1 |
NAME |
2 |
AnyEvent::Fork - everything you wanted to use fork() for, but couldn't |
3 |
|
4 |
ATTENTION, this is a very early release, and very untested. Consider it |
5 |
a technology preview. |
6 |
|
7 |
SYNOPSIS |
8 |
use AnyEvent::Fork; |
9 |
|
10 |
################################################################## |
11 |
# create a single new process, tell it to run your worker function |
12 |
|
13 |
AnyEvent::Fork |
14 |
->new |
15 |
->require ("MyModule") |
16 |
->run ("MyModule::worker, sub { |
17 |
my ($master_filehandle) = @_; |
18 |
|
19 |
# now $master_filehandle is connected to the |
20 |
# $slave_filehandle in the new process. |
21 |
}); |
22 |
|
23 |
# MyModule::worker might look like this |
24 |
sub MyModule::worker { |
25 |
my ($slave_filehandle) = @_; |
26 |
|
27 |
# now $slave_filehandle is connected to the $master_filehandle |
28 |
# in the original prorcess. have fun! |
29 |
} |
30 |
|
31 |
################################################################## |
32 |
# create a pool of server processes all accepting on the same socket |
33 |
|
34 |
# create listener socket |
35 |
my $listener = ...; |
36 |
|
37 |
# create a pool template, initialise it and give it the socket |
38 |
my $pool = AnyEvent::Fork |
39 |
->new |
40 |
->require ("Some::Stuff", "My::Server") |
41 |
->send_fh ($listener); |
42 |
|
43 |
# now create 10 identical workers |
44 |
for my $id (1..10) { |
45 |
$pool |
46 |
->fork |
47 |
->send_arg ($id) |
48 |
->run ("My::Server::run"); |
49 |
} |
50 |
|
51 |
# now do other things - maybe use the filehandle provided by run |
52 |
# to wait for the processes to die. or whatever. |
53 |
|
54 |
# My::Server::run might look like this |
55 |
sub My::Server::run { |
56 |
my ($slave, $listener, $id) = @_; |
57 |
|
58 |
close $slave; # we do not use the socket, so close it to save resources |
59 |
|
60 |
# we could go ballistic and use e.g. AnyEvent here, or IO::AIO, |
61 |
# or anything we usually couldn't do in a process forked normally. |
62 |
while (my $socket = $listener->accept) { |
63 |
# do sth. with new socket |
64 |
} |
65 |
} |
66 |
|
67 |
DESCRIPTION |
68 |
This module allows you to create new processes, without actually forking |
69 |
them from your current process (avoiding the problems of forking), but |
70 |
preserving most of the advantages of fork. |
71 |
|
72 |
It can be used to create new worker processes or new independent |
73 |
subprocesses for short- and long-running jobs, process pools (e.g. for |
74 |
use in pre-forked servers) but also to spawn new external processes |
75 |
(such as CGI scripts from a webserver), which can be faster (and more |
76 |
well behaved) than using fork+exec in big processes. |
77 |
|
78 |
Special care has been taken to make this module useful from other |
79 |
modules, while still supporting specialised environments such as |
80 |
App::Staticperl or PAR::Packer. |
81 |
|
82 |
PROBLEM STATEMENT |
83 |
There are two ways to implement parallel processing on UNIX like |
84 |
operating systems - fork and process, and fork+exec and process. They |
85 |
have different advantages and disadvantages that I describe below, |
86 |
together with how this module tries to mitigate the disadvantages. |
87 |
|
88 |
Forking from a big process can be very slow (a 5GB process needs 0.05s |
89 |
to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead is |
90 |
often shared with exec (because you have to fork first), but in some |
91 |
circumstances (e.g. when vfork is used), fork+exec can be much faster. |
92 |
This module can help here by telling a small(er) helper process to |
93 |
fork, or fork+exec instead. |
94 |
|
95 |
Forking usually creates a copy-on-write copy of the parent process. |
96 |
Memory (for example, modules or data files that have been will not take |
97 |
additional memory). When exec'ing a new process, modules and data files |
98 |
might need to be loaded again, at extra cpu and memory cost. Likewise |
99 |
when forking, all data structures are copied as well - if the program |
100 |
frees them and replaces them by new data, the child processes will |
101 |
retain the memory even if it isn't used. |
102 |
This module allows the main program to do a controlled fork, and |
103 |
allows modules to exec processes safely at any time. When creating a |
104 |
custom process pool you can take advantage of data sharing via fork |
105 |
without risking to share large dynamic data structures that will |
106 |
blow up child memory usage. |
107 |
|
108 |
Exec'ing a new perl process might be difficult and slow. For example, it |
109 |
is not easy to find the correct path to the perl interpreter, and all |
110 |
modules have to be loaded from disk again. Long running processes might |
111 |
run into problems when perl is upgraded for example. |
112 |
This module supports creating pre-initialised perl processes to be |
113 |
used as template, and also tries hard to identify the correct path |
114 |
to the perl interpreter. With a cooperative main program, exec'ing |
115 |
the interpreter might not even be necessary. |
116 |
|
117 |
Forking might be impossible when a program is running. For example, |
118 |
POSIX makes it almost impossible to fork from a multithreaded program |
119 |
and do anything useful in the child - strictly speaking, if your perl |
120 |
program uses posix threads (even indirectly via e.g. IO::AIO or |
121 |
threads), you cannot call fork on the perl level anymore, at all. |
122 |
This module can safely fork helper processes at any time, by caling |
123 |
fork+exec in C, in a POSIX-compatible way. |
124 |
|
125 |
Parallel processing with fork might be inconvenient or difficult to |
126 |
implement. For example, when a program uses an event loop and creates |
127 |
watchers it becomes very hard to use the event loop from a child |
128 |
program, as the watchers already exist but are only meaningful in the |
129 |
parent. Worse, a module might want to use such a system, not knowing |
130 |
whether another module or the main program also does, leading to |
131 |
problems. |
132 |
This module only lets the main program create pools by forking |
133 |
(because only the main program can know when it is still safe to do |
134 |
so) - all other pools are created by fork+exec, after which such |
135 |
modules can again be loaded. |
136 |
|
137 |
CONCEPTS |
138 |
This module can create new processes either by executing a new perl |
139 |
process, or by forking from an existing "template" process. |
140 |
|
141 |
Each such process comes with its own file handle that can be used to |
142 |
communicate with it (it's actually a socket - one end in the new |
143 |
process, one end in the main process), and among the things you can do |
144 |
in it are load modules, fork new processes, send file handles to it, and |
145 |
execute functions. |
146 |
|
147 |
There are multiple ways to create additional processes to execute some |
148 |
jobs: |
149 |
|
150 |
fork a new process from the "default" template process, load code, run |
151 |
it |
152 |
This module has a "default" template process which it executes when |
153 |
it is needed the first time. Forking from this process shares the |
154 |
memory used for the perl interpreter with the new process, but |
155 |
loading modules takes time, and the memory is not shared with |
156 |
anything else. |
157 |
|
158 |
This is ideal for when you only need one extra process of a kind, |
159 |
with the option of starting and stipping it on demand. |
160 |
|
161 |
Example: |
162 |
|
163 |
AnyEvent::Fork |
164 |
->new |
165 |
->require ("Some::Module") |
166 |
->run ("Some::Module::run", sub { |
167 |
my ($fork_fh) = @_; |
168 |
}); |
169 |
|
170 |
fork a new template process, load code, then fork processes off of it |
171 |
and run the code |
172 |
When you need to have a bunch of processes that all execute the same |
173 |
(or very similar) tasks, then a good way is to create a new template |
174 |
process for them, loading all the modules you need, and then create |
175 |
your worker processes from this new template process. |
176 |
|
177 |
This way, all code (and data structures) that can be shared (e.g. |
178 |
the modules you loaded) is shared between the processes, and each |
179 |
new process consumes relatively little memory of its own. |
180 |
|
181 |
The disadvantage of this approach is that you need to create a |
182 |
template process for the sole purpose of forking new processes from |
183 |
it, but if you only need a fixed number of proceses you can create |
184 |
them, and then destroy the template process. |
185 |
|
186 |
Example: |
187 |
|
188 |
my $template = AnyEvent::Fork->new->require ("Some::Module"); |
189 |
|
190 |
for (1..10) { |
191 |
$template->fork->run ("Some::Module::run", sub { |
192 |
my ($fork_fh) = @_; |
193 |
}); |
194 |
} |
195 |
|
196 |
# at this point, you can keep $template around to fork new processes |
197 |
# later, or you can destroy it, which causes it to vanish. |
198 |
|
199 |
execute a new perl interpreter, load some code, run it |
200 |
This is relatively slow, and doesn't allow you to share memory |
201 |
between multiple processes. |
202 |
|
203 |
The only advantage is that you don't have to have a template process |
204 |
hanging around all the time to fork off some new processes, which |
205 |
might be an advantage when there are long time spans where no extra |
206 |
processes are needed. |
207 |
|
208 |
Example: |
209 |
|
210 |
AnyEvent::Fork |
211 |
->new_exec |
212 |
->require ("Some::Module") |
213 |
->run ("Some::Module::run", sub { |
214 |
my ($fork_fh) = @_; |
215 |
}); |
216 |
|
217 |
FUNCTIONS |
218 |
my $pool = new AnyEvent::Fork key => value... |
219 |
Create a new process pool. The following named parameters are |
220 |
supported: |
221 |
|
222 |
my $proc = new AnyEvent::Fork |
223 |
Create a new "empty" perl interpreter process and returns its |
224 |
process object for further manipulation. |
225 |
|
226 |
The new process is forked from a template process that is kept |
227 |
around for this purpose. When it doesn't exist yet, it is created by |
228 |
a call to "new_exec" and kept around for future calls. |
229 |
|
230 |
When the process object is destroyed, it will release the file |
231 |
handle that connects it with the new process. When the new process |
232 |
has not yet called "run", then the process will exit. Otherwise, |
233 |
what happens depends entirely on the code that is executed. |
234 |
|
235 |
$new_proc = $proc->fork |
236 |
Forks $proc, creating a new process, and returns the process object |
237 |
of the new process. |
238 |
|
239 |
If any of the "send_" functions have been called before fork, then |
240 |
they will be cloned in the child. For example, in a pre-forked |
241 |
server, you might "send_fh" the listening socket into the template |
242 |
process, and then keep calling "fork" and "run". |
243 |
|
244 |
my $proc = new_exec AnyEvent::Fork |
245 |
Create a new "empty" perl interpreter process and returns its |
246 |
process object for further manipulation. |
247 |
|
248 |
Unlike the "new" method, this method *always* spawns a new perl |
249 |
process (except in some cases, see AnyEvent::Fork::Early for |
250 |
details). This reduces the amount of memory sharing that is |
251 |
possible, and is also slower. |
252 |
|
253 |
You should use "new" whenever possible, except when having a |
254 |
template process around is unacceptable. |
255 |
|
256 |
The path to the perl interpreter is divined usign various methods - |
257 |
first $^X is investigated to see if the path ends with something |
258 |
that sounds as if it were the perl interpreter. Failing this, the |
259 |
module falls back to using $Config::Config{perlpath}. |
260 |
|
261 |
$proc = $proc->eval ($perlcode, @args) |
262 |
Evaluates the given $perlcode as ... perl code, while setting @_ to |
263 |
the strings specified by @args. |
264 |
|
265 |
This call is meant to do any custom initialisation that might be |
266 |
required (for example, the "require" method uses it). It's not |
267 |
supposed to be used to completely take over the process, use "run" |
268 |
for that. |
269 |
|
270 |
The code will usually be executed after this call returns, and there |
271 |
is no way to pass anything back to the calling process. Any |
272 |
evaluation errors will be reported to stderr and cause the process |
273 |
to exit. |
274 |
|
275 |
Returns the process object for easy chaining of method calls. |
276 |
|
277 |
$proc = $proc->require ($module, ...) |
278 |
Tries to load the given module(s) into the process |
279 |
|
280 |
Returns the process object for easy chaining of method calls. |
281 |
|
282 |
$proc = $proc->send_fh ($handle, ...) |
283 |
Send one or more file handles (*not* file descriptors) to the |
284 |
process, to prepare a call to "run". |
285 |
|
286 |
The process object keeps a reference to the handles until this is |
287 |
done, so you must not explicitly close the handles. This is most |
288 |
easily accomplished by simply not storing the file handles anywhere |
289 |
after passing them to this method. |
290 |
|
291 |
Returns the process object for easy chaining of method calls. |
292 |
|
293 |
Example: pass an fh to a process, and release it without closing. it |
294 |
will be closed automatically when it is no longer used. |
295 |
|
296 |
$proc->send_fh ($my_fh); |
297 |
undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT |
298 |
|
299 |
$proc = $proc->send_arg ($string, ...) |
300 |
Send one or more argument strings to the process, to prepare a call |
301 |
to "run". The strings can be any octet string. |
302 |
|
303 |
Returns the process object for easy chaining of emthod calls. |
304 |
|
305 |
$proc->run ($func, $cb->($fh)) |
306 |
Enter the function specified by the fully qualified name in $func in |
307 |
the process. The function is called with the communication socket as |
308 |
first argument, followed by all file handles and string arguments |
309 |
sent earlier via "send_fh" and "send_arg" methods, in the order they |
310 |
were called. |
311 |
|
312 |
If the called function returns, the process exits. |
313 |
|
314 |
Preparing the process can take time - when the process is ready, the |
315 |
callback is invoked with the local communications socket as |
316 |
argument. |
317 |
|
318 |
The process object becomes unusable on return from this function. |
319 |
|
320 |
If the communication socket isn't used, it should be closed on both |
321 |
sides, to save on kernel memory. |
322 |
|
323 |
The socket is non-blocking in the parent, and blocking in the newly |
324 |
created process. The close-on-exec flag is set on both. Even if not |
325 |
used otherwise, the socket can be a good indicator for the existance |
326 |
of the process - if the other process exits, you get a readable |
327 |
event on it, because exiting the process closes the socket (if it |
328 |
didn't create any children using fork). |
329 |
|
330 |
Example: create a template for a process pool, pass a few strings, |
331 |
some file handles, then fork, pass one more string, and run some |
332 |
code. |
333 |
|
334 |
my $pool = AnyEvent::Fork |
335 |
->new |
336 |
->send_arg ("str1", "str2") |
337 |
->send_fh ($fh1, $fh2); |
338 |
|
339 |
for (1..2) { |
340 |
$pool |
341 |
->fork |
342 |
->send_arg ("str3") |
343 |
->run ("Some::function", sub { |
344 |
my ($fh) = @_; |
345 |
|
346 |
# fh is nonblocking, but we trust that the OS can accept these |
347 |
# extra 3 octets anyway. |
348 |
syswrite $fh, "hi #$_\n"; |
349 |
|
350 |
# $fh is being closed here, as we don't store it anywhere |
351 |
}); |
352 |
} |
353 |
|
354 |
# Some::function might look like this - all parameters passed before fork |
355 |
# and after will be passed, in order, after the communications socket. |
356 |
sub Some::function { |
357 |
my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; |
358 |
|
359 |
print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" |
360 |
} |
361 |
|
362 |
PORTABILITY NOTES |
363 |
Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a |
364 |
nop, and ::Template is not going to work), and it cost a lot of blood |
365 |
and sweat to make it so, mostly due to the bloody broken perl that |
366 |
nobody seems to care about. The fork emulation is a bad joke - I have |
367 |
yet to see something useful that you cna do with it without running into |
368 |
memory corruption issues or other braindamage. Hrrrr. |
369 |
|
370 |
Cygwin perl is not supported at the moment, as it should implement fd |
371 |
passing, but doesn't, and rolling my own is hard, as cygwin doesn't |
372 |
support enough functionality to do it. |
373 |
|
374 |
AUTHOR |
375 |
Marc Lehmann <schmorp@schmorp.de> |
376 |
http://home.schmorp.de/ |
377 |
|