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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,
… …
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 web server), 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
79WHAT THIS MODULE IS NOT 266 Two modules help here - AnyEvent::Fork::Early forks a template process
80 This module only creates processes and lets you pass file handles and 267 for all further calls to "new_exec", and AnyEvent::Fork::Template forks
81 strings to it, and run perl code. It does not implement any kind of RPC 268 the main program as a template process.
82 - there is no back channel from the process back to you, and there is no
83 RPC or message passing going on.
84 269
85 If you need some form of RPC, you can either implement it yourself in 270 Here is how your main program should look like:
86 whatever way you like, use some message-passing module such as
87 AnyEvent::MP, some pipe such as AnyEvent::ZeroMQ, use AnyEvent::Handle
88 on both sides to send e.g. JSON or Storable messages, and so on.
89 271
90PROBLEM STATEMENT 272 #! perl
91 There are two ways to implement parallel processing on UNIX like
92 operating systems - fork and process, and fork+exec and process. They
93 have different advantages and disadvantages that I describe below,
94 together with how this module tries to mitigate the disadvantages.
95 273
96 Forking from a big process can be very slow (a 5GB process needs 0.05s 274 # optional, as the very first thing.
97 to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead is 275 # in case modules want to create their own processes.
98 often shared with exec (because you have to fork first), but in some 276 use AnyEvent::Fork::Early;
99 circumstances (e.g. when vfork is used), fork+exec can be much faster.
100 This module can help here by telling a small(er) helper process to
101 fork, or fork+exec instead.
102 277
103 Forking usually creates a copy-on-write copy of the parent process. 278 # next, load all modules you need in your template process
104 Memory (for example, modules or data files that have been will not take 279 use Example::My::Module
105 additional memory). When exec'ing a new process, modules and data files 280 use Example::Whatever;
106 might need to be loaded again, at extra CPU and memory cost. Likewise
107 when forking, all data structures are copied as well - if the program
108 frees them and replaces them by new data, the child processes will
109 retain the memory even if it isn't used.
110 This module allows the main program to do a controlled fork, and
111 allows modules to exec processes safely at any time. When creating a
112 custom process pool you can take advantage of data sharing via fork
113 without risking to share large dynamic data structures that will
114 blow up child memory usage.
115 281
116 Exec'ing a new perl process might be difficult and slow. For example, it 282 # next, put your run function definition and anything else you
117 is not easy to find the correct path to the perl interpreter, and all 283 # need, but do not use code outside of BEGIN blocks.
118 modules have to be loaded from disk again. Long running processes might 284 sub worker_run {
119 run into problems when perl is upgraded for example. 285 my ($fh, @args) = @_;
120 This module supports creating pre-initialised perl processes to be 286 ...
121 used as template, and also tries hard to identify the correct path 287 }
122 to the perl interpreter. With a cooperative main program, exec'ing
123 the interpreter might not even be necessary.
124 288
125 Forking might be impossible when a program is running. For example, 289 # now preserve everything so far as AnyEvent::Fork object
126 POSIX makes it almost impossible to fork from a multi-threaded program 290 # in §TEMPLATE.
127 and do anything useful in the child - strictly speaking, if your perl 291 use AnyEvent::Fork::Template;
128 program uses posix threads (even indirectly via e.g. IO::AIO or
129 threads), you cannot call fork on the perl level anymore, at all.
130 This module can safely fork helper processes at any time, by calling
131 fork+exec in C, in a POSIX-compatible way.
132 292
133 Parallel processing with fork might be inconvenient or difficult to 293 # do not put code outside of BEGIN blocks until here
134 implement. For example, when a program uses an event loop and creates 294
135 watchers it becomes very hard to use the event loop from a child 295 # now use the $TEMPLATE process in any way you like
136 program, as the watchers already exist but are only meaningful in the 296
137 parent. Worse, a module might want to use such a system, not knowing 297 # for example: create 10 worker processes
138 whether another module or the main program also does, leading to 298 my @worker;
139 problems. 299 my $cv = AE::cv;
140 This module only lets the main program create pools by forking 300 for (1..10) {
141 (because only the main program can know when it is still safe to do 301 $cv->begin;
142 so) - all other pools are created by fork+exec, after which such 302 $TEMPLATE->fork->send_arg ($_)->run ("worker_run", sub {
143 modules can again be loaded. 303 push @worker, shift;
304 $cv->end;
305 });
306 }
307 $cv->recv;
144 308
145CONCEPTS 309CONCEPTS
146 This module can create new processes either by executing a new perl 310 This module can create new processes either by executing a new perl
147 process, or by forking from an existing "template" process. 311 process, or by forking from an existing "template" process.
312
313 All these processes are called "child processes" (whether they are
314 direct children or not), while the process that manages them is called
315 the "parent process".
148 316
149 Each such process comes with its own file handle that can be used to 317 Each such process comes with its own file handle that can be used to
150 communicate with it (it's actually a socket - one end in the new 318 communicate with it (it's actually a socket - one end in the new
151 process, one end in the main process), and among the things you can do 319 process, one end in the main process), and among the things you can do
152 in it are load modules, fork new processes, send file handles to it, and 320 in it are load modules, fork new processes, send file handles to it, and
… …
220 ->require ("Some::Module") 388 ->require ("Some::Module")
221 ->run ("Some::Module::run", sub { 389 ->run ("Some::Module::run", sub {
222 my ($fork_fh) = @_; 390 my ($fork_fh) = @_;
223 }); 391 });
224 392
225FUNCTIONS 393THE "AnyEvent::Fork" CLASS
226 my $pool = new AnyEvent::Fork key => value... 394 This module exports nothing, and only implements a single class -
227 Create a new process pool. The following named parameters are 395 "AnyEvent::Fork".
228 supported: 396
397 There are two class constructors that both create new processes - "new"
398 and "new_exec". The "fork" method creates a new process by forking an
399 existing one and could be considered a third constructor.
400
401 Most of the remaining methods deal with preparing the new process, by
402 loading code, evaluating code and sending data to the new process. They
403 usually return the process object, so you can chain method calls.
404
405 If a process object is destroyed before calling its "run" method, then
406 the process simply exits. After "run" is called, all responsibility is
407 passed to the specified function.
408
409 As long as there is any outstanding work to be done, process objects
410 resist being destroyed, so there is no reason to store them unless you
411 need them later - configure and forget works just fine.
229 412
230 my $proc = new AnyEvent::Fork 413 my $proc = new AnyEvent::Fork
231 Create a new "empty" perl interpreter process and returns its 414 Create a new "empty" perl interpreter process and returns its
232 process object for further manipulation. 415 process object for further manipulation.
233 416
234 The new process is forked from a template process that is kept 417 The new process is forked from a template process that is kept
235 around for this purpose. When it doesn't exist yet, it is created by 418 around for this purpose. When it doesn't exist yet, it is created by
236 a call to "new_exec" and kept around for future calls. 419 a call to "new_exec" first and then stays around for future calls.
237
238 When the process object is destroyed, it will release the file
239 handle that connects it with the new process. When the new process
240 has not yet called "run", then the process will exit. Otherwise,
241 what happens depends entirely on the code that is executed.
242 420
243 $new_proc = $proc->fork 421 $new_proc = $proc->fork
244 Forks $proc, creating a new process, and returns the process object 422 Forks $proc, creating a new process, and returns the process object
245 of the new process. 423 of the new process.
246 424
… …
264 The path to the perl interpreter is divined using various methods - 442 The path to the perl interpreter is divined using various methods -
265 first $^X is investigated to see if the path ends with something 443 first $^X is investigated to see if the path ends with something
266 that sounds as if it were the perl interpreter. Failing this, the 444 that sounds as if it were the perl interpreter. Failing this, the
267 module falls back to using $Config::Config{perlpath}. 445 module falls back to using $Config::Config{perlpath}.
268 446
447 The path to perl can also be overriden by setting the global
448 variable $AnyEvent::Fork::PERL - it's value will be used for all
449 subsequent invocations.
450
269 $pid = $proc->pid 451 $pid = $proc->pid
270 Returns the process id of the process *iff it is a direct child of 452 Returns the process id of the process *iff it is a direct child of
271 the process* running AnyEvent::Fork, and "undef" otherwise. 453 the process running AnyEvent::Fork*, and "undef" otherwise.
272 454
273 Normally, only processes created via "AnyEvent::Fork->new_exec" and 455 Normally, only processes created via "AnyEvent::Fork->new_exec" and
274 AnyEvent::Fork::Template are direct children, and you are 456 AnyEvent::Fork::Template are direct children, and you are
275 responsible to clean up their zombies when they die. 457 responsible to clean up their zombies when they die.
276 458
277 All other processes are not direct children, and will be cleaned up 459 All other processes are not direct children, and will be cleaned up
278 by AnyEvent::Fork. 460 by AnyEvent::Fork itself.
279 461
280 $proc = $proc->eval ($perlcode, @args) 462 $proc = $proc->eval ($perlcode, @args)
281 Evaluates the given $perlcode as ... perl code, while setting @_ to 463 Evaluates the given $perlcode as ... Perl code, while setting @_ to
282 the strings specified by @args. 464 the strings specified by @args, in the "main" package.
283 465
284 This call is meant to do any custom initialisation that might be 466 This call is meant to do any custom initialisation that might be
285 required (for example, the "require" method uses it). It's not 467 required (for example, the "require" method uses it). It's not
286 supposed to be used to completely take over the process, use "run" 468 supposed to be used to completely take over the process, use "run"
287 for that. 469 for that.
… …
289 The code will usually be executed after this call returns, and there 471 The code will usually be executed after this call returns, and there
290 is no way to pass anything back to the calling process. Any 472 is no way to pass anything back to the calling process. Any
291 evaluation errors will be reported to stderr and cause the process 473 evaluation errors will be reported to stderr and cause the process
292 to exit. 474 to exit.
293 475
476 If you want to execute some code (that isn't in a module) to take
477 over the process, you should compile a function via "eval" first,
478 and then call it via "run". This also gives you access to any
479 arguments passed via the "send_xxx" methods, such as file handles.
480 See the "use AnyEvent::Fork as a faster fork+exec" example to see it
481 in action.
482
294 Returns the process object for easy chaining of method calls. 483 Returns the process object for easy chaining of method calls.
295 484
296 $proc = $proc->require ($module, ...) 485 $proc = $proc->require ($module, ...)
297 Tries to load the given module(s) into the process 486 Tries to load the given module(s) into the process
298 487
… …
300 489
301 $proc = $proc->send_fh ($handle, ...) 490 $proc = $proc->send_fh ($handle, ...)
302 Send one or more file handles (*not* file descriptors) to the 491 Send one or more file handles (*not* file descriptors) to the
303 process, to prepare a call to "run". 492 process, to prepare a call to "run".
304 493
305 The process object keeps a reference to the handles until this is 494 The process object keeps a reference to the handles until they have
306 done, so you must not explicitly close the handles. This is most 495 been passed over to the process, so you must not explicitly close
307 easily accomplished by simply not storing the file handles anywhere 496 the handles. This is most easily accomplished by simply not storing
308 after passing them to this method. 497 the file handles anywhere after passing them to this method - when
498 AnyEvent::Fork is finished using them, perl will automatically close
499 them.
309 500
310 Returns the process object for easy chaining of method calls. 501 Returns the process object for easy chaining of method calls.
311 502
312 Example: pass a file handle to a process, and release it without 503 Example: pass a file handle to a process, and release it without
313 closing. It will be closed automatically when it is no longer used. 504 closing. It will be closed automatically when it is no longer used.
… …
315 $proc->send_fh ($my_fh); 506 $proc->send_fh ($my_fh);
316 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT 507 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT
317 508
318 $proc = $proc->send_arg ($string, ...) 509 $proc = $proc->send_arg ($string, ...)
319 Send one or more argument strings to the process, to prepare a call 510 Send one or more argument strings to the process, to prepare a call
320 to "run". The strings can be any octet string. 511 to "run". The strings can be any octet strings.
321 512
322 The protocol is optimised to pass a moderate number of relatively 513 The protocol is optimised to pass a moderate number of relatively
323 short strings - while you can pass up to 4GB of data in one go, this 514 short strings - while you can pass up to 4GB of data in one go, this
324 is more meant to pass some ID information or other startup info, not 515 is more meant to pass some ID information or other startup info, not
325 big chunks of data. 516 big chunks of data.
326 517
327 Returns the process object for easy chaining of method calls. 518 Returns the process object for easy chaining of method calls.
328 519
329 $proc->run ($func, $cb->($fh)) 520 $proc->run ($func, $cb->($fh))
330 Enter the function specified by the fully qualified name in $func in 521 Enter the function specified by the function name in $func in the
331 the process. The function is called with the communication socket as 522 process. The function is called with the communication socket as
332 first argument, followed by all file handles and string arguments 523 first argument, followed by all file handles and string arguments
333 sent earlier via "send_fh" and "send_arg" methods, in the order they 524 sent earlier via "send_fh" and "send_arg" methods, in the order they
334 were called. 525 were called.
335 526
336 If the called function returns, the process exits.
337
338 Preparing the process can take time - when the process is ready, the
339 callback is invoked with the local communications socket as
340 argument.
341
342 The process object becomes unusable on return from this function. 527 The process object becomes unusable on return from this function -
528 any further method calls result in undefined behaviour.
529
530 The function name should be fully qualified, but if it isn't, it
531 will be looked up in the "main" package.
532
533 If the called function returns, doesn't exist, or any error occurs,
534 the process exits.
535
536 Preparing the process is done in the background - when all commands
537 have been sent, the callback is invoked with the local
538 communications socket as argument. At this point you can start using
539 the socket in any way you like.
343 540
344 If the communication socket isn't used, it should be closed on both 541 If the communication socket isn't used, it should be closed on both
345 sides, to save on kernel memory. 542 sides, to save on kernel memory.
346 543
347 The socket is non-blocking in the parent, and blocking in the newly 544 The socket is non-blocking in the parent, and blocking in the newly
348 created process. The close-on-exec flag is set on both. Even if not 545 created process. The close-on-exec flag is set in both.
546
349 used otherwise, the socket can be a good indicator for the existence 547 Even if not used otherwise, the socket can be a good indicator for
350 of the process - if the other process exits, you get a readable 548 the existence of the process - if the other process exits, you get a
351 event on it, because exiting the process closes the socket (if it 549 readable event on it, because exiting the process closes the socket
352 didn't create any children using fork). 550 (if it didn't create any children using fork).
353 551
354 Example: create a template for a process pool, pass a few strings, 552 Example: create a template for a process pool, pass a few strings,
355 some file handles, then fork, pass one more string, and run some 553 some file handles, then fork, pass one more string, and run some
356 code. 554 code.
357 555
… …
366 ->send_arg ("str3") 564 ->send_arg ("str3")
367 ->run ("Some::function", sub { 565 ->run ("Some::function", sub {
368 my ($fh) = @_; 566 my ($fh) = @_;
369 567
370 # fh is nonblocking, but we trust that the OS can accept these 568 # fh is nonblocking, but we trust that the OS can accept these
371 # extra 3 octets anyway. 569 # few octets anyway.
372 syswrite $fh, "hi #$_\n"; 570 syswrite $fh, "hi #$_\n";
373 571
374 # $fh is being closed here, as we don't store it anywhere 572 # $fh is being closed here, as we don't store it anywhere
375 }); 573 });
376 } 574 }
… …
378 # Some::function might look like this - all parameters passed before fork 576 # Some::function might look like this - all parameters passed before fork
379 # and after will be passed, in order, after the communications socket. 577 # and after will be passed, in order, after the communications socket.
380 sub Some::function { 578 sub Some::function {
381 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; 579 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_;
382 580
383 print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" 581 print scalar <$fh>; # prints "hi #1\n" and "hi #2\n" in any order
384 } 582 }
583
584 EXPERIMENTAL METHODS
585 These methods might go away completely or change behaviour, at any time.
586
587 $proc->to_fh ($cb->($fh)) # EXPERIMENTAL, MIGHT BE REMOVED
588 Flushes all commands out to the process and then calls the callback
589 with the communications socket.
590
591 The process object becomes unusable on return from this function -
592 any further method calls result in undefined behaviour.
593
594 The point of this method is to give you a file handle thta you cna
595 pass to another process. In that other process, you can call
596 "new_from_fh AnyEvent::Fork" to create a new "AnyEvent::Fork" object
597 from it, thereby effectively passing a fork object to another
598 process.
599
600 new_from_fh AnyEvent::Fork $fh # EXPERIMENTAL, MIGHT BE REMOVED
601 Takes a file handle originally rceeived by the "to_fh" method and
602 creates a new "AnyEvent:Fork" object. The child process itself will
603 not change in any way, i.e. it will keep all the modifications done
604 to it before calling "to_fh".
605
606 The new object is very much like the original object, except that
607 the "pid" method will return "undef" even if the process is a direct
608 child.
385 609
386PERFORMANCE 610PERFORMANCE
387 Now for some unscientific benchmark numbers (all done on an amd64 611 Now for some unscientific benchmark numbers (all done on an amd64
388 GNU/Linux box). These are intended to give you an idea of the relative 612 GNU/Linux box). These are intended to give you an idea of the relative
389 performance you can expect, they are not meant to be absolute 613 performance you can expect, they are not meant to be absolute
… …
396 620
397 2079 new processes per second, using manual socketpair + fork 621 2079 new processes per second, using manual socketpair + fork
398 622
399 Then I did the same thing, but instead of calling fork, I called 623 Then I did the same thing, but instead of calling fork, I called
400 AnyEvent::Fork->new->run ("CORE::exit") and then again waited for the 624 AnyEvent::Fork->new->run ("CORE::exit") and then again waited for the
401 socket form the child to close on exit. This does the same thing as 625 socket from the child to close on exit. This does the same thing as
402 manual socket pair + fork, except that what is forked is the template 626 manual socket pair + fork, except that what is forked is the template
403 process (2440kB), and the socket needs to be passed to the server at the 627 process (2440kB), and the socket needs to be passed to the server at the
404 other end of the socket first. 628 other end of the socket first.
405 629
406 2307 new processes per second, using AnyEvent::Fork->new 630 2307 new processes per second, using AnyEvent::Fork->new
… …
411 479 vfork+execs per second, using AnyEvent::Fork->new_exec 635 479 vfork+execs per second, using AnyEvent::Fork->new_exec
412 636
413 So how can "AnyEvent->new" be faster than a standard fork, even though 637 So how can "AnyEvent->new" be faster than a standard fork, even though
414 it uses the same operations, but adds a lot of overhead? 638 it uses the same operations, but adds a lot of overhead?
415 639
416 The difference is simply the process size: forking the 6MB process takes 640 The difference is simply the process size: forking the 5MB process takes
417 so much longer than forking the 2.5MB template process that the overhead 641 so much longer than forking the 2.5MB template process that the extra
418 introduced is canceled out. 642 overhead is canceled out.
419 643
420 If the benchmark process grows, the normal fork becomes even slower: 644 If the benchmark process grows, the normal fork becomes even slower:
421 645
422 1340 new processes, manual fork in a 20MB process 646 1340 new processes, manual fork of a 20MB process
423 731 new processes, manual fork in a 200MB process 647 731 new processes, manual fork of a 200MB process
424 235 new processes, manual fork in a 2000MB process 648 235 new processes, manual fork of a 2000MB process
425 649
426 What that means (to me) is that I can use this module without having a 650 What that means (to me) is that I can use this module without having a
427 very bad conscience because of the extra overhead required to start new 651 bad conscience because of the extra overhead required to start new
428 processes. 652 processes.
429 653
430TYPICAL PROBLEMS 654TYPICAL PROBLEMS
431 This section lists typical problems that remain. I hope by recognising 655 This section lists typical problems that remain. I hope by recognising
432 them, most can be avoided. 656 them, most can be avoided.
433 657
434 "leaked" file descriptors for exec'ed processes 658 leaked file descriptors for exec'ed processes
435 POSIX systems inherit file descriptors by default when exec'ing a 659 POSIX systems inherit file descriptors by default when exec'ing a
436 new process. While perl itself laudably sets the close-on-exec flags 660 new process. While perl itself laudably sets the close-on-exec flags
437 on new file handles, most C libraries don't care, and even if all 661 on new file handles, most C libraries don't care, and even if all
438 cared, it's often not possible to set the flag in a race-free 662 cared, it's often not possible to set the flag in a race-free
439 manner. 663 manner.
… …
459 libraries or the code that leaks those file descriptors. 683 libraries or the code that leaks those file descriptors.
460 684
461 Fortunately, most of these leaked descriptors do no harm, other than 685 Fortunately, most of these leaked descriptors do no harm, other than
462 sitting on some resources. 686 sitting on some resources.
463 687
464 "leaked" file descriptors for fork'ed processes 688 leaked file descriptors for fork'ed processes
465 Normally, AnyEvent::Fork does start new processes by exec'ing them, 689 Normally, AnyEvent::Fork does start new processes by exec'ing them,
466 which closes file descriptors not marked for being inherited. 690 which closes file descriptors not marked for being inherited.
467 691
468 However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a 692 However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a
469 way to create these processes by forking, and this leaks more file 693 way to create these processes by forking, and this leaks more file
… …
477 701
478 The solution is to either not load these modules before use'ing 702 The solution is to either not load these modules before use'ing
479 AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay 703 AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay
480 initialising them, for example, by calling "init Gtk2" manually. 704 initialising them, for example, by calling "init Gtk2" manually.
481 705
482 exit runs destructors 706 exiting calls object destructors
483 This only applies to users of Lc<AnyEvent::Fork:Early> and 707 This only applies to users of AnyEvent::Fork:Early and
484 AnyEvent::Fork::Template. 708 AnyEvent::Fork::Template, or when initialising code creates objects
709 that reference external resources.
485 710
486 When a process created by AnyEvent::Fork exits, it might do so by 711 When a process created by AnyEvent::Fork exits, it might do so by
487 calling exit, or simply letting perl reach the end of the program. 712 calling exit, or simply letting perl reach the end of the program.
488 At which point Perl runs all destructors. 713 At which point Perl runs all destructors.
489 714
… …
505 and sweat to make it so, mostly due to the bloody broken perl that 730 and sweat to make it so, mostly due to the bloody broken perl that
506 nobody seems to care about. The fork emulation is a bad joke - I have 731 nobody seems to care about. The fork emulation is a bad joke - I have
507 yet to see something useful that you can do with it without running into 732 yet to see something useful that you can do with it without running into
508 memory corruption issues or other braindamage. Hrrrr. 733 memory corruption issues or other braindamage. Hrrrr.
509 734
510 Cygwin perl is not supported at the moment, as it should implement fd 735 Since fork is endlessly broken on win32 perls (it doesn't even remotely
511 passing, but doesn't, and rolling my own is hard, as cygwin doesn't 736 work within it's documented limits) and quite obviously it's not getting
512 support enough functionality to do it. 737 improved any time soon, the best way to proceed on windows would be to
738 always use "new_exec" and thus never rely on perl's fork "emulation".
739
740 Cygwin perl is not supported at the moment due to some hilarious
741 shortcomings of its API - see IO::FDPoll for more details. If you never
742 use "send_fh" and always use "new_exec" to create processes, it should
743 work though.
513 744
514SEE ALSO 745SEE ALSO
515 AnyEvent::Fork::Early (to avoid executing a perl interpreter), 746 AnyEvent::Fork::Early, to avoid executing a perl interpreter at all
747 (part of this distribution).
748
516 AnyEvent::Fork::Template (to create a process by forking the main 749 AnyEvent::Fork::Template, to create a process by forking the main
517 program at a convenient time). 750 program at a convenient time (part of this distribution).
518 751
519AUTHOR 752 AnyEvent::Fork::RPC, for simple RPC to child processes (on CPAN).
753
754 AnyEvent::Fork::Pool, for simple worker process pool (on CPAN).
755
756AUTHOR AND CONTACT INFORMATION
520 Marc Lehmann <schmorp@schmorp.de> 757 Marc Lehmann <schmorp@schmorp.de>
521 http://home.schmorp.de/ 758 http://software.schmorp.de/pkg/AnyEvent-Fork
522 759

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