use some
44 message-passing module such as AnyEvent::MP, some pipe such as
45 AnyEvent::ZeroMQ, use AnyEvent::Handle on both sides to send e.g. JSON
46 or Storable messages, and so on.
47
48 COMPARISON TO OTHER MODULES
49 There is an abundance of modules on CPAN that do "something fork", such
50 as Parallel::ForkManager, AnyEvent::ForkManager, AnyEvent::Worker or
51 AnyEvent::Subprocess. There are modules that implement their own process
52 management, such as AnyEvent::DBI.
53
54 The problems that all these modules try to solve are real, however, none
55 of them (from what I have seen) tackle the very real problems of
56 unwanted memory sharing, efficiency or not being able to use event
57 processing, GUI toolkits or similar modules in the processes they
58 create.
59
60 This module doesn't try to replace any of them - instead it tries to
61 solve the problem of creating processes with a minimum of fuss and
62 overhead (and also luxury). Ideally, most of these would use
63 AnyEvent::Fork internally, except they were written before AnyEvent:Fork
64 was available, so obviously had to roll their own.
65
66 PROBLEM STATEMENT
67 There are two traditional ways to implement parallel processing on UNIX
68 like operating systems - fork and process, and fork+exec and process.
69 They have different advantages and disadvantages that I describe below,
70 together with how this module tries to mitigate the disadvantages.
71
72 Forking from a big process can be very slow.
73 A 5GB process needs 0.05s to fork on my 3.6GHz amd64 GNU/Linux box.
74 This overhead is often shared with exec (because you have to fork
75 first), but in some circumstances (e.g. when vfork is used),
76 fork+exec can be much faster.
77
78 This module can help here by telling a small(er) helper process to
79 fork, which is faster then forking the main process, and also uses
80 vfork where possible. This gives the speed of vfork, with the
81 flexibility of fork.
82
83 Forking usually creates a copy-on-write copy of the parent process.
84 For example, modules or data files that are loaded will not use
85 additional memory after a fork. Exec'ing a new process, in contrast,
86 means modules and data files might need to be loaded again, at extra
87 CPU and memory cost.
88
89 But when forking, you still create a copy of your data structures -
90 if the program frees them and replaces them by new data, the child
91 processes will retain the old version even if it isn't used, which
92 can suddenly and unexpectedly increase memory usage when freeing
93 memory.
94
95 For example, Gtk2::CV is an image viewer optimised for large
96 directories (millions of pictures). It also forks subprocesses for
97 thumbnail generation, which inherit the data structure that stores
98 all file information. If the user changes the directory, it gets
99 freed in the main process, leaving a copy in the thumbnailer
100 processes. This can lead to many times the memory usage that would
101 actually be required. The solution is to fork early (and being
102 unable to dynamically generate more subprocesses or do this from a
103 module)... or to use <AnyEvent:Fork>.
104
105 There is a trade-off between more sharing with fork (which can be
106 good or bad), and no sharing with exec.
107
108 This module allows the main program to do a controlled fork, and
109 allows modules to exec processes safely at any time. When creating a
110 custom process pool you can take advantage of data sharing via fork
111 without risking to share large dynamic data structures that will
112 blow up child memory usage.
113
114 In other words, this module puts you into control over what is being
115 shared and what isn't, at all times.
116
117 Exec'ing a new perl process might be difficult.
118 For example, it is not easy to find the correct path to the perl
119 interpreter - $^X might not be a perl interpreter at all. Worse,
120 there might not even be a perl binary installed on the system.
121
122 This module tries hard to identify the correct path to the perl
123 interpreter. With a cooperative main program, exec'ing the
124 interpreter might not even be necessary, but even without help from
125 the main program, it will still work when used from a module.
126
127 Exec'ing a new perl process might be slow, as all necessary modules have
128 to be loaded from disk again, with no guarantees of success.
129 Long running processes might run into problems when perl is upgraded
130 and modules are no longer loadable because they refer to a different
131 perl version, or parts of a distribution are newer than the ones
132 already loaded.
133
134 This module supports creating pre-initialised perl processes to be
135 used as a template for new processes at a later time, e.g. for use
136 in a process pool.
137
138 Forking might be impossible when a program is running.
139 For example, POSIX makes it almost impossible to fork from a
140 multi-threaded program while doing anything useful in the child - in
141 fact, if your perl program uses POSIX threads (even indirectly via
142 e.g. IO::AIO or threads), you cannot call fork on the perl level
143 anymore without risking memory corruption or worse on a number of
144 operating systems.
145
146 This module can safely fork helper processes at any time, by calling
147 fork+exec in C, in a POSIX-compatible way (via Proc::FastSpawn).
148
149 Parallel processing with fork might be inconvenient or difficult to
150 implement. Modules might not work in both parent and child.
151 For example, when a program uses an event loop and creates watchers
152 it becomes very hard to use the event loop from a child program, as
153 the watchers already exist but are only meaningful in the parent.
154 Worse, a module might want to use such a module, not knowing whether
155 another module or the main program also does, leading to problems.
156
157 Apart from event loops, graphical toolkits also commonly fall into
158 the "unsafe module" category, or just about anything that
159 communicates with the external world, such as network libraries and
160 file I/O modules, which usually don't like being copied and then
161 allowed to continue in two processes.
162
163 With this module only the main program is allowed to create new
164 processes by forking (because only the main program can know when it
165 is still safe to do so) - all other processes are created via
166 fork+exec, which makes it possible to use modules such as event
167 loops or window interfaces safely.
168
169EXAMPLES
170 This is where the wall of text ends and code speaks.
171
11 # create a single new process, tell it to run your worker function 172 Create a single new process, tell it to run your worker function.
12
13 AnyEvent::Fork 173 AnyEvent::Fork
14 ->new 174 ->new
15 ->require ("MyModule") 175 ->require ("MyModule")
16 ->run ("MyModule::worker, sub { 176 ->run ("MyModule::worker, sub {
17 my ($master_filehandle) = @_; 177 my ($master_filehandle) = @_;
18 178
19 # now $master_filehandle is connected to the 179 # now $master_filehandle is connected to the
20 # $slave_filehandle in the new process. 180 # $slave_filehandle in the new process.
21 }); 181 });
22 182
23 # MyModule::worker might look like this 183 "MyModule" might look like this:
184
185 package MyModule;
186
24 sub MyModule::worker { 187 sub worker {
25 my ($slave_filehandle) = @_; 188 my ($slave_filehandle) = @_;
26 189
27 # now $slave_filehandle is connected to the $master_filehandle 190 # now $slave_filehandle is connected to the $master_filehandle
28 # in the original prorcess. have fun! 191 # in the original process. have fun!
29 } 192 }
30 193
31 ##################################################################
32 # create a pool of server processes all accepting on the same socket 194 Create a pool of server processes all accepting on the same socket.
33
34 # create listener socket 195 # create listener socket
35 my $listener = ...; 196 my $listener = ...;
36 197
37 # create a pool template, initialise it and give it the socket 198 # create a pool template, initialise it and give it the socket
38 my $pool = AnyEvent::Fork 199 my $pool = AnyEvent::Fork
… …
49 } 210 }
50 211
51 # now do other things - maybe use the filehandle provided by run 212 # now do other things - maybe use the filehandle provided by run
52 # to wait for the processes to die. or whatever. 213 # to wait for the processes to die. or whatever.
53 214
54 # My::Server::run might look like this 215 "My::Server" might look like this:
55 sub My::Server::run { 216
217 package My::Server;
218
219 sub run {
56 my ($slave, $listener, $id) = @_; 220 my ($slave, $listener, $id) = @_;
57 221
58 close $slave; # we do not use the socket, so close it to save resources 222 close $slave; # we do not use the socket, so close it to save resources
59 223
60 # we could go ballistic and use e.g. AnyEvent here, or IO::AIO, 224 # we could go ballistic and use e.g. AnyEvent here, or IO::AIO,
… …
62 while (my $socket = $listener->accept) { 226 while (my $socket = $listener->accept) {
63 # do sth. with new socket 227 # do sth. with new socket
64 } 228 }
65 } 229 }
66 230
67DESCRIPTION 231 use AnyEvent::Fork as a faster fork+exec
68 This module allows you to create new processes, without actually forking 232 This runs "/bin/echo hi", with standard output redirected to /tmp/log
69 them from your current process (avoiding the problems of forking), but 233 and standard error redirected to the communications socket. It is
70 preserving most of the advantages of fork. 234 usually faster than fork+exec, but still lets you prepare the
235 environment.
71 236
72 It can be used to create new worker processes or new independent 237 open my $output, ">/tmp/log" or die "$!";
73 subprocesses for short- and long-running jobs, process pools (e.g. for
74 use in pre-forked servers) but also to spawn new external processes
75 (such as CGI scripts from a webserver), which can be faster (and more
76 well behaved) than using fork+exec in big processes.
77 238
78 Special care has been taken to make this module useful from other 239 AnyEvent::Fork
79 modules, while still supporting specialised environments such as 240 ->new
241 ->eval ('
242 # compile a helper function for later use
243 sub run {
244 my ($fh, $output, @cmd) = @_;
245
246 # perl will clear close-on-exec on STDOUT/STDERR
247 open STDOUT, ">&", $output or die;
248 open STDERR, ">&", $fh or die;
249
250 exec @cmd;
251 }
252 ')
253 ->send_fh ($output)
254 ->send_arg ("/bin/echo", "hi")
255 ->run ("run", my $cv = AE::cv);
256
257 my $stderr = $cv->recv;
258
259 For stingy users: put the worker code into a "DATA" section.
260 When you want to be stingy with files, you can put your code into the
261 "DATA" section of your module (or program):
262
263 use AnyEvent::Fork;
264
265 AnyEvent::Fork
266 ->new
267 ->eval (do { local $/; <DATA> })
268 ->run ("doit", sub { ... });
269
270 __DATA__
271
272 sub doit {
273 ... do something!
274 }
275
276 For stingy standalone programs: do not rely on external files at
277all.
278 For single-file scripts it can be inconvenient to rely on external files
279 - even when using a "DATA" section, you still need to "exec" an external
280 perl interpreter, which might not be available when using
80 App::Staticperl or PAR::Packer. 281 App::Staticperl, Urlader or PAR::Packer for example.
81 282
82PROBLEM STATEMENT 283 Two modules help here - AnyEvent::Fork::Early forks a template process
83 There are two ways to implement parallel processing on UNIX like 284 for all further calls to "new_exec", and AnyEvent::Fork::Template forks
84 operating systems - fork and process, and fork+exec and process. They 285 the main program as a template process.
85 have different advantages and disadvantages that I describe below,
86 together with how this module tries to mitigate the disadvantages.
87 286
88 Forking from a big process can be very slow (a 5GB process needs 0.05s 287 Here is how your main program should look like:
89 to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead is
90 often shared with exec (because you have to fork first), but in some
91 circumstances (e.g. when vfork is used), fork+exec can be much faster.
92 This module can help here by telling a small(er) helper process to
93 fork, or fork+exec instead.
94 288
95 Forking usually creates a copy-on-write copy of the parent process. 289 #! perl
96 Memory (for example, modules or data files that have been will not take
97 additional memory). When exec'ing a new process, modules and data files
98 might need to be loaded again, at extra cpu and memory cost. Likewise
99 when forking, all data structures are copied as well - if the program
100 frees them and replaces them by new data, the child processes will
101 retain the memory even if it isn't used.
102 This module allows the main program to do a controlled fork, and
103 allows modules to exec processes safely at any time. When creating a
104 custom process pool you can take advantage of data sharing via fork
105 without risking to share large dynamic data structures that will
106 blow up child memory usage.
107 290
108 Exec'ing a new perl process might be difficult and slow. For example, it 291 # optional, as the very first thing.
109 is not easy to find the correct path to the perl interpreter, and all 292 # in case modules want to create their own processes.
110 modules have to be loaded from disk again. Long running processes might 293 use AnyEvent::Fork::Early;
111 run into problems when perl is upgraded for example.
112 This module supports creating pre-initialised perl processes to be
113 used as template, and also tries hard to identify the correct path
114 to the perl interpreter. With a cooperative main program, exec'ing
115 the interpreter might not even be necessary.
116 294
117 Forking might be impossible when a program is running. For example, 295 # next, load all modules you need in your template process
118 POSIX makes it almost impossible to fork from a multithreaded program 296 use Example::My::Module
119 and do anything useful in the child - strictly speaking, if your perl 297 use Example::Whatever;
120 program uses posix threads (even indirectly via e.g. IO::AIO or
121 threads), you cannot call fork on the perl level anymore, at all.
122 This module can safely fork helper processes at any time, by caling
123 fork+exec in C, in a POSIX-compatible way.
124 298
125 Parallel processing with fork might be inconvenient or difficult to 299 # next, put your run function definition and anything else you
126 implement. For example, when a program uses an event loop and creates 300 # need, but do not use code outside of BEGIN blocks.
127 watchers it becomes very hard to use the event loop from a child 301 sub worker_run {
128 program, as the watchers already exist but are only meaningful in the 302 my ($fh, @args) = @_;
129 parent. Worse, a module might want to use such a system, not knowing 303 ...
130 whether another module or the main program also does, leading to 304 }
131 problems. 305
132 This module only lets the main program create pools by forking 306 # now preserve everything so far as AnyEvent::Fork object
133 (because only the main program can know when it is still safe to do 307 # in $TEMPLATE.
134 so) - all other pools are created by fork+exec, after which such 308 use AnyEvent::Fork::Template;
135 modules can again be loaded. 309
310 # do not put code outside of BEGIN blocks until here
311
312 # now use the $TEMPLATE process in any way you like
313
314 # for example: create 10 worker processes
315 my @worker;
316 my $cv = AE::cv;
317 for (1..10) {
318 $cv->begin;
319 $TEMPLATE->fork->send_arg ($_)->run ("worker_run", sub {
320 push @worker, shift;
321 $cv->end;
322 });
323 }
324 $cv->recv;
136 325
137CONCEPTS 326CONCEPTS
138 This module can create new processes either by executing a new perl 327 This module can create new processes either by executing a new perl
139 process, or by forking from an existing "template" process. 328 process, or by forking from an existing "template" process.
329
330 All these processes are called "child processes" (whether they are
331 direct children or not), while the process that manages them is called
332 the "parent process".
140 333
141 Each such process comes with its own file handle that can be used to 334 Each such process comes with its own file handle that can be used to
142 communicate with it (it's actually a socket - one end in the new 335 communicate with it (it's actually a socket - one end in the new
143 process, one end in the main process), and among the things you can do 336 process, one end in the main process), and among the things you can do
144 in it are load modules, fork new processes, send file handles to it, and 337 in it are load modules, fork new processes, send file handles to it, and
… …
154 memory used for the perl interpreter with the new process, but 347 memory used for the perl interpreter with the new process, but
155 loading modules takes time, and the memory is not shared with 348 loading modules takes time, and the memory is not shared with
156 anything else. 349 anything else.
157 350
158 This is ideal for when you only need one extra process of a kind, 351 This is ideal for when you only need one extra process of a kind,
159 with the option of starting and stipping it on demand. 352 with the option of starting and stopping it on demand.
160 353
161 Example: 354 Example:
162 355
163 AnyEvent::Fork 356 AnyEvent::Fork
164 ->new 357 ->new
… …
178 the modules you loaded) is shared between the processes, and each 371 the modules you loaded) is shared between the processes, and each
179 new process consumes relatively little memory of its own. 372 new process consumes relatively little memory of its own.
180 373
181 The disadvantage of this approach is that you need to create a 374 The disadvantage of this approach is that you need to create a
182 template process for the sole purpose of forking new processes from 375 template process for the sole purpose of forking new processes from
183 it, but if you only need a fixed number of proceses you can create 376 it, but if you only need a fixed number of processes you can create
184 them, and then destroy the template process. 377 them, and then destroy the template process.
185 378
186 Example: 379 Example:
187 380
188 my $template = AnyEvent::Fork->new->require ("Some::Module"); 381 my $template = AnyEvent::Fork->new->require ("Some::Module");
… …
212 ->require ("Some::Module") 405 ->require ("Some::Module")
213 ->run ("Some::Module::run", sub { 406 ->run ("Some::Module::run", sub {
214 my ($fork_fh) = @_; 407 my ($fork_fh) = @_;
215 }); 408 });
216 409
217FUNCTIONS 410THE "AnyEvent::Fork" CLASS
218 my $pool = new AnyEvent::Fork key => value... 411 This module exports nothing, and only implements a single class -
219 Create a new process pool. The following named parameters are 412 "AnyEvent::Fork".
220 supported: 413
414 There are two class constructors that both create new processes - "new"
415 and "new_exec". The "fork" method creates a new process by forking an
416 existing one and could be considered a third constructor.
417
418 Most of the remaining methods deal with preparing the new process, by
419 loading code, evaluating code and sending data to the new process. They
420 usually return the process object, so you can chain method calls.
421
422 If a process object is destroyed before calling its "run" method, then
423 the process simply exits. After "run" is called, all responsibility is
424 passed to the specified function.
425
426 As long as there is any outstanding work to be done, process objects
427 resist being destroyed, so there is no reason to store them unless you
428 need them later - configure and forget works just fine.
221 429
222 my $proc = new AnyEvent::Fork 430 my $proc = new AnyEvent::Fork
223 Create a new "empty" perl interpreter process and returns its 431 Create a new "empty" perl interpreter process and returns its
224 process object for further manipulation. 432 process object for further manipulation.
225 433
226 The new process is forked from a template process that is kept 434 The new process is forked from a template process that is kept
227 around for this purpose. When it doesn't exist yet, it is created by 435 around for this purpose. When it doesn't exist yet, it is created by
228 a call to "new_exec" and kept around for future calls. 436 a call to "new_exec" first and then stays around for future calls.
229
230 When the process object is destroyed, it will release the file
231 handle that connects it with the new process. When the new process
232 has not yet called "run", then the process will exit. Otherwise,
233 what happens depends entirely on the code that is executed.
234 437
235 $new_proc = $proc->fork 438 $new_proc = $proc->fork
236 Forks $proc, creating a new process, and returns the process object 439 Forks $proc, creating a new process, and returns the process object
237 of the new process. 440 of the new process.
238 441
… …
251 possible, and is also slower. 454 possible, and is also slower.
252 455
253 You should use "new" whenever possible, except when having a 456 You should use "new" whenever possible, except when having a
254 template process around is unacceptable. 457 template process around is unacceptable.
255 458
256 The path to the perl interpreter is divined usign various methods - 459 The path to the perl interpreter is divined using various methods -
257 first $^X is investigated to see if the path ends with something 460 first $^X is investigated to see if the path ends with something
258 that sounds as if it were the perl interpreter. Failing this, the 461 that looks as if it were the perl interpreter. Failing this, the
259 module falls back to using $Config::Config{perlpath}. 462 module falls back to using $Config::Config{perlpath}.
260 463
464 The path to perl can also be overridden by setting the global
465 variable $AnyEvent::Fork::PERL - it's value will be used for all
466 subsequent invocations.
467
468 $pid = $proc->pid
469 Returns the process id of the process *iff it is a direct child of
470 the process running AnyEvent::Fork*, and "undef" otherwise. As a
471 general rule (that you cannot rely upon), processes created via
472 "new_exec", AnyEvent::Fork::Early or AnyEvent::Fork::Template are
473 direct children, while all other processes are not.
474
475 Or in other words, you do not normally have to take care of zombies
476 for processes created via "new", but when in doubt, or zombies are a
477 problem, you need to check whether a process is a diretc child by
478 calling this method, and possibly creating a child watcher or reap
479 it manually.
480
261 $proc = $proc->eval ($perlcode, @args) 481 $proc = $proc->eval ($perlcode, @args)
262 Evaluates the given $perlcode as ... perl code, while setting @_ to 482 Evaluates the given $perlcode as ... Perl code, while setting @_ to
263 the strings specified by @args. 483 the strings specified by @args, in the "main" package (so you can
484 access the args using $_[0] and so on, but not using implicit "shit"
485 as the latter works on @ARGV).
264 486
265 This call is meant to do any custom initialisation that might be 487 This call is meant to do any custom initialisation that might be
266 required (for example, the "require" method uses it). It's not 488 required (for example, the "require" method uses it). It's not
267 supposed to be used to completely take over the process, use "run" 489 supposed to be used to completely take over the process, use "run"
268 for that. 490 for that.
… …
270 The code will usually be executed after this call returns, and there 492 The code will usually be executed after this call returns, and there
271 is no way to pass anything back to the calling process. Any 493 is no way to pass anything back to the calling process. Any
272 evaluation errors will be reported to stderr and cause the process 494 evaluation errors will be reported to stderr and cause the process
273 to exit. 495 to exit.
274 496
497 If you want to execute some code (that isn't in a module) to take
498 over the process, you should compile a function via "eval" first,
499 and then call it via "run". This also gives you access to any
500 arguments passed via the "send_xxx" methods, such as file handles.
501 See the "use AnyEvent::Fork as a faster fork+exec" example to see it
502 in action.
503
275 Returns the process object for easy chaining of method calls. 504 Returns the process object for easy chaining of method calls.
505
506 It's common to want to call an iniitalisation function with some
507 arguments. Make sure you actually pass @_ to that function (for
508 example by using &name syntax), and do not just specify a function
509 name:
510
511 $proc->eval ('&MyModule::init', $string1, $string2);
276 512
277 $proc = $proc->require ($module, ...) 513 $proc = $proc->require ($module, ...)
278 Tries to load the given module(s) into the process 514 Tries to load the given module(s) into the process
279 515
280 Returns the process object for easy chaining of method calls. 516 Returns the process object for easy chaining of method calls.
281 517
282 $proc = $proc->send_fh ($handle, ...) 518 $proc = $proc->send_fh ($handle, ...)
283 Send one or more file handles (*not* file descriptors) to the 519 Send one or more file handles (*not* file descriptors) to the
284 process, to prepare a call to "run". 520 process, to prepare a call to "run".
285 521
286 The process object keeps a reference to the handles until this is 522 The process object keeps a reference to the handles until they have
287 done, so you must not explicitly close the handles. This is most 523 been passed over to the process, so you must not explicitly close
288 easily accomplished by simply not storing the file handles anywhere 524 the handles. This is most easily accomplished by simply not storing
289 after passing them to this method. 525 the file handles anywhere after passing them to this method - when
526 AnyEvent::Fork is finished using them, perl will automatically close
527 them.
290 528
291 Returns the process object for easy chaining of method calls. 529 Returns the process object for easy chaining of method calls.
292 530
293 Example: pass an fh to a process, and release it without closing. it 531 Example: pass a file handle to a process, and release it without
294 will be closed automatically when it is no longer used. 532 closing. It will be closed automatically when it is no longer used.
295 533
296 $proc->send_fh ($my_fh); 534 $proc->send_fh ($my_fh);
297 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT 535 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT
298 536
299 $proc = $proc->send_arg ($string, ...) 537 $proc = $proc->send_arg ($string, ...)
300 Send one or more argument strings to the process, to prepare a call 538 Send one or more argument strings to the process, to prepare a call
301 to "run". The strings can be any octet string. 539 to "run". The strings can be any octet strings.
302 540
541 The protocol is optimised to pass a moderate number of relatively
542 short strings - while you can pass up to 4GB of data in one go, this
543 is more meant to pass some ID information or other startup info, not
544 big chunks of data.
545
303 Returns the process object for easy chaining of emthod calls. 546 Returns the process object for easy chaining of method calls.
304 547
305 $proc->run ($func, $cb->($fh)) 548 $proc->run ($func, $cb->($fh))
306 Enter the function specified by the fully qualified name in $func in 549 Enter the function specified by the function name in $func in the
307 the process. The function is called with the communication socket as 550 process. The function is called with the communication socket as
308 first argument, followed by all file handles and string arguments 551 first argument, followed by all file handles and string arguments
309 sent earlier via "send_fh" and "send_arg" methods, in the order they 552 sent earlier via "send_fh" and "send_arg" methods, in the order they
310 were called. 553 were called.
311 554
312 If the called function returns, the process exits.
313
314 Preparing the process can take time - when the process is ready, the
315 callback is invoked with the local communications socket as
316 argument.
317
318 The process object becomes unusable on return from this function. 555 The process object becomes unusable on return from this function -
556 any further method calls result in undefined behaviour.
557
558 The function name should be fully qualified, but if it isn't, it
559 will be looked up in the "main" package.
560
561 If the called function returns, doesn't exist, or any error occurs,
562 the process exits.
563
564 Preparing the process is done in the background - when all commands
565 have been sent, the callback is invoked with the local
566 communications socket as argument. At this point you can start using
567 the socket in any way you like.
319 568
320 If the communication socket isn't used, it should be closed on both 569 If the communication socket isn't used, it should be closed on both
321 sides, to save on kernel memory. 570 sides, to save on kernel memory.
322 571
323 The socket is non-blocking in the parent, and blocking in the newly 572 The socket is non-blocking in the parent, and blocking in the newly
324 created process. The close-on-exec flag is set on both. Even if not 573 created process. The close-on-exec flag is set in both.
574
325 used otherwise, the socket can be a good indicator for the existance 575 Even if not used otherwise, the socket can be a good indicator for
326 of the process - if the other process exits, you get a readable 576 the existence of the process - if the other process exits, you get a
327 event on it, because exiting the process closes the socket (if it 577 readable event on it, because exiting the process closes the socket
328 didn't create any children using fork). 578 (if it didn't create any children using fork).
579
580 Compatibility to AnyEvent::Fork::Remote
581 If you want to write code that works with both this module and
582 AnyEvent::Fork::Remote, you need to write your code so that it
583 assumes there are two file handles for communications, which
584 might not be unix domain sockets. The "run" function should
585 start like this:
586
587 sub run {
588 my ($rfh, @args) = @_; # @args is your normal arguments
589 my $wfh = fileno $rfh ? $rfh : *STDOUT;
590
591 # now use $rfh for reading and $wfh for writing
592 }
593
594 This checks whether the passed file handle is, in fact, the
595 process "STDIN" handle. If it is, then the function was invoked
596 visa AnyEvent::Fork::Remote, so STDIN should be used for reading
597 and "STDOUT" should be used for writing.
598
599 In all other cases, the function was called via this module, and
600 there is only one file handle that should be sued for reading
601 and writing.
329 602
330 Example: create a template for a process pool, pass a few strings, 603 Example: create a template for a process pool, pass a few strings,
331 some file handles, then fork, pass one more string, and run some 604 some file handles, then fork, pass one more string, and run some
332 code. 605 code.
333 606
… …
342 ->send_arg ("str3") 615 ->send_arg ("str3")
343 ->run ("Some::function", sub { 616 ->run ("Some::function", sub {
344 my ($fh) = @_; 617 my ($fh) = @_;
345 618
346 # fh is nonblocking, but we trust that the OS can accept these 619 # fh is nonblocking, but we trust that the OS can accept these
347 # extra 3 octets anyway. 620 # few octets anyway.
348 syswrite $fh, "hi #$_\n"; 621 syswrite $fh, "hi #$_\n";
349 622
350 # $fh is being closed here, as we don't store it anywhere 623 # $fh is being closed here, as we don't store it anywhere
351 }); 624 });
352 } 625 }
… …
354 # Some::function might look like this - all parameters passed before fork 627 # Some::function might look like this - all parameters passed before fork
355 # and after will be passed, in order, after the communications socket. 628 # and after will be passed, in order, after the communications socket.
356 sub Some::function { 629 sub Some::function {
357 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; 630 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_;
358 631
359 print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" 632 print scalar <$fh>; # prints "hi #1\n" and "hi #2\n" in any order
360 } 633 }
634
635 CHILD PROCESS INTERFACE
636 This module has a limited API for use in child processes.
637
638 @args = AnyEvent::Fork::Serve::run_args
639 This function, which only exists before the "run" method is called,
640 returns the arguments that would be passed to the run function, and
641 clears them.
642
643 This is mainly useful to get any file handles passed via "send_fh",
644 but works for any arguments passed via "send_*xxx*" methods.
645
646 EXPERIMENTAL METHODS
647 These methods might go away completely or change behaviour, at any time.
648
649 $proc->to_fh ($cb->($fh)) # EXPERIMENTAL, MIGHT BE REMOVED
650 Flushes all commands out to the process and then calls the callback
651 with the communications socket.
652
653 The process object becomes unusable on return from this function -
654 any further method calls result in undefined behaviour.
655
656 The point of this method is to give you a file handle that you can
657 pass to another process. In that other process, you can call
658 "new_from_fh AnyEvent::Fork $fh" to create a new "AnyEvent::Fork"
659 object from it, thereby effectively passing a fork object to another
660 process.
661
662 new_from_fh AnyEvent::Fork $fh # EXPERIMENTAL, MIGHT BE REMOVED
663 Takes a file handle originally rceeived by the "to_fh" method and
664 creates a new "AnyEvent:Fork" object. The child process itself will
665 not change in any way, i.e. it will keep all the modifications done
666 to it before calling "to_fh".
667
668 The new object is very much like the original object, except that
669 the "pid" method will return "undef" even if the process is a direct
670 child.
671
672PERFORMANCE
673 Now for some unscientific benchmark numbers (all done on an amd64
674 GNU/Linux box). These are intended to give you an idea of the relative
675 performance you can expect, they are not meant to be absolute
676 performance numbers.
677
678 OK, so, I ran a simple benchmark that creates a socket pair, forks,
679 calls exit in the child and waits for the socket to close in the parent.
680 I did load AnyEvent, EV and AnyEvent::Fork, for a total process size of
681 5100kB.
682
683 2079 new processes per second, using manual socketpair + fork
684
685 Then I did the same thing, but instead of calling fork, I called
686 AnyEvent::Fork->new->run ("CORE::exit") and then again waited for the
687 socket from the child to close on exit. This does the same thing as
688 manual socket pair + fork, except that what is forked is the template
689 process (2440kB), and the socket needs to be passed to the server at the
690 other end of the socket first.
691
692 2307 new processes per second, using AnyEvent::Fork->new
693
694 And finally, using "new_exec" instead "new", using vforks+execs to exec
695 a new perl interpreter and compile the small server each time, I get:
696
697 479 vfork+execs per second, using AnyEvent::Fork->new_exec
698
699 So how can "AnyEvent->new" be faster than a standard fork, even though
700 it uses the same operations, but adds a lot of overhead?
701
702 The difference is simply the process size: forking the 5MB process takes
703 so much longer than forking the 2.5MB template process that the extra
704 overhead is canceled out.
705
706 If the benchmark process grows, the normal fork becomes even slower:
707
708 1340 new processes, manual fork of a 20MB process
709 731 new processes, manual fork of a 200MB process
710 235 new processes, manual fork of a 2000MB process
711
712 What that means (to me) is that I can use this module without having a
713 bad conscience because of the extra overhead required to start new
714 processes.
715
716TYPICAL PROBLEMS
717 This section lists typical problems that remain. I hope by recognising
718 them, most can be avoided.
719
720 leaked file descriptors for exec'ed processes
721 POSIX systems inherit file descriptors by default when exec'ing a
722 new process. While perl itself laudably sets the close-on-exec flags
723 on new file handles, most C libraries don't care, and even if all
724 cared, it's often not possible to set the flag in a race-free
725 manner.
726
727 That means some file descriptors can leak through. And since it
728 isn't possible to know which file descriptors are "good" and
729 "necessary" (or even to know which file descriptors are open), there
730 is no good way to close the ones that might harm.
731
732 As an example of what "harm" can be done consider a web server that
733 accepts connections and afterwards some module uses AnyEvent::Fork
734 for the first time, causing it to fork and exec a new process, which
735 might inherit the network socket. When the server closes the socket,
736 it is still open in the child (which doesn't even know that) and the
737 client might conclude that the connection is still fine.
738
739 For the main program, there are multiple remedies available -
740 AnyEvent::Fork::Early is one, creating a process early and not using
741 "new_exec" is another, as in both cases, the first process can be
742 exec'ed well before many random file descriptors are open.
743
744 In general, the solution for these kind of problems is to fix the
745 libraries or the code that leaks those file descriptors.
746
747 Fortunately, most of these leaked descriptors do no harm, other than
748 sitting on some resources.
749
750 leaked file descriptors for fork'ed processes
751 Normally, AnyEvent::Fork does start new processes by exec'ing them,
752 which closes file descriptors not marked for being inherited.
753
754 However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a
755 way to create these processes by forking, and this leaks more file
756 descriptors than exec'ing them, as there is no way to mark
757 descriptors as "close on fork".
758
759 An example would be modules like EV, IO::AIO or Gtk2. Both create
760 pipes for internal uses, and Gtk2 might open a connection to the X
761 server. EV and IO::AIO can deal with fork, but Gtk2 might have
762 trouble with a fork.
763
764 The solution is to either not load these modules before use'ing
765 AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay
766 initialising them, for example, by calling "init Gtk2" manually.
767
768 exiting calls object destructors
769 This only applies to users of AnyEvent::Fork:Early and
770 AnyEvent::Fork::Template, or when initialising code creates objects
771 that reference external resources.
772
773 When a process created by AnyEvent::Fork exits, it might do so by
774 calling exit, or simply letting perl reach the end of the program.
775 At which point Perl runs all destructors.
776
777 Not all destructors are fork-safe - for example, an object that
778 represents the connection to an X display might tell the X server to
779 free resources, which is inconvenient when the "real" object in the
780 parent still needs to use them.
781
782 This is obviously not a problem for AnyEvent::Fork::Early, as you
783 used it as the very first thing, right?
784
785 It is a problem for AnyEvent::Fork::Template though - and the
786 solution is to not create objects with nontrivial destructors that
787 might have an effect outside of Perl.
361 788
362PORTABILITY NOTES 789PORTABILITY NOTES
363 Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a 790 Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a
364 nop, and ::Template is not going to work), and it cost a lot of blood 791 nop, and ::Template is not going to work), and it cost a lot of blood
365 and sweat to make it so, mostly due to the bloody broken perl that 792 and sweat to make it so, mostly due to the bloody broken perl that
366 nobody seems to care about. The fork emulation is a bad joke - I have 793 nobody seems to care about. The fork emulation is a bad joke - I have
367 yet to see something useful that you cna do with it without running into 794 yet to see something useful that you can do with it without running into
368 memory corruption issues or other braindamage. Hrrrr. 795 memory corruption issues or other braindamage. Hrrrr.
369 796
370 Cygwin perl is not supported at the moment, as it should implement fd 797 Since fork is endlessly broken on win32 perls (it doesn't even remotely
371 passing, but doesn't, and rolling my own is hard, as cygwin doesn't 798 work within it's documented limits) and quite obviously it's not getting
372 support enough functionality to do it. 799 improved any time soon, the best way to proceed on windows would be to
800 always use "new_exec" and thus never rely on perl's fork "emulation".
373 801
374AUTHOR 802 Cygwin perl is not supported at the moment due to some hilarious
803 shortcomings of its API - see IO::FDPoll for more details. If you never
804 use "send_fh" and always use "new_exec" to create processes, it should
805 work though.
806
807USING AnyEvent::Fork IN SUBPROCESSES
808 AnyEvent::Fork itself cannot generally be used in subprocesses. As long
809 as only one process ever forks new processes, sharing the template
810 processes is possible (you could use a pipe as a lock by writing a byte
811 into it to unlock, and reading the byte to lock for example)
812
813 To make concurrent calls possible after fork, you should get rid of the
814 template and early fork processes. AnyEvent::Fork will create a new
815 template process as needed.
816
817 undef $AnyEvent::Fork::EARLY;
818 undef $AnyEvent::Fork::TEMPLATE;
819
820 It doesn't matter whether you get rid of them in the parent or child
821 after a fork.
822
823SEE ALSO
824 AnyEvent::Fork::Early, to avoid executing a perl interpreter at all
825 (part of this distribution).
826
827 AnyEvent::Fork::Template, to create a process by forking the main
828 program at a convenient time (part of this distribution).
829
830 AnyEvent::Fork::Remote, for another way to create processes that is
831 mostly compatible to this module and modules building on top of it, but
832 works better with remote processes.
833
834 AnyEvent::Fork::RPC, for simple RPC to child processes (on CPAN).
835
836 AnyEvent::Fork::Pool, for simple worker process pool (on CPAN).
837
838AUTHOR AND CONTACT INFORMATION
375 Marc Lehmann <schmorp@schmorp.de> 839 Marc Lehmann <schmorp@schmorp.de>
376 http://home.schmorp.de/ 840 http://software.schmorp.de/pkg/AnyEvent-Fork
377 841

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