ViewVC Help
View File | Revision Log | Show Annotations | Download File
/cvs/AnyEvent-Fork/README
(Generate patch)

Comparing AnyEvent-Fork/README (file contents):
Revision 1.3 by root, Fri Apr 5 19:10:10 2013 UTC vs.
Revision 1.5 by root, Sat Apr 6 22:41:56 2013 UTC

… …
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 can either implement it yourself in
36 whatever way you like, use some message-passing module such as
37 AnyEvent::MP, some pipe such as AnyEvent::ZeroMQ, use AnyEvent::Handle
38 on both sides to send e.g. JSON or Storable messages, and so on.
39
40 COMPARISON TO OTHER MODULES
41 There is an abundance of modules on CPAN that do "something fork", such
42 as Parallel::ForkManager, AnyEvent::ForkManager, AnyEvent::Worker or
43 AnyEvent::Subprocess. There are modules that implement their own process
44 management, such as AnyEvent::DBI.
45
46 The problems that all these modules try to solve are real, however, none
47 of them (from what I have seen) tackle the very real problems of
48 unwanted memory sharing, efficiency, not being able to use event
49 processing or similar modules in the processes they create.
50
51 This module doesn't try to replace any of them - instead it tries to
52 solve the problem of creating processes with a minimum of fuss and
53 overhead (and also luxury). Ideally, most of these would use
54 AnyEvent::Fork internally, except they were written before AnyEvent:Fork
55 was available, so obviously had to roll their own.
56
57 PROBLEM STATEMENT
58 There are two traditional ways to implement parallel processing on UNIX
59 like operating systems - fork and process, and fork+exec and process.
60 They have different advantages and disadvantages that I describe below,
61 together with how this module tries to mitigate the disadvantages.
62
63 Forking from a big process can be very slow.
64 A 5GB process needs 0.05s to fork on my 3.6GHz amd64 GNU/Linux box.
65 This overhead is often shared with exec (because you have to fork
66 first), but in some circumstances (e.g. when vfork is used),
67 fork+exec can be much faster.
68
69 This module can help here by telling a small(er) helper process to
70 fork, which is faster then forking the main process, and also uses
71 vfork where possible. This gives the speed of vfork, with the
72 flexibility of fork.
73
74 Forking usually creates a copy-on-write copy of the parent process.
75 For example, modules or data files that are loaded will not use
76 additional memory after a fork. When exec'ing a new process, modules
77 and data files might need to be loaded again, at extra CPU and
78 memory cost. But when forking, literally all data structures are
79 copied - if the program frees them and replaces them by new data,
80 the child processes will retain the old version even if it isn't
81 used, which can suddenly and unexpectedly increase memory usage when
82 freeing memory.
83
84 The trade-off is between more sharing with fork (which can be good
85 or bad), and no sharing with exec.
86
87 This module allows the main program to do a controlled fork, and
88 allows modules to exec processes safely at any time. When creating a
89 custom process pool you can take advantage of data sharing via fork
90 without risking to share large dynamic data structures that will
91 blow up child memory usage.
92
93 In other words, this module puts you into control over what is being
94 shared and what isn't, at all times.
95
96 Exec'ing a new perl process might be difficult.
97 For example, it is not easy to find the correct path to the perl
98 interpreter - $^X might not be a perl interpreter at all.
99
100 This module tries hard to identify the correct path to the perl
101 interpreter. With a cooperative main program, exec'ing the
102 interpreter might not even be necessary, but even without help from
103 the main program, it will still work when used from a module.
104
105 Exec'ing a new perl process might be slow, as all necessary modules have
106 to be loaded from disk again, with no guarantees of success.
107 Long running processes might run into problems when perl is upgraded
108 and modules are no longer loadable because they refer to a different
109 perl version, or parts of a distribution are newer than the ones
110 already loaded.
111
112 This module supports creating pre-initialised perl processes to be
113 used as a template for new processes.
114
115 Forking might be impossible when a program is running.
116 For example, POSIX makes it almost impossible to fork from a
117 multi-threaded program while doing anything useful in the child - in
118 fact, if your perl program uses POSIX threads (even indirectly via
119 e.g. IO::AIO or threads), you cannot call fork on the perl level
120 anymore without risking corruption issues on a number of operating
121 systems.
122
123 This module can safely fork helper processes at any time, by calling
124 fork+exec in C, in a POSIX-compatible way (via Proc::FastSpawn).
125
126 Parallel processing with fork might be inconvenient or difficult to
127 implement. Modules might not work in both parent and child.
128 For example, when a program uses an event loop and creates watchers
129 it becomes very hard to use the event loop from a child program, as
130 the watchers already exist but are only meaningful in the parent.
131 Worse, a module might want to use such a module, not knowing whether
132 another module or the main program also does, leading to problems.
133
134 Apart from event loops, graphical toolkits also commonly fall into
135 the "unsafe module" category, or just about anything that
136 communicates with the external world, such as network libraries and
137 file I/O modules, which usually don't like being copied and then
138 allowed to continue in two processes.
139
140 With this module only the main program is allowed to create new
141 processes by forking (because only the main program can know when it
142 is still safe to do so) - all other processes are created via
143 fork+exec, which makes it possible to use modules such as event
144 loops or window interfaces safely.
145
146EXAMPLES
8 # create a single new process, tell it to run your worker function 147 Create a single new process, tell it to run your worker function.
9
10 AnyEvent::Fork 148 AnyEvent::Fork
11 ->new 149 ->new
12 ->require ("MyModule") 150 ->require ("MyModule")
13 ->run ("MyModule::worker, sub { 151 ->run ("MyModule::worker, sub {
14 my ($master_filehandle) = @_; 152 my ($master_filehandle) = @_;
15 153
16 # now $master_filehandle is connected to the 154 # now $master_filehandle is connected to the
17 # $slave_filehandle in the new process. 155 # $slave_filehandle in the new process.
18 }); 156 });
19 157
20 # MyModule::worker might look like this 158 "MyModule" might look like this:
159
160 package MyModule;
161
21 sub MyModule::worker { 162 sub worker {
22 my ($slave_filehandle) = @_; 163 my ($slave_filehandle) = @_;
23 164
24 # now $slave_filehandle is connected to the $master_filehandle 165 # now $slave_filehandle is connected to the $master_filehandle
25 # in the original prorcess. have fun! 166 # in the original prorcess. have fun!
26 } 167 }
27 168
28 ##################################################################
29 # create a pool of server processes all accepting on the same socket 169 Create a pool of server processes all accepting on the same socket.
30
31 # create listener socket 170 # create listener socket
32 my $listener = ...; 171 my $listener = ...;
33 172
34 # create a pool template, initialise it and give it the socket 173 # create a pool template, initialise it and give it the socket
35 my $pool = AnyEvent::Fork 174 my $pool = AnyEvent::Fork
… …
46 } 185 }
47 186
48 # now do other things - maybe use the filehandle provided by run 187 # now do other things - maybe use the filehandle provided by run
49 # to wait for the processes to die. or whatever. 188 # to wait for the processes to die. or whatever.
50 189
51 # My::Server::run might look like this 190 "My::Server" might look like this:
52 sub My::Server::run { 191
192 package My::Server;
193
194 sub run {
53 my ($slave, $listener, $id) = @_; 195 my ($slave, $listener, $id) = @_;
54 196
55 close $slave; # we do not use the socket, so close it to save resources 197 close $slave; # we do not use the socket, so close it to save resources
56 198
57 # we could go ballistic and use e.g. AnyEvent here, or IO::AIO, 199 # we could go ballistic and use e.g. AnyEvent here, or IO::AIO,
… …
59 while (my $socket = $listener->accept) { 201 while (my $socket = $listener->accept) {
60 # do sth. with new socket 202 # do sth. with new socket
61 } 203 }
62 } 204 }
63 205
64DESCRIPTION 206 use AnyEvent::Fork as a faster fork+exec
65 This module allows you to create new processes, without actually forking 207 This runs "/bin/echo hi", with stdandard output redirected to /tmp/log
66 them from your current process (avoiding the problems of forking), but 208 and standard error redirected to the communications socket. It is
67 preserving most of the advantages of fork. 209 usually faster than fork+exec, but still lets you prepare the
210 environment.
68 211
69 It can be used to create new worker processes or new independent 212 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.
74 213
75 Special care has been taken to make this module useful from other 214 AnyEvent::Fork
76 modules, while still supporting specialised environments such as 215 ->new
77 App::Staticperl or PAR::Packer. 216 ->eval ('
217 # compile a helper function for later use
218 sub run {
219 my ($fh, $output, @cmd) = @_;
78 220
79PROBLEM STATEMENT 221 # perl will clear close-on-exec on STDOUT/STDERR
80 There are two ways to implement parallel processing on UNIX like 222 open STDOUT, ">&", $output or die;
81 operating systems - fork and process, and fork+exec and process. They 223 open STDERR, ">&", $fh or die;
82 have different advantages and disadvantages that I describe below,
83 together with how this module tries to mitigate the disadvantages.
84 224
85 Forking from a big process can be very slow (a 5GB process needs 0.05s 225 exec @cmd;
86 to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead is 226 }
87 often shared with exec (because you have to fork first), but in some 227 ')
88 circumstances (e.g. when vfork is used), fork+exec can be much faster. 228 ->send_fh ($output)
89 This module can help here by telling a small(er) helper process to 229 ->send_arg ("/bin/echo", "hi")
90 fork, or fork+exec instead. 230 ->run ("run", my $cv = AE::cv);
91 231
92 Forking usually creates a copy-on-write copy of the parent process. 232 my $stderr = $cv->recv;
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
105 Exec'ing a new perl process might be difficult and slow. For example, it
106 is not easy to find the correct path to the perl interpreter, and all
107 modules have to be loaded from disk again. Long running processes might
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
114 Forking might be impossible when a program is running. For example,
115 POSIX makes it almost impossible to fork from a multithreaded program
116 and do anything useful in the child - strictly speaking, if your perl
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
122 Parallel processing with fork might be inconvenient or difficult to
123 implement. For example, when a program uses an event loop and creates
124 watchers it becomes very hard to use the event loop from a child
125 program, as the watchers already exist but are only meaningful in the
126 parent. Worse, a module might want to use such a system, not knowing
127 whether another module or the main program also does, leading to
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.
133 233
134CONCEPTS 234CONCEPTS
135 This module can create new processes either by executing a new perl 235 This module can create new processes either by executing a new perl
136 process, or by forking from an existing "template" process. 236 process, or by forking from an existing "template" process.
137 237
… …
151 memory used for the perl interpreter with the new process, but 251 memory used for the perl interpreter with the new process, but
152 loading modules takes time, and the memory is not shared with 252 loading modules takes time, and the memory is not shared with
153 anything else. 253 anything else.
154 254
155 This is ideal for when you only need one extra process of a kind, 255 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. 256 with the option of starting and stopping it on demand.
157 257
158 Example: 258 Example:
159 259
160 AnyEvent::Fork 260 AnyEvent::Fork
161 ->new 261 ->new
… …
175 the modules you loaded) is shared between the processes, and each 275 the modules you loaded) is shared between the processes, and each
176 new process consumes relatively little memory of its own. 276 new process consumes relatively little memory of its own.
177 277
178 The disadvantage of this approach is that you need to create a 278 The disadvantage of this approach is that you need to create a
179 template process for the sole purpose of forking new processes from 279 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 280 it, but if you only need a fixed number of processes you can create
181 them, and then destroy the template process. 281 them, and then destroy the template process.
182 282
183 Example: 283 Example:
184 284
185 my $template = AnyEvent::Fork->new->require ("Some::Module"); 285 my $template = AnyEvent::Fork->new->require ("Some::Module");
… …
209 ->require ("Some::Module") 309 ->require ("Some::Module")
210 ->run ("Some::Module::run", sub { 310 ->run ("Some::Module::run", sub {
211 my ($fork_fh) = @_; 311 my ($fork_fh) = @_;
212 }); 312 });
213 313
214FUNCTIONS 314THE "AnyEvent::Fork" CLASS
215 my $pool = new AnyEvent::Fork key => value... 315 This module exports nothing, and only implements a single class -
216 Create a new process pool. The following named parameters are 316 "AnyEvent::Fork".
217 supported:
218 317
318 There are two class constructors that both create new processes - "new"
319 and "new_exec". The "fork" method creates a new process by forking an
320 existing one and could be considered a third constructor.
321
322 Most of the remaining methods deal with preparing the new process, by
323 loading code, evaluating code and sending data to the new process. They
324 usually return the process object, so you can chain method calls.
325
326 If a process object is destroyed before calling its "run" method, then
327 the process simply exits. After "run" is called, all responsibility is
328 passed to the specified function.
329
330 As long as there is any outstanding work to be done, process objects
331 resist being destroyed, so there is no reason to store them unless you
332 need them later - configure and forget works just fine.
333
219 my $proc = new AnyEvent::Fork 334 my $proc = new AnyEvent::Fork
335
220 Create a new "empty" perl interpreter process and returns its 336 Create a new "empty" perl interpreter process and returns its
221 process object for further manipulation. 337 process object for further manipulation.
222 338
223 The new process is forked from a template process that is kept 339 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 340 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. 341 a call to "new_exec" first and then stays around for future calls.
226 342
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
232 $new_proc = $proc->fork 343 $new_proc = $proc->fork
344
233 Forks $proc, creating a new process, and returns the process object 345 Forks $proc, creating a new process, and returns the process object
234 of the new process. 346 of the new process.
235 347
236 If any of the "send_" functions have been called before fork, then 348 If any of the "send_" functions have been called before fork, then
237 they will be cloned in the child. For example, in a pre-forked 349 they will be cloned in the child. For example, in a pre-forked
238 server, you might "send_fh" the listening socket into the template 350 server, you might "send_fh" the listening socket into the template
239 process, and then keep calling "fork" and "run". 351 process, and then keep calling "fork" and "run".
240 352
241 my $proc = new_exec AnyEvent::Fork 353 my $proc = new_exec AnyEvent::Fork
354
242 Create a new "empty" perl interpreter process and returns its 355 Create a new "empty" perl interpreter process and returns its
243 process object for further manipulation. 356 process object for further manipulation.
244 357
245 Unlike the "new" method, this method *always* spawns a new perl 358 Unlike the "new" method, this method *always* spawns a new perl
246 process (except in some cases, see AnyEvent::Fork::Early for 359 process (except in some cases, see AnyEvent::Fork::Early for
… …
248 possible, and is also slower. 361 possible, and is also slower.
249 362
250 You should use "new" whenever possible, except when having a 363 You should use "new" whenever possible, except when having a
251 template process around is unacceptable. 364 template process around is unacceptable.
252 365
253 The path to the perl interpreter is divined usign various methods - 366 The path to the perl interpreter is divined using various methods -
254 first $^X is investigated to see if the path ends with something 367 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 368 that sounds as if it were the perl interpreter. Failing this, the
256 module falls back to using $Config::Config{perlpath}. 369 module falls back to using $Config::Config{perlpath}.
257 370
371 $pid = $proc->pid
372
373 Returns the process id of the process *iff it is a direct child of
374 the process running AnyEvent::Fork*, and "undef" otherwise.
375
376 Normally, only processes created via "AnyEvent::Fork->new_exec" and
377 AnyEvent::Fork::Template are direct children, and you are
378 responsible to clean up their zombies when they die.
379
380 All other processes are not direct children, and will be cleaned up
381 by AnyEvent::Fork itself.
382
258 $proc = $proc->eval ($perlcode, @args) 383 $proc = $proc->eval ($perlcode, @args)
384
259 Evaluates the given $perlcode as ... perl code, while setting @_ to 385 Evaluates the given $perlcode as ... perl code, while setting @_ to
260 the strings specified by @args. 386 the strings specified by @args, in the "main" package.
261 387
262 This call is meant to do any custom initialisation that might be 388 This call is meant to do any custom initialisation that might be
263 required (for example, the "require" method uses it). It's not 389 required (for example, the "require" method uses it). It's not
264 supposed to be used to completely take over the process, use "run" 390 supposed to be used to completely take over the process, use "run"
265 for that. 391 for that.
… …
267 The code will usually be executed after this call returns, and there 393 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 394 is no way to pass anything back to the calling process. Any
269 evaluation errors will be reported to stderr and cause the process 395 evaluation errors will be reported to stderr and cause the process
270 to exit. 396 to exit.
271 397
398 If you want to execute some code (that isn't in a module) to take
399 over the process, you should compile a function via "eval" first,
400 and then call it via "run". This also gives you access to any
401 arguments passed via the "send_xxx" methods, such as file handles.
402 See the "use AnyEvent::Fork as a faster fork+exec" example to see it
403 in action.
404
272 Returns the process object for easy chaining of method calls. 405 Returns the process object for easy chaining of method calls.
273 406
274 $proc = $proc->require ($module, ...) 407 $proc = $proc->require ($module, ...)
408
275 Tries to load the given module(s) into the process 409 Tries to load the given module(s) into the process
276 410
277 Returns the process object for easy chaining of method calls. 411 Returns the process object for easy chaining of method calls.
278 412
279 $proc = $proc->send_fh ($handle, ...) 413 $proc = $proc->send_fh ($handle, ...)
414
280 Send one or more file handles (*not* file descriptors) to the 415 Send one or more file handles (*not* file descriptors) to the
281 process, to prepare a call to "run". 416 process, to prepare a call to "run".
282 417
283 The process object keeps a reference to the handles until this is 418 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 419 been passed over to the process, so you must not explicitly close
285 easily accomplished by simply not storing the file handles anywhere 420 the handles. This is most easily accomplished by simply not storing
286 after passing them to this method. 421 the file handles anywhere after passing them to this method - when
422 AnyEvent::Fork is finished using them, perl will automatically close
423 them.
287 424
288 Returns the process object for easy chaining of method calls. 425 Returns the process object for easy chaining of method calls.
289 426
290 Example: pass an fh to a process, and release it without closing. it 427 Example: pass a file handle to a process, and release it without
291 will be closed automatically when it is no longer used. 428 closing. It will be closed automatically when it is no longer used.
292 429
293 $proc->send_fh ($my_fh); 430 $proc->send_fh ($my_fh);
294 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT 431 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT
295 432
296 $proc = $proc->send_arg ($string, ...) 433 $proc = $proc->send_arg ($string, ...)
434
297 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
298 to "run". The strings can be any octet string. 436 to "run". The strings can be any octet strings.
299 437
438 The protocol is optimised to pass a moderate number of relatively
439 short strings - while you can pass up to 4GB of data in one go, this
440 is more meant to pass some ID information or other startup info, not
441 big chunks of data.
442
300 Returns the process object for easy chaining of emthod calls. 443 Returns the process object for easy chaining of method calls.
301 444
302 $proc->run ($func, $cb->($fh)) 445 $proc->run ($func, $cb->($fh))
446
303 Enter the function specified by the fully qualified name in $func in 447 Enter the function specified by the function name in $func in the
304 the process. The function is called with the communication socket as 448 process. The function is called with the communication socket as
305 first argument, followed by all file handles and string arguments 449 first argument, followed by all file handles and string arguments
306 sent earlier via "send_fh" and "send_arg" methods, in the order they 450 sent earlier via "send_fh" and "send_arg" methods, in the order they
307 were called. 451 were called.
308 452
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. 453 The process object becomes unusable on return from this function -
454 any further method calls result in undefined behaviour.
455
456 The function name should be fully qualified, but if it isn't, it
457 will be looked up in the "main" package.
458
459 If the called function returns, doesn't exist, or any error occurs,
460 the process exits.
461
462 Preparing the process is done in the background - when all commands
463 have been sent, the callback is invoked with the local
464 communications socket as argument. At this point you can start using
465 the socket in any way you like.
316 466
317 If the communication socket isn't used, it should be closed on both 467 If the communication socket isn't used, it should be closed on both
318 sides, to save on kernel memory. 468 sides, to save on kernel memory.
319 469
320 The socket is non-blocking in the parent, and blocking in the newly 470 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 471 created process. The close-on-exec flag is set in both.
472
322 used otherwise, the socket can be a good indicator for the existance 473 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 474 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 475 readable event on it, because exiting the process closes the socket
325 didn't create any children using fork). 476 (if it didn't create any children using fork).
326 477
327 Example: create a template for a process pool, pass a few strings, 478 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 479 some file handles, then fork, pass one more string, and run some
329 code. 480 code.
330 481
… …
339 ->send_arg ("str3") 490 ->send_arg ("str3")
340 ->run ("Some::function", sub { 491 ->run ("Some::function", sub {
341 my ($fh) = @_; 492 my ($fh) = @_;
342 493
343 # fh is nonblocking, but we trust that the OS can accept these 494 # fh is nonblocking, but we trust that the OS can accept these
344 # extra 3 octets anyway. 495 # few octets anyway.
345 syswrite $fh, "hi #$_\n"; 496 syswrite $fh, "hi #$_\n";
346 497
347 # $fh is being closed here, as we don't store it anywhere 498 # $fh is being closed here, as we don't store it anywhere
348 }); 499 });
349 } 500 }
… …
351 # Some::function might look like this - all parameters passed before fork 502 # Some::function might look like this - all parameters passed before fork
352 # and after will be passed, in order, after the communications socket. 503 # and after will be passed, in order, after the communications socket.
353 sub Some::function { 504 sub Some::function {
354 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; 505 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_;
355 506
356 print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" 507 print scalar <$fh>; # prints "hi #1\n" and "hi #2\n" in any order
357 } 508 }
509
510PERFORMANCE
511 Now for some unscientific benchmark numbers (all done on an amd64
512 GNU/Linux box). These are intended to give you an idea of the relative
513 performance you can expect, they are not meant to be absolute
514 performance numbers.
515
516 OK, so, I ran a simple benchmark that creates a socket pair, forks,
517 calls exit in the child and waits for the socket to close in the parent.
518 I did load AnyEvent, EV and AnyEvent::Fork, for a total process size of
519 5100kB.
520
521 2079 new processes per second, using manual socketpair + fork
522
523 Then I did the same thing, but instead of calling fork, I called
524 AnyEvent::Fork->new->run ("CORE::exit") and then again waited for the
525 socket form the child to close on exit. This does the same thing as
526 manual socket pair + fork, except that what is forked is the template
527 process (2440kB), and the socket needs to be passed to the server at the
528 other end of the socket first.
529
530 2307 new processes per second, using AnyEvent::Fork->new
531
532 And finally, using "new_exec" instead "new", using vforks+execs to exec
533 a new perl interpreter and compile the small server each time, I get:
534
535 479 vfork+execs per second, using AnyEvent::Fork->new_exec
536
537 So how can "AnyEvent->new" be faster than a standard fork, even though
538 it uses the same operations, but adds a lot of overhead?
539
540 The difference is simply the process size: forking the 5MB process takes
541 so much longer than forking the 2.5MB template process that the extra
542 overhead introduced is canceled out.
543
544 If the benchmark process grows, the normal fork becomes even slower:
545
546 1340 new processes, manual fork of a 20MB process
547 731 new processes, manual fork of a 200MB process
548 235 new processes, manual fork of a 2000MB process
549
550 What that means (to me) is that I can use this module without having a
551 bad conscience because of the extra overhead required to start new
552 processes.
358 553
359TYPICAL PROBLEMS 554TYPICAL PROBLEMS
360 This section lists typical problems that remain. I hope by recognising 555 This section lists typical problems that remain. I hope by recognising
361 them, most can be avoided. 556 them, most can be avoided.
362 557
363 "leaked" file descriptors for exec'ed processes 558 leaked file descriptors for exec'ed processes
364 POSIX systems inherit file descriptors by default when exec'ing a 559 POSIX systems inherit file descriptors by default when exec'ing a
365 new process. While perl itself laudably sets the close-on-exec flags 560 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 561 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 562 cared, it's often not possible to set the flag in a race-free
368 manner. 563 manner.
369 564
370 That means some file descriptors can leak through. And since it 565 That means some file descriptors can leak through. And since it
371 isn't possible to know which file descriptors are "good" and 566 isn't possible to know which file descriptors are "good" and
372 "neccessary" (or even to know which file descreiptors are open), 567 "necessary" (or even to know which file descriptors are open), there
373 there is no good way to close the ones that might harm. 568 is no good way to close the ones that might harm.
374 569
375 As an example of what "harm" can be done consider a web server that 570 As an example of what "harm" can be done consider a web server that
376 accepts connections and afterwards some module uses AnyEvent::Fork 571 accepts connections and afterwards some module uses AnyEvent::Fork
377 for the first time, causing it to fork and exec a new process, which 572 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, 573 might inherit the network socket. When the server closes the socket,
… …
385 exec'ed well before many random file descriptors are open. 580 exec'ed well before many random file descriptors are open.
386 581
387 In general, the solution for these kind of problems is to fix the 582 In general, the solution for these kind of problems is to fix the
388 libraries or the code that leaks those file descriptors. 583 libraries or the code that leaks those file descriptors.
389 584
390 Fortunately, most of these lekaed descriptors do no harm, other than 585 Fortunately, most of these leaked descriptors do no harm, other than
391 sitting on some resources. 586 sitting on some resources.
392 587
393 "leaked" file descriptors for fork'ed processes 588 leaked file descriptors for fork'ed processes
394 Normally, AnyEvent::Fork does start new processes by exec'ing them, 589 Normally, AnyEvent::Fork does start new processes by exec'ing them,
395 which closes file descriptors not marked for being inherited. 590 which closes file descriptors not marked for being inherited.
396 591
397 However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a 592 However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a
398 way to create these processes by forking, and this leaks more file 593 way to create these processes by forking, and this leaks more file
… …
405 trouble with a fork. 600 trouble with a fork.
406 601
407 The solution is to either not load these modules before use'ing 602 The solution is to either not load these modules before use'ing
408 AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay 603 AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay
409 initialising them, for example, by calling "init Gtk2" manually. 604 initialising them, for example, by calling "init Gtk2" manually.
605
606 exiting calls object destructors
607 This only applies to users of AnyEvent::Fork:Early and
608 AnyEvent::Fork::Template, or when initialiasing code creates objects
609 that reference external resources.
610
611 When a process created by AnyEvent::Fork exits, it might do so by
612 calling exit, or simply letting perl reach the end of the program.
613 At which point Perl runs all destructors.
614
615 Not all destructors are fork-safe - for example, an object that
616 represents the connection to an X display might tell the X server to
617 free resources, which is inconvenient when the "real" object in the
618 parent still needs to use them.
619
620 This is obviously not a problem for AnyEvent::Fork::Early, as you
621 used it as the very first thing, right?
622
623 It is a problem for AnyEvent::Fork::Template though - and the
624 solution is to not create objects with nontrivial destructors that
625 might have an effect outside of Perl.
410 626
411PORTABILITY NOTES 627PORTABILITY NOTES
412 Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a 628 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 629 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 630 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 631 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 632 yet to see something useful that you can do with it without running into
417 memory corruption issues or other braindamage. Hrrrr. 633 memory corruption issues or other braindamage. Hrrrr.
418 634
419 Cygwin perl is not supported at the moment, as it should implement fd 635 Cygwin perl is not supported at the moment due to some hilarious
420 passing, but doesn't, and rolling my own is hard, as cygwin doesn't 636 shortcomings of its API - see IO::FDPoll for more details.
421 support enough functionality to do it.
422 637
423SEE ALSO 638SEE ALSO
424 AnyEvent::Fork::Early (to avoid executing a perl interpreter), 639 AnyEvent::Fork::Early (to avoid executing a perl interpreter),
425 AnyEvent::Fork::Template (to create a process by forking the main 640 AnyEvent::Fork::Template (to create a process by forking the main
426 program at a convenient time). 641 program at a convenient time).
427 642
428AUTHOR 643AUTHOR
429 Marc Lehmann <schmorp@schmorp.de> 644 Marc Lehmann <schmorp@schmorp.de>
430 http://home.schmorp.de/ 645 http://home.schmorp.de/
431 646
647POD ERRORS
648 Hey! The above document had some coding errors, which are explained
649 below:
650
651 Around line 360:
652 You can't have =items (as at line 476) unless the first thing after
653 the =over is an =item
654

Diff Legend

– Removed lines
+ Added lines
< Changed lines
> Changed lines