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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
4SYNOPSIS 4SYNOPSIS
5 use AnyEvent::Fork; 5 use AnyEvent::Fork;
6 6
7 ################################################################## 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, not being able to use event
57 processing or similar modules in the processes they create.
58
59 This module doesn't try to replace any of them - instead it tries to
60 solve the problem of creating processes with a minimum of fuss and
61 overhead (and also luxury). Ideally, most of these would use
62 AnyEvent::Fork internally, except they were written before AnyEvent:Fork
63 was available, so obviously had to roll their own.
64
65 PROBLEM STATEMENT
66 There are two traditional ways to implement parallel processing on UNIX
67 like operating systems - fork and process, and fork+exec and process.
68 They have different advantages and disadvantages that I describe below,
69 together with how this module tries to mitigate the disadvantages.
70
71 Forking from a big process can be very slow.
72 A 5GB process needs 0.05s to fork on my 3.6GHz amd64 GNU/Linux box.
73 This overhead is often shared with exec (because you have to fork
74 first), but in some circumstances (e.g. when vfork is used),
75 fork+exec can be much faster.
76
77 This module can help here by telling a small(er) helper process to
78 fork, which is faster then forking the main process, and also uses
79 vfork where possible. This gives the speed of vfork, with the
80 flexibility of fork.
81
82 Forking usually creates a copy-on-write copy of the parent process.
83 For example, modules or data files that are loaded will not use
84 additional memory after a fork. When exec'ing a new process, modules
85 and data files might need to be loaded again, at extra CPU and
86 memory cost. But when forking, literally all data structures are
87 copied - if the program frees them and replaces them by new data,
88 the child processes will retain the old version even if it isn't
89 used, which can suddenly and unexpectedly increase memory usage when
90 freeing memory.
91
92 The trade-off is between more sharing with fork (which can be good
93 or bad), and no sharing with exec.
94
95 This module allows the main program to do a controlled fork, and
96 allows modules to exec processes safely at any time. When creating a
97 custom process pool you can take advantage of data sharing via fork
98 without risking to share large dynamic data structures that will
99 blow up child memory usage.
100
101 In other words, this module puts you into control over what is being
102 shared and what isn't, at all times.
103
104 Exec'ing a new perl process might be difficult.
105 For example, it is not easy to find the correct path to the perl
106 interpreter - $^X might not be a perl interpreter at all.
107
108 This module tries hard to identify the correct path to the perl
109 interpreter. With a cooperative main program, exec'ing the
110 interpreter might not even be necessary, but even without help from
111 the main program, it will still work when used from a module.
112
113 Exec'ing a new perl process might be slow, as all necessary modules have
114 to be loaded from disk again, with no guarantees of success.
115 Long running processes might run into problems when perl is upgraded
116 and modules are no longer loadable because they refer to a different
117 perl version, or parts of a distribution are newer than the ones
118 already loaded.
119
120 This module supports creating pre-initialised perl processes to be
121 used as a template for new processes.
122
123 Forking might be impossible when a program is running.
124 For example, POSIX makes it almost impossible to fork from a
125 multi-threaded program while doing anything useful in the child - in
126 fact, if your perl program uses POSIX threads (even indirectly via
127 e.g. IO::AIO or threads), you cannot call fork on the perl level
128 anymore without risking corruption issues on a number of operating
129 systems.
130
131 This module can safely fork helper processes at any time, by calling
132 fork+exec in C, in a POSIX-compatible way (via Proc::FastSpawn).
133
134 Parallel processing with fork might be inconvenient or difficult to
135 implement. Modules might not work in both parent and child.
136 For example, when a program uses an event loop and creates watchers
137 it becomes very hard to use the event loop from a child program, as
138 the watchers already exist but are only meaningful in the parent.
139 Worse, a module might want to use such a module, not knowing whether
140 another module or the main program also does, leading to problems.
141
142 Apart from event loops, graphical toolkits also commonly fall into
143 the "unsafe module" category, or just about anything that
144 communicates with the external world, such as network libraries and
145 file I/O modules, which usually don't like being copied and then
146 allowed to continue in two processes.
147
148 With this module only the main program is allowed to create new
149 processes by forking (because only the main program can know when it
150 is still safe to do so) - all other processes are created via
151 fork+exec, which makes it possible to use modules such as event
152 loops or window interfaces safely.
153
154EXAMPLES
8 # create a single new process, tell it to run your worker function 155 Create a single new process, tell it to run your worker function.
9
10 AnyEvent::Fork 156 AnyEvent::Fork
11 ->new 157 ->new
12 ->require ("MyModule") 158 ->require ("MyModule")
13 ->run ("MyModule::worker, sub { 159 ->run ("MyModule::worker, sub {
14 my ($master_filehandle) = @_; 160 my ($master_filehandle) = @_;
15 161
16 # now $master_filehandle is connected to the 162 # now $master_filehandle is connected to the
17 # $slave_filehandle in the new process. 163 # $slave_filehandle in the new process.
18 }); 164 });
19 165
20 # MyModule::worker might look like this 166 "MyModule" might look like this:
167
168 package MyModule;
169
21 sub MyModule::worker { 170 sub worker {
22 my ($slave_filehandle) = @_; 171 my ($slave_filehandle) = @_;
23 172
24 # now $slave_filehandle is connected to the $master_filehandle 173 # now $slave_filehandle is connected to the $master_filehandle
25 # in the original prorcess. have fun! 174 # in the original prorcess. have fun!
26 } 175 }
27 176
28 ##################################################################
29 # create a pool of server processes all accepting on the same socket 177 Create a pool of server processes all accepting on the same socket.
30
31 # create listener socket 178 # create listener socket
32 my $listener = ...; 179 my $listener = ...;
33 180
34 # create a pool template, initialise it and give it the socket 181 # create a pool template, initialise it and give it the socket
35 my $pool = AnyEvent::Fork 182 my $pool = AnyEvent::Fork
… …
46 } 193 }
47 194
48 # now do other things - maybe use the filehandle provided by run 195 # now do other things - maybe use the filehandle provided by run
49 # to wait for the processes to die. or whatever. 196 # to wait for the processes to die. or whatever.
50 197
51 # My::Server::run might look like this 198 "My::Server" might look like this:
52 sub My::Server::run { 199
200 package My::Server;
201
202 sub run {
53 my ($slave, $listener, $id) = @_; 203 my ($slave, $listener, $id) = @_;
54 204
55 close $slave; # we do not use the socket, so close it to save resources 205 close $slave; # we do not use the socket, so close it to save resources
56 206
57 # we could go ballistic and use e.g. AnyEvent here, or IO::AIO, 207 # we could go ballistic and use e.g. AnyEvent here, or IO::AIO,
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59 while (my $socket = $listener->accept) { 209 while (my $socket = $listener->accept) {
60 # do sth. with new socket 210 # do sth. with new socket
61 } 211 }
62 } 212 }
63 213
64DESCRIPTION 214 use AnyEvent::Fork as a faster fork+exec
65 This module allows you to create new processes, without actually forking 215 This runs "/bin/echo hi", with standard output redirected to /tmp/log
66 them from your current process (avoiding the problems of forking), but 216 and standard error redirected to the communications socket. It is
67 preserving most of the advantages of fork. 217 usually faster than fork+exec, but still lets you prepare the
218 environment.
68 219
69 It can be used to create new worker processes or new independent 220 open my $output, ">/tmp/log" or die "$!";
70 subprocesses for short- and long-running jobs, process pools (e.g. for
71 use in pre-forked servers) but also to spawn new external processes
72 (such as CGI scripts from a webserver), which can be faster (and more
73 well behaved) than using fork+exec in big processes.
74 221
75 Special care has been taken to make this module useful from other 222 AnyEvent::Fork
76 modules, while still supporting specialised environments such as 223 ->new
224 ->eval ('
225 # compile a helper function for later use
226 sub run {
227 my ($fh, $output, @cmd) = @_;
228
229 # perl will clear close-on-exec on STDOUT/STDERR
230 open STDOUT, ">&", $output or die;
231 open STDERR, ">&", $fh or die;
232
233 exec @cmd;
234 }
235 ')
236 ->send_fh ($output)
237 ->send_arg ("/bin/echo", "hi")
238 ->run ("run", my $cv = AE::cv);
239
240 my $stderr = $cv->recv;
241
242 For stingy users: put the worker code into a "DATA" section.
243 When you want to be stingy with files, you cna put your code into the
244 "DATA" section of your module (or program):
245
246 use AnyEvent::Fork;
247
248 AnyEvent::Fork
249 ->new
250 ->eval (do { local $/; <DATA> })
251 ->run ("doit", sub { ... });
252
253 __DATA__
254
255 sub doit {
256 ... do something!
257 }
258
259 For stingy standalone programs: do not rely on external files at
260all.
261 For single-file scripts it can be inconvenient to rely on external files
262 - even when using < "DATA" section, you still need to "exec" an external
263 perl interpreter, which might not be available when using
77 App::Staticperl or PAR::Packer. 264 App::Staticperl, Urlader or PAR::Packer for example.
78 265
79PROBLEM STATEMENT 266 Two modules help here - AnyEvent::Fork::Early forks a template process
80 There are two ways to implement parallel processing on UNIX like 267 for all further calls to "new_exec", and AnyEvent::Fork::Template forks
81 operating systems - fork and process, and fork+exec and process. They 268 the main program as a template process.
82 have different advantages and disadvantages that I describe below,
83 together with how this module tries to mitigate the disadvantages.
84 269
85 Forking from a big process can be very slow (a 5GB process needs 0.05s 270 Here is how your main program should look like:
86 to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead is
87 often shared with exec (because you have to fork first), but in some
88 circumstances (e.g. when vfork is used), fork+exec can be much faster.
89 This module can help here by telling a small(er) helper process to
90 fork, or fork+exec instead.
91 271
92 Forking usually creates a copy-on-write copy of the parent process. 272 #! perl
93 Memory (for example, modules or data files that have been will not take
94 additional memory). When exec'ing a new process, modules and data files
95 might need to be loaded again, at extra cpu and memory cost. Likewise
96 when forking, all data structures are copied as well - if the program
97 frees them and replaces them by new data, the child processes will
98 retain the memory even if it isn't used.
99 This module allows the main program to do a controlled fork, and
100 allows modules to exec processes safely at any time. When creating a
101 custom process pool you can take advantage of data sharing via fork
102 without risking to share large dynamic data structures that will
103 blow up child memory usage.
104 273
105 Exec'ing a new perl process might be difficult and slow. For example, it 274 # optional, as the very first thing.
106 is not easy to find the correct path to the perl interpreter, and all 275 # in case modules want to create their own processes.
107 modules have to be loaded from disk again. Long running processes might 276 use AnyEvent::Fork::Early;
108 run into problems when perl is upgraded for example.
109 This module supports creating pre-initialised perl processes to be
110 used as template, and also tries hard to identify the correct path
111 to the perl interpreter. With a cooperative main program, exec'ing
112 the interpreter might not even be necessary.
113 277
114 Forking might be impossible when a program is running. For example, 278 # next, load all modules you need in your template process
115 POSIX makes it almost impossible to fork from a multithreaded program 279 use Example::My::Module
116 and do anything useful in the child - strictly speaking, if your perl 280 use Example::Whatever;
117 program uses posix threads (even indirectly via e.g. IO::AIO or
118 threads), you cannot call fork on the perl level anymore, at all.
119 This module can safely fork helper processes at any time, by caling
120 fork+exec in C, in a POSIX-compatible way.
121 281
122 Parallel processing with fork might be inconvenient or difficult to 282 # next, put your run function definition and anything else you
123 implement. For example, when a program uses an event loop and creates 283 # need, but do not use code outside of BEGIN blocks.
124 watchers it becomes very hard to use the event loop from a child 284 sub worker_run {
125 program, as the watchers already exist but are only meaningful in the 285 my ($fh, @args) = @_;
126 parent. Worse, a module might want to use such a system, not knowing 286 ...
127 whether another module or the main program also does, leading to 287 }
128 problems.
129 This module only lets the main program create pools by forking
130 (because only the main program can know when it is still safe to do
131 so) - all other pools are created by fork+exec, after which such
132 modules can again be loaded.
133 288
134CONCEPTS 289 # now preserve everything so far as AnyEvent::Fork object
290 # in §TEMPLATE.
291 use AnyEvent::Fork::Template;
292
293 # do not put code outside of BEGIN blocks until here
294
295 # now use the $TEMPLATE process in any way you like
296
297 # for example: create 10 worker processes
298 my @worker;
299 my $cv = AE::cv;
300 for (1..10) {
301 $cv->begin;
302 $TEMPLATE->fork->send_arg ($_)->run ("worker_run", sub {
303 push @worker, shift;
304 $cv->end;
305 });
306 }
307 $cv->recv;
308
309 lhead1 CONCEPTS
310
135 This module can create new processes either by executing a new perl 311 This module can create new processes either by executing a new perl
136 process, or by forking from an existing "template" process. 312 process, or by forking from an existing "template" process.
313
314 All these processes are called "child processes" (whether they are
315 direct children or not), while the process that manages them is called
316 the "parent process".
137 317
138 Each such process comes with its own file handle that can be used to 318 Each such process comes with its own file handle that can be used to
139 communicate with it (it's actually a socket - one end in the new 319 communicate with it (it's actually a socket - one end in the new
140 process, one end in the main process), and among the things you can do 320 process, one end in the main process), and among the things you can do
141 in it are load modules, fork new processes, send file handles to it, and 321 in it are load modules, fork new processes, send file handles to it, and
… …
151 memory used for the perl interpreter with the new process, but 331 memory used for the perl interpreter with the new process, but
152 loading modules takes time, and the memory is not shared with 332 loading modules takes time, and the memory is not shared with
153 anything else. 333 anything else.
154 334
155 This is ideal for when you only need one extra process of a kind, 335 This is ideal for when you only need one extra process of a kind,
156 with the option of starting and stipping it on demand. 336 with the option of starting and stopping it on demand.
157 337
158 Example: 338 Example:
159 339
160 AnyEvent::Fork 340 AnyEvent::Fork
161 ->new 341 ->new
… …
175 the modules you loaded) is shared between the processes, and each 355 the modules you loaded) is shared between the processes, and each
176 new process consumes relatively little memory of its own. 356 new process consumes relatively little memory of its own.
177 357
178 The disadvantage of this approach is that you need to create a 358 The disadvantage of this approach is that you need to create a
179 template process for the sole purpose of forking new processes from 359 template process for the sole purpose of forking new processes from
180 it, but if you only need a fixed number of proceses you can create 360 it, but if you only need a fixed number of processes you can create
181 them, and then destroy the template process. 361 them, and then destroy the template process.
182 362
183 Example: 363 Example:
184 364
185 my $template = AnyEvent::Fork->new->require ("Some::Module"); 365 my $template = AnyEvent::Fork->new->require ("Some::Module");
… …
209 ->require ("Some::Module") 389 ->require ("Some::Module")
210 ->run ("Some::Module::run", sub { 390 ->run ("Some::Module::run", sub {
211 my ($fork_fh) = @_; 391 my ($fork_fh) = @_;
212 }); 392 });
213 393
214FUNCTIONS 394THE "AnyEvent::Fork" CLASS
215 my $pool = new AnyEvent::Fork key => value... 395 This module exports nothing, and only implements a single class -
216 Create a new process pool. The following named parameters are 396 "AnyEvent::Fork".
217 supported: 397
398 There are two class constructors that both create new processes - "new"
399 and "new_exec". The "fork" method creates a new process by forking an
400 existing one and could be considered a third constructor.
401
402 Most of the remaining methods deal with preparing the new process, by
403 loading code, evaluating code and sending data to the new process. They
404 usually return the process object, so you can chain method calls.
405
406 If a process object is destroyed before calling its "run" method, then
407 the process simply exits. After "run" is called, all responsibility is
408 passed to the specified function.
409
410 As long as there is any outstanding work to be done, process objects
411 resist being destroyed, so there is no reason to store them unless you
412 need them later - configure and forget works just fine.
218 413
219 my $proc = new AnyEvent::Fork 414 my $proc = new AnyEvent::Fork
220 Create a new "empty" perl interpreter process and returns its 415 Create a new "empty" perl interpreter process and returns its
221 process object for further manipulation. 416 process object for further manipulation.
222 417
223 The new process is forked from a template process that is kept 418 The new process is forked from a template process that is kept
224 around for this purpose. When it doesn't exist yet, it is created by 419 around for this purpose. When it doesn't exist yet, it is created by
225 a call to "new_exec" and kept around for future calls. 420 a call to "new_exec" first and then stays around for future calls.
226
227 When the process object is destroyed, it will release the file
228 handle that connects it with the new process. When the new process
229 has not yet called "run", then the process will exit. Otherwise,
230 what happens depends entirely on the code that is executed.
231 421
232 $new_proc = $proc->fork 422 $new_proc = $proc->fork
233 Forks $proc, creating a new process, and returns the process object 423 Forks $proc, creating a new process, and returns the process object
234 of the new process. 424 of the new process.
235 425
… …
248 possible, and is also slower. 438 possible, and is also slower.
249 439
250 You should use "new" whenever possible, except when having a 440 You should use "new" whenever possible, except when having a
251 template process around is unacceptable. 441 template process around is unacceptable.
252 442
253 The path to the perl interpreter is divined usign various methods - 443 The path to the perl interpreter is divined using various methods -
254 first $^X is investigated to see if the path ends with something 444 first $^X is investigated to see if the path ends with something
255 that sounds as if it were the perl interpreter. Failing this, the 445 that sounds as if it were the perl interpreter. Failing this, the
256 module falls back to using $Config::Config{perlpath}. 446 module falls back to using $Config::Config{perlpath}.
257 447
448 $pid = $proc->pid
449 Returns the process id of the process *iff it is a direct child of
450 the process running AnyEvent::Fork*, and "undef" otherwise.
451
452 Normally, only processes created via "AnyEvent::Fork->new_exec" and
453 AnyEvent::Fork::Template are direct children, and you are
454 responsible to clean up their zombies when they die.
455
456 All other processes are not direct children, and will be cleaned up
457 by AnyEvent::Fork itself.
458
258 $proc = $proc->eval ($perlcode, @args) 459 $proc = $proc->eval ($perlcode, @args)
259 Evaluates the given $perlcode as ... perl code, while setting @_ to 460 Evaluates the given $perlcode as ... Perl code, while setting @_ to
260 the strings specified by @args. 461 the strings specified by @args, in the "main" package.
261 462
262 This call is meant to do any custom initialisation that might be 463 This call is meant to do any custom initialisation that might be
263 required (for example, the "require" method uses it). It's not 464 required (for example, the "require" method uses it). It's not
264 supposed to be used to completely take over the process, use "run" 465 supposed to be used to completely take over the process, use "run"
265 for that. 466 for that.
… …
267 The code will usually be executed after this call returns, and there 468 The code will usually be executed after this call returns, and there
268 is no way to pass anything back to the calling process. Any 469 is no way to pass anything back to the calling process. Any
269 evaluation errors will be reported to stderr and cause the process 470 evaluation errors will be reported to stderr and cause the process
270 to exit. 471 to exit.
271 472
473 If you want to execute some code (that isn't in a module) to take
474 over the process, you should compile a function via "eval" first,
475 and then call it via "run". This also gives you access to any
476 arguments passed via the "send_xxx" methods, such as file handles.
477 See the "use AnyEvent::Fork as a faster fork+exec" example to see it
478 in action.
479
272 Returns the process object for easy chaining of method calls. 480 Returns the process object for easy chaining of method calls.
273 481
274 $proc = $proc->require ($module, ...) 482 $proc = $proc->require ($module, ...)
275 Tries to load the given module(s) into the process 483 Tries to load the given module(s) into the process
276 484
… …
278 486
279 $proc = $proc->send_fh ($handle, ...) 487 $proc = $proc->send_fh ($handle, ...)
280 Send one or more file handles (*not* file descriptors) to the 488 Send one or more file handles (*not* file descriptors) to the
281 process, to prepare a call to "run". 489 process, to prepare a call to "run".
282 490
283 The process object keeps a reference to the handles until this is 491 The process object keeps a reference to the handles until they have
284 done, so you must not explicitly close the handles. This is most 492 been passed over to the process, so you must not explicitly close
285 easily accomplished by simply not storing the file handles anywhere 493 the handles. This is most easily accomplished by simply not storing
286 after passing them to this method. 494 the file handles anywhere after passing them to this method - when
495 AnyEvent::Fork is finished using them, perl will automatically close
496 them.
287 497
288 Returns the process object for easy chaining of method calls. 498 Returns the process object for easy chaining of method calls.
289 499
290 Example: pass an fh to a process, and release it without closing. it 500 Example: pass a file handle to a process, and release it without
291 will be closed automatically when it is no longer used. 501 closing. It will be closed automatically when it is no longer used.
292 502
293 $proc->send_fh ($my_fh); 503 $proc->send_fh ($my_fh);
294 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT 504 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT
295 505
296 $proc = $proc->send_arg ($string, ...) 506 $proc = $proc->send_arg ($string, ...)
297 Send one or more argument strings to the process, to prepare a call 507 Send one or more argument strings to the process, to prepare a call
298 to "run". The strings can be any octet string. 508 to "run". The strings can be any octet strings.
299 509
510 The protocol is optimised to pass a moderate number of relatively
511 short strings - while you can pass up to 4GB of data in one go, this
512 is more meant to pass some ID information or other startup info, not
513 big chunks of data.
514
300 Returns the process object for easy chaining of emthod calls. 515 Returns the process object for easy chaining of method calls.
301 516
302 $proc->run ($func, $cb->($fh)) 517 $proc->run ($func, $cb->($fh))
303 Enter the function specified by the fully qualified name in $func in 518 Enter the function specified by the function name in $func in the
304 the process. The function is called with the communication socket as 519 process. The function is called with the communication socket as
305 first argument, followed by all file handles and string arguments 520 first argument, followed by all file handles and string arguments
306 sent earlier via "send_fh" and "send_arg" methods, in the order they 521 sent earlier via "send_fh" and "send_arg" methods, in the order they
307 were called. 522 were called.
308 523
309 If the called function returns, the process exits.
310
311 Preparing the process can take time - when the process is ready, the
312 callback is invoked with the local communications socket as
313 argument.
314
315 The process object becomes unusable on return from this function. 524 The process object becomes unusable on return from this function -
525 any further method calls result in undefined behaviour.
526
527 The function name should be fully qualified, but if it isn't, it
528 will be looked up in the "main" package.
529
530 If the called function returns, doesn't exist, or any error occurs,
531 the process exits.
532
533 Preparing the process is done in the background - when all commands
534 have been sent, the callback is invoked with the local
535 communications socket as argument. At this point you can start using
536 the socket in any way you like.
316 537
317 If the communication socket isn't used, it should be closed on both 538 If the communication socket isn't used, it should be closed on both
318 sides, to save on kernel memory. 539 sides, to save on kernel memory.
319 540
320 The socket is non-blocking in the parent, and blocking in the newly 541 The socket is non-blocking in the parent, and blocking in the newly
321 created process. The close-on-exec flag is set on both. Even if not 542 created process. The close-on-exec flag is set in both.
543
322 used otherwise, the socket can be a good indicator for the existance 544 Even if not used otherwise, the socket can be a good indicator for
323 of the process - if the other process exits, you get a readable 545 the existence of the process - if the other process exits, you get a
324 event on it, because exiting the process closes the socket (if it 546 readable event on it, because exiting the process closes the socket
325 didn't create any children using fork). 547 (if it didn't create any children using fork).
326 548
327 Example: create a template for a process pool, pass a few strings, 549 Example: create a template for a process pool, pass a few strings,
328 some file handles, then fork, pass one more string, and run some 550 some file handles, then fork, pass one more string, and run some
329 code. 551 code.
330 552
… …
339 ->send_arg ("str3") 561 ->send_arg ("str3")
340 ->run ("Some::function", sub { 562 ->run ("Some::function", sub {
341 my ($fh) = @_; 563 my ($fh) = @_;
342 564
343 # fh is nonblocking, but we trust that the OS can accept these 565 # fh is nonblocking, but we trust that the OS can accept these
344 # extra 3 octets anyway. 566 # few octets anyway.
345 syswrite $fh, "hi #$_\n"; 567 syswrite $fh, "hi #$_\n";
346 568
347 # $fh is being closed here, as we don't store it anywhere 569 # $fh is being closed here, as we don't store it anywhere
348 }); 570 });
349 } 571 }
… …
351 # Some::function might look like this - all parameters passed before fork 573 # Some::function might look like this - all parameters passed before fork
352 # and after will be passed, in order, after the communications socket. 574 # and after will be passed, in order, after the communications socket.
353 sub Some::function { 575 sub Some::function {
354 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; 576 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_;
355 577
356 print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" 578 print scalar <$fh>; # prints "hi #1\n" and "hi #2\n" in any order
357 } 579 }
580
581 $proc->to_fh ($cb->($fh)) # EXPERIMENTAL, MIGHT BE REMOVED
582 Flushes all commands out to the process and then calls the callback
583 with the communications socket.
584
585 The process object becomes unusable on return from this function -
586 any further method calls result in undefined behaviour.
587
588 The point of this method is to give you a file handle thta you cna
589 pass to another process. In that other process, you can call
590 "new_from_fh AnyEvent::Fork" to create a new "AnyEvent::Fork" object
591 from it, thereby effectively passing a fork object to another
592 process.
593
594 new_from_fh AnyEvent::Fork $fh # EXPERIMENTAL, MIGHT BE REMOVED
595 Takes a file handle originally rceeived by the "to_fh" method and
596 creates a new "AnyEvent:Fork" object. The child process itself will
597 not change in any way, i.e. it will keep all the modifications done
598 to it before calling "to_fh".
599
600 The new object is very much like the original object, except that
601 the "pid" method will return "undef" even if the process is a direct
602 child.
603
604PERFORMANCE
605 Now for some unscientific benchmark numbers (all done on an amd64
606 GNU/Linux box). These are intended to give you an idea of the relative
607 performance you can expect, they are not meant to be absolute
608 performance numbers.
609
610 OK, so, I ran a simple benchmark that creates a socket pair, forks,
611 calls exit in the child and waits for the socket to close in the parent.
612 I did load AnyEvent, EV and AnyEvent::Fork, for a total process size of
613 5100kB.
614
615 2079 new processes per second, using manual socketpair + fork
616
617 Then I did the same thing, but instead of calling fork, I called
618 AnyEvent::Fork->new->run ("CORE::exit") and then again waited for the
619 socket from the child to close on exit. This does the same thing as
620 manual socket pair + fork, except that what is forked is the template
621 process (2440kB), and the socket needs to be passed to the server at the
622 other end of the socket first.
623
624 2307 new processes per second, using AnyEvent::Fork->new
625
626 And finally, using "new_exec" instead "new", using vforks+execs to exec
627 a new perl interpreter and compile the small server each time, I get:
628
629 479 vfork+execs per second, using AnyEvent::Fork->new_exec
630
631 So how can "AnyEvent->new" be faster than a standard fork, even though
632 it uses the same operations, but adds a lot of overhead?
633
634 The difference is simply the process size: forking the 5MB process takes
635 so much longer than forking the 2.5MB template process that the extra
636 overhead is canceled out.
637
638 If the benchmark process grows, the normal fork becomes even slower:
639
640 1340 new processes, manual fork of a 20MB process
641 731 new processes, manual fork of a 200MB process
642 235 new processes, manual fork of a 2000MB process
643
644 What that means (to me) is that I can use this module without having a
645 bad conscience because of the extra overhead required to start new
646 processes.
358 647
359TYPICAL PROBLEMS 648TYPICAL PROBLEMS
360 This section lists typical problems that remain. I hope by recognising 649 This section lists typical problems that remain. I hope by recognising
361 them, most can be avoided. 650 them, most can be avoided.
362 651
363 "leaked" file descriptors for exec'ed processes 652 leaked file descriptors for exec'ed processes
364 POSIX systems inherit file descriptors by default when exec'ing a 653 POSIX systems inherit file descriptors by default when exec'ing a
365 new process. While perl itself laudably sets the close-on-exec flags 654 new process. While perl itself laudably sets the close-on-exec flags
366 on new file handles, most C libraries don't care, and even if all 655 on new file handles, most C libraries don't care, and even if all
367 cared, it's often not possible to set the flag in a race-free 656 cared, it's often not possible to set the flag in a race-free
368 manner. 657 manner.
369 658
370 That means some file descriptors can leak through. And since it 659 That means some file descriptors can leak through. And since it
371 isn't possible to know which file descriptors are "good" and 660 isn't possible to know which file descriptors are "good" and
372 "neccessary" (or even to know which file descreiptors are open), 661 "necessary" (or even to know which file descriptors are open), there
373 there is no good way to close the ones that might harm. 662 is no good way to close the ones that might harm.
374 663
375 As an example of what "harm" can be done consider a web server that 664 As an example of what "harm" can be done consider a web server that
376 accepts connections and afterwards some module uses AnyEvent::Fork 665 accepts connections and afterwards some module uses AnyEvent::Fork
377 for the first time, causing it to fork and exec a new process, which 666 for the first time, causing it to fork and exec a new process, which
378 might inherit the network socket. When the server closes the socket, 667 might inherit the network socket. When the server closes the socket,
… …
385 exec'ed well before many random file descriptors are open. 674 exec'ed well before many random file descriptors are open.
386 675
387 In general, the solution for these kind of problems is to fix the 676 In general, the solution for these kind of problems is to fix the
388 libraries or the code that leaks those file descriptors. 677 libraries or the code that leaks those file descriptors.
389 678
390 Fortunately, most of these lekaed descriptors do no harm, other than 679 Fortunately, most of these leaked descriptors do no harm, other than
391 sitting on some resources. 680 sitting on some resources.
392 681
393 "leaked" file descriptors for fork'ed processes 682 leaked file descriptors for fork'ed processes
394 Normally, AnyEvent::Fork does start new processes by exec'ing them, 683 Normally, AnyEvent::Fork does start new processes by exec'ing them,
395 which closes file descriptors not marked for being inherited. 684 which closes file descriptors not marked for being inherited.
396 685
397 However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a 686 However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a
398 way to create these processes by forking, and this leaks more file 687 way to create these processes by forking, and this leaks more file
… …
405 trouble with a fork. 694 trouble with a fork.
406 695
407 The solution is to either not load these modules before use'ing 696 The solution is to either not load these modules before use'ing
408 AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay 697 AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay
409 initialising them, for example, by calling "init Gtk2" manually. 698 initialising them, for example, by calling "init Gtk2" manually.
699
700 exiting calls object destructors
701 This only applies to users of AnyEvent::Fork:Early and
702 AnyEvent::Fork::Template, or when initialising code creates objects
703 that reference external resources.
704
705 When a process created by AnyEvent::Fork exits, it might do so by
706 calling exit, or simply letting perl reach the end of the program.
707 At which point Perl runs all destructors.
708
709 Not all destructors are fork-safe - for example, an object that
710 represents the connection to an X display might tell the X server to
711 free resources, which is inconvenient when the "real" object in the
712 parent still needs to use them.
713
714 This is obviously not a problem for AnyEvent::Fork::Early, as you
715 used it as the very first thing, right?
716
717 It is a problem for AnyEvent::Fork::Template though - and the
718 solution is to not create objects with nontrivial destructors that
719 might have an effect outside of Perl.
410 720
411PORTABILITY NOTES 721PORTABILITY NOTES
412 Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a 722 Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a
413 nop, and ::Template is not going to work), and it cost a lot of blood 723 nop, and ::Template is not going to work), and it cost a lot of blood
414 and sweat to make it so, mostly due to the bloody broken perl that 724 and sweat to make it so, mostly due to the bloody broken perl that
415 nobody seems to care about. The fork emulation is a bad joke - I have 725 nobody seems to care about. The fork emulation is a bad joke - I have
416 yet to see something useful that you cna do with it without running into 726 yet to see something useful that you can do with it without running into
417 memory corruption issues or other braindamage. Hrrrr. 727 memory corruption issues or other braindamage. Hrrrr.
418 728
419 Cygwin perl is not supported at the moment, as it should implement fd 729 Since fork is endlessly broken on win32 perls (it doesn't even remotely
420 passing, but doesn't, and rolling my own is hard, as cygwin doesn't 730 work within it's documented limits) and quite obviously it's not getting
421 support enough functionality to do it. 731 improved any time soon, the best way to proceed on windows would be to
732 always use "new_exec" and thus never rely on perl's fork "emulation".
733
734 Cygwin perl is not supported at the moment due to some hilarious
735 shortcomings of its API - see IO::FDPoll for more details. If you never
736 use "send_fh" and always use "new_exec" to create processes, it should
737 work though.
422 738
423SEE ALSO 739SEE ALSO
424 AnyEvent::Fork::Early (to avoid executing a perl interpreter), 740 AnyEvent::Fork::Early, to avoid executing a perl interpreter at all
741 (part of this distribution).
742
425 AnyEvent::Fork::Template (to create a process by forking the main 743 AnyEvent::Fork::Template, to create a process by forking the main
426 program at a convenient time). 744 program at a convenient time (part of this distribution).
427 745
428AUTHOR 746 AnyEvent::Fork::RPC, for simple RPC to child processes (on CPAN).
747
748 AnyEvent::Fork::Pool, for simple worker process pool (on CPAN).
749
750AUTHOR AND CONTACT INFORMATION
429 Marc Lehmann <schmorp@schmorp.de> 751 Marc Lehmann <schmorp@schmorp.de>
430 http://home.schmorp.de/ 752 http://software.schmorp.de/pkg/AnyEvent-Fork
431 753

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