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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 Or you can implement it yourself in whatever way you like, use some
40 message-passing module such as AnyEvent::MP, some pipe such as
41 AnyEvent::ZeroMQ, use AnyEvent::Handle on both sides to send e.g. JSON
42 or Storable messages, and so on.
43
44 COMPARISON TO OTHER MODULES
45 There is an abundance of modules on CPAN that do "something fork", such
46 as Parallel::ForkManager, AnyEvent::ForkManager, AnyEvent::Worker or
47 AnyEvent::Subprocess. There are modules that implement their own process
48 management, such as AnyEvent::DBI.
49
50 The problems that all these modules try to solve are real, however, none
51 of them (from what I have seen) tackle the very real problems of
52 unwanted memory sharing, efficiency, not being able to use event
53 processing or similar modules in the processes they create.
54
55 This module doesn't try to replace any of them - instead it tries to
56 solve the problem of creating processes with a minimum of fuss and
57 overhead (and also luxury). Ideally, most of these would use
58 AnyEvent::Fork internally, except they were written before AnyEvent:Fork
59 was available, so obviously had to roll their own.
60
61 PROBLEM STATEMENT
62 There are two traditional ways to implement parallel processing on UNIX
63 like operating systems - fork and process, and fork+exec and process.
64 They have different advantages and disadvantages that I describe below,
65 together with how this module tries to mitigate the disadvantages.
66
67 Forking from a big process can be very slow.
68 A 5GB process needs 0.05s to fork on my 3.6GHz amd64 GNU/Linux box.
69 This overhead is often shared with exec (because you have to fork
70 first), but in some circumstances (e.g. when vfork is used),
71 fork+exec can be much faster.
72
73 This module can help here by telling a small(er) helper process to
74 fork, which is faster then forking the main process, and also uses
75 vfork where possible. This gives the speed of vfork, with the
76 flexibility of fork.
77
78 Forking usually creates a copy-on-write copy of the parent process.
79 For example, modules or data files that are loaded will not use
80 additional memory after a fork. When exec'ing a new process, modules
81 and data files might need to be loaded again, at extra CPU and
82 memory cost. But when forking, literally all data structures are
83 copied - if the program frees them and replaces them by new data,
84 the child processes will retain the old version even if it isn't
85 used, which can suddenly and unexpectedly increase memory usage when
86 freeing memory.
87
88 The trade-off is between more sharing with fork (which can be good
89 or bad), and no sharing with exec.
90
91 This module allows the main program to do a controlled fork, and
92 allows modules to exec processes safely at any time. When creating a
93 custom process pool you can take advantage of data sharing via fork
94 without risking to share large dynamic data structures that will
95 blow up child memory usage.
96
97 In other words, this module puts you into control over what is being
98 shared and what isn't, at all times.
99
100 Exec'ing a new perl process might be difficult.
101 For example, it is not easy to find the correct path to the perl
102 interpreter - $^X might not be a perl interpreter at all.
103
104 This module tries hard to identify the correct path to the perl
105 interpreter. With a cooperative main program, exec'ing the
106 interpreter might not even be necessary, but even without help from
107 the main program, it will still work when used from a module.
108
109 Exec'ing a new perl process might be slow, as all necessary modules have
110 to be loaded from disk again, with no guarantees of success.
111 Long running processes might run into problems when perl is upgraded
112 and modules are no longer loadable because they refer to a different
113 perl version, or parts of a distribution are newer than the ones
114 already loaded.
115
116 This module supports creating pre-initialised perl processes to be
117 used as a template for new processes.
118
119 Forking might be impossible when a program is running.
120 For example, POSIX makes it almost impossible to fork from a
121 multi-threaded program while doing anything useful in the child - in
122 fact, if your perl program uses POSIX threads (even indirectly via
123 e.g. IO::AIO or threads), you cannot call fork on the perl level
124 anymore without risking corruption issues on a number of operating
125 systems.
126
127 This module can safely fork helper processes at any time, by calling
128 fork+exec in C, in a POSIX-compatible way (via Proc::FastSpawn).
129
130 Parallel processing with fork might be inconvenient or difficult to
131 implement. Modules might not work in both parent and child.
132 For example, when a program uses an event loop and creates watchers
133 it becomes very hard to use the event loop from a child program, as
134 the watchers already exist but are only meaningful in the parent.
135 Worse, a module might want to use such a module, not knowing whether
136 another module or the main program also does, leading to problems.
137
138 Apart from event loops, graphical toolkits also commonly fall into
139 the "unsafe module" category, or just about anything that
140 communicates with the external world, such as network libraries and
141 file I/O modules, which usually don't like being copied and then
142 allowed to continue in two processes.
143
144 With this module only the main program is allowed to create new
145 processes by forking (because only the main program can know when it
146 is still safe to do so) - all other processes are created via
147 fork+exec, which makes it possible to use modules such as event
148 loops or window interfaces safely.
149
150EXAMPLES
8 # create a single new process, tell it to run your worker function 151 Create a single new process, tell it to run your worker function.
9
10 AnyEvent::Fork 152 AnyEvent::Fork
11 ->new 153 ->new
12 ->require ("MyModule") 154 ->require ("MyModule")
13 ->run ("MyModule::worker, sub { 155 ->run ("MyModule::worker, sub {
14 my ($master_filehandle) = @_; 156 my ($master_filehandle) = @_;
15 157
16 # now $master_filehandle is connected to the 158 # now $master_filehandle is connected to the
17 # $slave_filehandle in the new process. 159 # $slave_filehandle in the new process.
18 }); 160 });
19 161
20 # MyModule::worker might look like this 162 "MyModule" might look like this:
163
164 package MyModule;
165
21 sub MyModule::worker { 166 sub worker {
22 my ($slave_filehandle) = @_; 167 my ($slave_filehandle) = @_;
23 168
24 # now $slave_filehandle is connected to the $master_filehandle 169 # now $slave_filehandle is connected to the $master_filehandle
25 # in the original prorcess. have fun! 170 # in the original prorcess. have fun!
26 } 171 }
27 172
28 ##################################################################
29 # create a pool of server processes all accepting on the same socket 173 Create a pool of server processes all accepting on the same socket.
30
31 # create listener socket 174 # create listener socket
32 my $listener = ...; 175 my $listener = ...;
33 176
34 # create a pool template, initialise it and give it the socket 177 # create a pool template, initialise it and give it the socket
35 my $pool = AnyEvent::Fork 178 my $pool = AnyEvent::Fork
… …
46 } 189 }
47 190
48 # now do other things - maybe use the filehandle provided by run 191 # now do other things - maybe use the filehandle provided by run
49 # to wait for the processes to die. or whatever. 192 # to wait for the processes to die. or whatever.
50 193
51 # My::Server::run might look like this 194 "My::Server" might look like this:
52 sub My::Server::run { 195
196 package My::Server;
197
198 sub run {
53 my ($slave, $listener, $id) = @_; 199 my ($slave, $listener, $id) = @_;
54 200
55 close $slave; # we do not use the socket, so close it to save resources 201 close $slave; # we do not use the socket, so close it to save resources
56 202
57 # we could go ballistic and use e.g. AnyEvent here, or IO::AIO, 203 # we could go ballistic and use e.g. AnyEvent here, or IO::AIO,
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59 while (my $socket = $listener->accept) { 205 while (my $socket = $listener->accept) {
60 # do sth. with new socket 206 # do sth. with new socket
61 } 207 }
62 } 208 }
63 209
64DESCRIPTION 210 use AnyEvent::Fork as a faster fork+exec
65 This module allows you to create new processes, without actually forking 211 This runs "/bin/echo hi", with standard output redirected to /tmp/log
66 them from your current process (avoiding the problems of forking), but 212 and standard error redirected to the communications socket. It is
67 preserving most of the advantages of fork. 213 usually faster than fork+exec, but still lets you prepare the
214 environment.
68 215
69 It can be used to create new worker processes or new independent 216 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.
74 217
75 Special care has been taken to make this module useful from other 218 AnyEvent::Fork
76 modules, while still supporting specialised environments such as 219 ->new
77 App::Staticperl or PAR::Packer. 220 ->eval ('
221 # compile a helper function for later use
222 sub run {
223 my ($fh, $output, @cmd) = @_;
78 224
79WHAT THIS MODULE IS NOT 225 # perl will clear close-on-exec on STDOUT/STDERR
80 This module only creates processes and lets you pass file handles and 226 open STDOUT, ">&", $output or die;
81 strings to it, and run perl code. It does not implement any kind of RPC 227 open STDERR, ">&", $fh or die;
82 - there is no back channel from the process back to you, and there is no
83 RPC or message passing going on.
84 228
85 If you need some form of RPC, you can either implement it yourself in 229 exec @cmd;
86 whatever way you like, use some message-passing module such as 230 }
87 AnyEvent::MP, some pipe such as AnyEvent::ZeroMQ, use AnyEvent::Handle 231 ')
88 on both sides to send e.g. JSON or Storable messages, and so on. 232 ->send_fh ($output)
233 ->send_arg ("/bin/echo", "hi")
234 ->run ("run", my $cv = AE::cv);
89 235
90PROBLEM STATEMENT 236 my $stderr = $cv->recv;
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
96 Forking from a big process can be very slow (a 5GB process needs 0.05s
97 to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead is
98 often shared with exec (because you have to fork first), but in some
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
103 Forking usually creates a copy-on-write copy of the parent process.
104 Memory (for example, modules or data files that have been will not take
105 additional memory). When exec'ing a new process, modules and data files
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
116 Exec'ing a new perl process might be difficult and slow. For example, it
117 is not easy to find the correct path to the perl interpreter, and all
118 modules have to be loaded from disk again. Long running processes might
119 run into problems when perl is upgraded for example.
120 This module supports creating pre-initialised perl processes to be
121 used as template, and also tries hard to identify the correct path
122 to the perl interpreter. With a cooperative main program, exec'ing
123 the interpreter might not even be necessary.
124
125 Forking might be impossible when a program is running. For example,
126 POSIX makes it almost impossible to fork from a multi-threaded program
127 and do anything useful in the child - strictly speaking, if your perl
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
133 Parallel processing with fork might be inconvenient or difficult to
134 implement. For example, when a program uses an event loop and creates
135 watchers it becomes very hard to use the event loop from a child
136 program, as the watchers already exist but are only meaningful in the
137 parent. Worse, a module might want to use such a system, not knowing
138 whether another module or the main program also does, leading to
139 problems.
140 This module only lets the main program create pools by forking
141 (because only the main program can know when it is still safe to do
142 so) - all other pools are created by fork+exec, after which such
143 modules can again be loaded.
144 237
145CONCEPTS 238CONCEPTS
146 This module can create new processes either by executing a new perl 239 This module can create new processes either by executing a new perl
147 process, or by forking from an existing "template" process. 240 process, or by forking from an existing "template" process.
241
242 All these processes are called "child processes" (whether they are
243 direct children or not), while the process that manages them is called
244 the "parent process".
148 245
149 Each such process comes with its own file handle that can be used to 246 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 247 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 248 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 249 in it are load modules, fork new processes, send file handles to it, and
… …
220 ->require ("Some::Module") 317 ->require ("Some::Module")
221 ->run ("Some::Module::run", sub { 318 ->run ("Some::Module::run", sub {
222 my ($fork_fh) = @_; 319 my ($fork_fh) = @_;
223 }); 320 });
224 321
225FUNCTIONS 322THE "AnyEvent::Fork" CLASS
226 my $pool = new AnyEvent::Fork key => value... 323 This module exports nothing, and only implements a single class -
227 Create a new process pool. The following named parameters are 324 "AnyEvent::Fork".
228 supported: 325
326 There are two class constructors that both create new processes - "new"
327 and "new_exec". The "fork" method creates a new process by forking an
328 existing one and could be considered a third constructor.
329
330 Most of the remaining methods deal with preparing the new process, by
331 loading code, evaluating code and sending data to the new process. They
332 usually return the process object, so you can chain method calls.
333
334 If a process object is destroyed before calling its "run" method, then
335 the process simply exits. After "run" is called, all responsibility is
336 passed to the specified function.
337
338 As long as there is any outstanding work to be done, process objects
339 resist being destroyed, so there is no reason to store them unless you
340 need them later - configure and forget works just fine.
229 341
230 my $proc = new AnyEvent::Fork 342 my $proc = new AnyEvent::Fork
231 Create a new "empty" perl interpreter process and returns its 343 Create a new "empty" perl interpreter process and returns its
232 process object for further manipulation. 344 process object for further manipulation.
233 345
234 The new process is forked from a template process that is kept 346 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 347 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. 348 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.
242 349
243 $new_proc = $proc->fork 350 $new_proc = $proc->fork
244 Forks $proc, creating a new process, and returns the process object 351 Forks $proc, creating a new process, and returns the process object
245 of the new process. 352 of the new process.
246 353
… …
266 that sounds as if it were the perl interpreter. Failing this, the 373 that sounds as if it were the perl interpreter. Failing this, the
267 module falls back to using $Config::Config{perlpath}. 374 module falls back to using $Config::Config{perlpath}.
268 375
269 $pid = $proc->pid 376 $pid = $proc->pid
270 Returns the process id of the process *iff it is a direct child of 377 Returns the process id of the process *iff it is a direct child of
271 the process* running AnyEvent::Fork, and "undef" otherwise. 378 the process running AnyEvent::Fork*, and "undef" otherwise.
272 379
273 Normally, only processes created via "AnyEvent::Fork->new_exec" and 380 Normally, only processes created via "AnyEvent::Fork->new_exec" and
274 AnyEvent::Fork::Template are direct children, and you are 381 AnyEvent::Fork::Template are direct children, and you are
275 responsible to clean up their zombies when they die. 382 responsible to clean up their zombies when they die.
276 383
277 All other processes are not direct children, and will be cleaned up 384 All other processes are not direct children, and will be cleaned up
278 by AnyEvent::Fork. 385 by AnyEvent::Fork itself.
279 386
280 $proc = $proc->eval ($perlcode, @args) 387 $proc = $proc->eval ($perlcode, @args)
281 Evaluates the given $perlcode as ... perl code, while setting @_ to 388 Evaluates the given $perlcode as ... Perl code, while setting @_ to
282 the strings specified by @args. 389 the strings specified by @args, in the "main" package.
283 390
284 This call is meant to do any custom initialisation that might be 391 This call is meant to do any custom initialisation that might be
285 required (for example, the "require" method uses it). It's not 392 required (for example, the "require" method uses it). It's not
286 supposed to be used to completely take over the process, use "run" 393 supposed to be used to completely take over the process, use "run"
287 for that. 394 for that.
… …
289 The code will usually be executed after this call returns, and there 396 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 397 is no way to pass anything back to the calling process. Any
291 evaluation errors will be reported to stderr and cause the process 398 evaluation errors will be reported to stderr and cause the process
292 to exit. 399 to exit.
293 400
401 If you want to execute some code (that isn't in a module) to take
402 over the process, you should compile a function via "eval" first,
403 and then call it via "run". This also gives you access to any
404 arguments passed via the "send_xxx" methods, such as file handles.
405 See the "use AnyEvent::Fork as a faster fork+exec" example to see it
406 in action.
407
294 Returns the process object for easy chaining of method calls. 408 Returns the process object for easy chaining of method calls.
295 409
296 $proc = $proc->require ($module, ...) 410 $proc = $proc->require ($module, ...)
297 Tries to load the given module(s) into the process 411 Tries to load the given module(s) into the process
298 412
… …
300 414
301 $proc = $proc->send_fh ($handle, ...) 415 $proc = $proc->send_fh ($handle, ...)
302 Send one or more file handles (*not* file descriptors) to the 416 Send one or more file handles (*not* file descriptors) to the
303 process, to prepare a call to "run". 417 process, to prepare a call to "run".
304 418
305 The process object keeps a reference to the handles until this is 419 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 420 been passed over to the process, so you must not explicitly close
307 easily accomplished by simply not storing the file handles anywhere 421 the handles. This is most easily accomplished by simply not storing
308 after passing them to this method. 422 the file handles anywhere after passing them to this method - when
423 AnyEvent::Fork is finished using them, perl will automatically close
424 them.
309 425
310 Returns the process object for easy chaining of method calls. 426 Returns the process object for easy chaining of method calls.
311 427
312 Example: pass a file handle to a process, and release it without 428 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. 429 closing. It will be closed automatically when it is no longer used.
… …
315 $proc->send_fh ($my_fh); 431 $proc->send_fh ($my_fh);
316 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT 432 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT
317 433
318 $proc = $proc->send_arg ($string, ...) 434 $proc = $proc->send_arg ($string, ...)
319 Send one or more argument strings to the process, to prepare a call 435 Send one or more argument strings to the process, to prepare a call
320 to "run". The strings can be any octet string. 436 to "run". The strings can be any octet strings.
321 437
322 The protocol is optimised to pass a moderate number of relatively 438 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 439 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 440 is more meant to pass some ID information or other startup info, not
325 big chunks of data. 441 big chunks of data.
326 442
327 Returns the process object for easy chaining of method calls. 443 Returns the process object for easy chaining of method calls.
328 444
329 $proc->run ($func, $cb->($fh)) 445 $proc->run ($func, $cb->($fh))
330 Enter the function specified by the fully qualified name in $func in 446 Enter the function specified by the function name in $func in the
331 the process. The function is called with the communication socket as 447 process. The function is called with the communication socket as
332 first argument, followed by all file handles and string arguments 448 first argument, followed by all file handles and string arguments
333 sent earlier via "send_fh" and "send_arg" methods, in the order they 449 sent earlier via "send_fh" and "send_arg" methods, in the order they
334 were called. 450 were called.
335 451
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. 452 The process object becomes unusable on return from this function -
453 any further method calls result in undefined behaviour.
454
455 The function name should be fully qualified, but if it isn't, it
456 will be looked up in the "main" package.
457
458 If the called function returns, doesn't exist, or any error occurs,
459 the process exits.
460
461 Preparing the process is done in the background - when all commands
462 have been sent, the callback is invoked with the local
463 communications socket as argument. At this point you can start using
464 the socket in any way you like.
343 465
344 If the communication socket isn't used, it should be closed on both 466 If the communication socket isn't used, it should be closed on both
345 sides, to save on kernel memory. 467 sides, to save on kernel memory.
346 468
347 The socket is non-blocking in the parent, and blocking in the newly 469 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 470 created process. The close-on-exec flag is set in both.
471
349 used otherwise, the socket can be a good indicator for the existence 472 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 473 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 474 readable event on it, because exiting the process closes the socket
352 didn't create any children using fork). 475 (if it didn't create any children using fork).
353 476
354 Example: create a template for a process pool, pass a few strings, 477 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 478 some file handles, then fork, pass one more string, and run some
356 code. 479 code.
357 480
… …
366 ->send_arg ("str3") 489 ->send_arg ("str3")
367 ->run ("Some::function", sub { 490 ->run ("Some::function", sub {
368 my ($fh) = @_; 491 my ($fh) = @_;
369 492
370 # fh is nonblocking, but we trust that the OS can accept these 493 # fh is nonblocking, but we trust that the OS can accept these
371 # extra 3 octets anyway. 494 # few octets anyway.
372 syswrite $fh, "hi #$_\n"; 495 syswrite $fh, "hi #$_\n";
373 496
374 # $fh is being closed here, as we don't store it anywhere 497 # $fh is being closed here, as we don't store it anywhere
375 }); 498 });
376 } 499 }
… …
378 # Some::function might look like this - all parameters passed before fork 501 # Some::function might look like this - all parameters passed before fork
379 # and after will be passed, in order, after the communications socket. 502 # and after will be passed, in order, after the communications socket.
380 sub Some::function { 503 sub Some::function {
381 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; 504 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_;
382 505
383 print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" 506 print scalar <$fh>; # prints "hi #1\n" and "hi #2\n" in any order
384 } 507 }
385 508
386PERFORMANCE 509PERFORMANCE
387 Now for some unscientific benchmark numbers (all done on an amd64 510 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 511 GNU/Linux box). These are intended to give you an idea of the relative
… …
411 479 vfork+execs per second, using AnyEvent::Fork->new_exec 534 479 vfork+execs per second, using AnyEvent::Fork->new_exec
412 535
413 So how can "AnyEvent->new" be faster than a standard fork, even though 536 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? 537 it uses the same operations, but adds a lot of overhead?
415 538
416 The difference is simply the process size: forking the 6MB process takes 539 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 540 so much longer than forking the 2.5MB template process that the extra
418 introduced is canceled out. 541 overhead is canceled out.
419 542
420 If the benchmark process grows, the normal fork becomes even slower: 543 If the benchmark process grows, the normal fork becomes even slower:
421 544
422 1340 new processes, manual fork in a 20MB process 545 1340 new processes, manual fork of a 20MB process
423 731 new processes, manual fork in a 200MB process 546 731 new processes, manual fork of a 200MB process
424 235 new processes, manual fork in a 2000MB process 547 235 new processes, manual fork of a 2000MB process
425 548
426 What that means (to me) is that I can use this module without having a 549 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 550 bad conscience because of the extra overhead required to start new
428 processes. 551 processes.
429 552
430TYPICAL PROBLEMS 553TYPICAL PROBLEMS
431 This section lists typical problems that remain. I hope by recognising 554 This section lists typical problems that remain. I hope by recognising
432 them, most can be avoided. 555 them, most can be avoided.
433 556
434 "leaked" file descriptors for exec'ed processes 557 leaked file descriptors for exec'ed processes
435 POSIX systems inherit file descriptors by default when exec'ing a 558 POSIX systems inherit file descriptors by default when exec'ing a
436 new process. While perl itself laudably sets the close-on-exec flags 559 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 560 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 561 cared, it's often not possible to set the flag in a race-free
439 manner. 562 manner.
… …
459 libraries or the code that leaks those file descriptors. 582 libraries or the code that leaks those file descriptors.
460 583
461 Fortunately, most of these leaked descriptors do no harm, other than 584 Fortunately, most of these leaked descriptors do no harm, other than
462 sitting on some resources. 585 sitting on some resources.
463 586
464 "leaked" file descriptors for fork'ed processes 587 leaked file descriptors for fork'ed processes
465 Normally, AnyEvent::Fork does start new processes by exec'ing them, 588 Normally, AnyEvent::Fork does start new processes by exec'ing them,
466 which closes file descriptors not marked for being inherited. 589 which closes file descriptors not marked for being inherited.
467 590
468 However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a 591 However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a
469 way to create these processes by forking, and this leaks more file 592 way to create these processes by forking, and this leaks more file
… …
477 600
478 The solution is to either not load these modules before use'ing 601 The solution is to either not load these modules before use'ing
479 AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay 602 AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay
480 initialising them, for example, by calling "init Gtk2" manually. 603 initialising them, for example, by calling "init Gtk2" manually.
481 604
482 exit runs destructors 605 exiting calls object destructors
483 This only applies to users of Lc<AnyEvent::Fork:Early> and 606 This only applies to users of AnyEvent::Fork:Early and
484 AnyEvent::Fork::Template. 607 AnyEvent::Fork::Template, or when initialising code creates objects
608 that reference external resources.
485 609
486 When a process created by AnyEvent::Fork exits, it might do so by 610 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. 611 calling exit, or simply letting perl reach the end of the program.
488 At which point Perl runs all destructors. 612 At which point Perl runs all destructors.
489 613
… …
505 and sweat to make it so, mostly due to the bloody broken perl that 629 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 630 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 631 yet to see something useful that you can do with it without running into
508 memory corruption issues or other braindamage. Hrrrr. 632 memory corruption issues or other braindamage. Hrrrr.
509 633
510 Cygwin perl is not supported at the moment, as it should implement fd 634 Cygwin perl is not supported at the moment due to some hilarious
511 passing, but doesn't, and rolling my own is hard, as cygwin doesn't 635 shortcomings of its API - see IO::FDPoll for more details.
512 support enough functionality to do it.
513 636
514SEE ALSO 637SEE ALSO
515 AnyEvent::Fork::Early (to avoid executing a perl interpreter), 638 AnyEvent::Fork::Early, to avoid executing a perl interpreter at all
639 (part of this distribution).
640
516 AnyEvent::Fork::Template (to create a process by forking the main 641 AnyEvent::Fork::Template, to create a process by forking the main
517 program at a convenient time). 642 program at a convenient time (part of this distribution).
518 643
519AUTHOR 644 AnyEvent::Fork::RPC, for simple RPC to child processes (on CPAN).
645
646AUTHOR AND CONTACT INFORMATION
520 Marc Lehmann <schmorp@schmorp.de> 647 Marc Lehmann <schmorp@schmorp.de>
521 http://home.schmorp.de/ 648 http://software.schmorp.de/pkg/AnyEvent-Fork
522 649

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