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Revision 1.10 by root, Thu Apr 4 06:09:15 2013 UTC vs.
Revision 1.28 by root, Sat Apr 6 09:06:43 2013 UTC

1=head1 NAME 1=head1 NAME
2 2
3AnyEvent::Fork - everything you wanted to use fork() for, but couldn't 3AnyEvent::Fork - everything you wanted to use fork() for, but couldn't
4 4
5ATTENTION, this is a very early release, and very untested. Consider it a
6technology preview.
7
8=head1 SYNOPSIS 5=head1 SYNOPSIS
9 6
10 use AnyEvent::Fork; 7 use AnyEvent::Fork;
11 8
12 ################################################################## 9 AnyEvent::Fork
10 ->new
11 ->require ("MyModule")
12 ->run ("MyModule::server", my $cv = AE::cv);
13
14 my $fh = $cv->recv;
15
16=head1 DESCRIPTION
17
18This module allows you to create new processes, without actually forking
19them from your current process (avoiding the problems of forking), but
20preserving most of the advantages of fork.
21
22It can be used to create new worker processes or new independent
23subprocesses for short- and long-running jobs, process pools (e.g. for use
24in pre-forked servers) but also to spawn new external processes (such as
25CGI scripts from a web server), which can be faster (and more well behaved)
26than using fork+exec in big processes.
27
28Special care has been taken to make this module useful from other modules,
29while still supporting specialised environments such as L<App::Staticperl>
30or L<PAR::Packer>.
31
32=head1 WHAT THIS MODULE IS NOT
33
34This module only creates processes and lets you pass file handles and
35strings to it, and run perl code. It does not implement any kind of RPC -
36there is no back channel from the process back to you, and there is no RPC
37or message passing going on.
38
39If you need some form of RPC, you can either implement it yourself
40in whatever way you like, use some message-passing module such
41as L<AnyEvent::MP>, some pipe such as L<AnyEvent::ZeroMQ>, use
42L<AnyEvent::Handle> on both sides to send e.g. JSON or Storable messages,
43and so on.
44
45=head1 PROBLEM STATEMENT
46
47There are two traditional ways to implement parallel processing on UNIX
48like operating systems - fork and process, and fork+exec and process. They
49have different advantages and disadvantages that I describe below,
50together with how this module tries to mitigate the disadvantages.
51
52=over 4
53
54=item Forking from a big process can be very slow.
55
56A 5GB process needs 0.05s to fork on my 3.6GHz amd64 GNU/Linux box. This
57overhead is often shared with exec (because you have to fork first), but
58in some circumstances (e.g. when vfork is used), fork+exec can be much
59faster.
60
61This module can help here by telling a small(er) helper process to fork,
62which is faster then forking the main process, and also uses vfork where
63possible. This gives the speed of vfork, with the flexibility of fork.
64
65=item Forking usually creates a copy-on-write copy of the parent
66process.
67
68For example, modules or data files that are loaded will not use additional
69memory after a fork. When exec'ing a new process, modules and data files
70might need to be loaded again, at extra CPU and memory cost. But when
71forking, literally all data structures are copied - if the program frees
72them and replaces them by new data, the child processes will retain the
73old version even if it isn't used, which can suddenly and unexpectedly
74increase memory usage when freeing memory.
75
76The trade-off is between more sharing with fork (which can be good or
77bad), and no sharing with exec.
78
79This module allows the main program to do a controlled fork, and allows
80modules to exec processes safely at any time. When creating a custom
81process pool you can take advantage of data sharing via fork without
82risking to share large dynamic data structures that will blow up child
83memory usage.
84
85In other words, this module puts you into control over what is being
86shared and what isn't, at all times.
87
88=item Exec'ing a new perl process might be difficult.
89
90For example, it is not easy to find the correct path to the perl
91interpreter - C<$^X> might not be a perl interpreter at all.
92
93This module tries hard to identify the correct path to the perl
94interpreter. With a cooperative main program, exec'ing the interpreter
95might not even be necessary, but even without help from the main program,
96it will still work when used from a module.
97
98=item Exec'ing a new perl process might be slow, as all necessary modules
99have to be loaded from disk again, with no guarantees of success.
100
101Long running processes might run into problems when perl is upgraded
102and modules are no longer loadable because they refer to a different
103perl version, or parts of a distribution are newer than the ones already
104loaded.
105
106This module supports creating pre-initialised perl processes to be used as
107a template for new processes.
108
109=item Forking might be impossible when a program is running.
110
111For example, POSIX makes it almost impossible to fork from a
112multi-threaded program while doing anything useful in the child - in
113fact, if your perl program uses POSIX threads (even indirectly via
114e.g. L<IO::AIO> or L<threads>), you cannot call fork on the perl level
115anymore without risking corruption issues on a number of operating
116systems.
117
118This module can safely fork helper processes at any time, by calling
119fork+exec in C, in a POSIX-compatible way (via L<Proc::FastSpawn>).
120
121=item Parallel processing with fork might be inconvenient or difficult
122to implement. Modules might not work in both parent and child.
123
124For example, when a program uses an event loop and creates watchers it
125becomes very hard to use the event loop from a child program, as the
126watchers already exist but are only meaningful in the parent. Worse, a
127module might want to use such a module, not knowing whether another module
128or the main program also does, leading to problems.
129
130Apart from event loops, graphical toolkits also commonly fall into the
131"unsafe module" category, or just about anything that communicates with
132the external world, such as network libraries and file I/O modules, which
133usually don't like being copied and then allowed to continue in two
134processes.
135
136With this module only the main program is allowed to create new processes
137by forking (because only the main program can know when it is still safe
138to do so) - all other processes are created via fork+exec, which makes it
139possible to use modules such as event loops or window interfaces safely.
140
141=back
142
143=head1 EXAMPLES
144
13 # create a single new process, tell it to run your worker function 145=head2 Create a single new process, tell it to run your worker function.
14 146
15 AnyEvent::Fork 147 AnyEvent::Fork
16 ->new 148 ->new
17 ->require ("MyModule") 149 ->require ("MyModule")
18 ->run ("MyModule::worker, sub { 150 ->run ("MyModule::worker, sub {
… …
28 160
29 # now $slave_filehandle is connected to the $master_filehandle 161 # now $slave_filehandle is connected to the $master_filehandle
30 # in the original prorcess. have fun! 162 # in the original prorcess. have fun!
31 } 163 }
32 164
33 ##################################################################
34 # create a pool of server processes all accepting on the same socket 165=head2 Create a pool of server processes all accepting on the same socket.
35 166
36 # create listener socket 167 # create listener socket
37 my $listener = ...; 168 my $listener = ...;
38 169
39 # create a pool template, initialise it and give it the socket 170 # create a pool template, initialise it and give it the socket
… …
64 while (my $socket = $listener->accept) { 195 while (my $socket = $listener->accept) {
65 # do sth. with new socket 196 # do sth. with new socket
66 } 197 }
67 } 198 }
68 199
69=head1 DESCRIPTION 200=head2 use AnyEvent::Fork as a faster fork+exec
70 201
71This module allows you to create new processes, without actually forking 202This runs /bin/echo hi, with stdout redirected to /tmp/log and stderr to
72them from your current process (avoiding the problems of forking), but 203the communications socket. It is usually faster than fork+exec, but still
73preserving most of the advantages of fork. 204let's you prepare the environment.
74 205
75It can be used to create new worker processes or new independent 206 open my $output, ">/tmp/log" or die "$!";
76subprocesses for short- and long-running jobs, process pools (e.g. for use
77in pre-forked servers) but also to spawn new external processes (such as
78CGI scripts from a webserver), which can be faster (and more well behaved)
79than using fork+exec in big processes.
80 207
81Special care has been taken to make this module useful from other modules, 208 AnyEvent::Fork
82while still supporting specialised environments such as L<App::Staticperl> 209 ->new
83or L<PAR::Packer>. 210 ->eval ('
211 sub run {
212 my ($fh, $output, @cmd) = @_;
84 213
85=head1 PROBLEM STATEMENT 214 # perl will clear close-on-exec on STDOUT/STDERR
215 open STDOUT, ">&", $output or die;
216 open STDERR, ">&", $fh or die;
86 217
87There are two ways to implement parallel processing on UNIX like operating 218 exec @cmd;
88systems - fork and process, and fork+exec and process. They have different 219 }
89advantages and disadvantages that I describe below, together with how this 220 ')
90module tries to mitigate the disadvantages. 221 ->send_fh ($output)
222 ->send_arg ("/bin/echo", "hi")
223 ->run ("run", my $cv = AE::cv);
91 224
92=over 4 225 my $stderr = $cv->recv;
93
94=item Forking from a big process can be very slow (a 5GB process needs
950.05s to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead
96is often shared with exec (because you have to fork first), but in some
97circumstances (e.g. when vfork is used), fork+exec can be much faster.
98
99This module can help here by telling a small(er) helper process to fork,
100or fork+exec instead.
101
102=item Forking usually creates a copy-on-write copy of the parent
103process. Memory (for example, modules or data files that have been
104will not take additional memory). When exec'ing a new process, modules
105and data files might need to be loaded again, at extra cpu and memory
106cost. Likewise when forking, all data structures are copied as well - if
107the program frees them and replaces them by new data, the child processes
108will retain the memory even if it isn't used.
109
110This module allows the main program to do a controlled fork, and allows
111modules to exec processes safely at any time. When creating a custom
112process pool you can take advantage of data sharing via fork without
113risking to share large dynamic data structures that will blow up child
114memory usage.
115
116=item Exec'ing a new perl process might be difficult and slow. For
117example, it is not easy to find the correct path to the perl interpreter,
118and all modules have to be loaded from disk again. Long running processes
119might run into problems when perl is upgraded for example.
120
121This module supports creating pre-initialised perl processes to be used
122as template, and also tries hard to identify the correct path to the perl
123interpreter. With a cooperative main program, exec'ing the interpreter
124might not even be necessary.
125
126=item Forking might be impossible when a program is running. For example,
127POSIX makes it almost impossible to fork from a multithreaded program and
128do anything useful in the child - strictly speaking, if your perl program
129uses posix threads (even indirectly via e.g. L<IO::AIO> or L<threads>),
130you cannot call fork on the perl level anymore, at all.
131
132This module can safely fork helper processes at any time, by caling
133fork+exec in C, in a POSIX-compatible way.
134
135=item Parallel processing with fork might be inconvenient or difficult
136to implement. For example, when a program uses an event loop and creates
137watchers it becomes very hard to use the event loop from a child
138program, as the watchers already exist but are only meaningful in the
139parent. Worse, a module might want to use such a system, not knowing
140whether another module or the main program also does, leading to problems.
141
142This module only lets the main program create pools by forking (because
143only the main program can know when it is still safe to do so) - all other
144pools are created by fork+exec, after which such modules can again be
145loaded.
146
147=back
148 226
149=head1 CONCEPTS 227=head1 CONCEPTS
150 228
151This module can create new processes either by executing a new perl 229This module can create new processes either by executing a new perl
152process, or by forking from an existing "template" process. 230process, or by forking from an existing "template" process.
… …
169needed the first time. Forking from this process shares the memory used 247needed the first time. Forking from this process shares the memory used
170for the perl interpreter with the new process, but loading modules takes 248for the perl interpreter with the new process, but loading modules takes
171time, and the memory is not shared with anything else. 249time, and the memory is not shared with anything else.
172 250
173This is ideal for when you only need one extra process of a kind, with the 251This is ideal for when you only need one extra process of a kind, with the
174option of starting and stipping it on demand. 252option of starting and stopping it on demand.
175 253
176Example: 254Example:
177 255
178 AnyEvent::Fork 256 AnyEvent::Fork
179 ->new 257 ->new
… …
194modules you loaded) is shared between the processes, and each new process 272modules you loaded) is shared between the processes, and each new process
195consumes relatively little memory of its own. 273consumes relatively little memory of its own.
196 274
197The disadvantage of this approach is that you need to create a template 275The disadvantage of this approach is that you need to create a template
198process for the sole purpose of forking new processes from it, but if you 276process for the sole purpose of forking new processes from it, but if you
199only need a fixed number of proceses you can create them, and then destroy 277only need a fixed number of processes you can create them, and then destroy
200the template process. 278the template process.
201 279
202Example: 280Example:
203 281
204 my $template = AnyEvent::Fork->new->require ("Some::Module"); 282 my $template = AnyEvent::Fork->new->require ("Some::Module");
… …
231 my ($fork_fh) = @_; 309 my ($fork_fh) = @_;
232 }); 310 });
233 311
234=back 312=back
235 313
236=head1 FUNCTIONS 314=head1 THE C<AnyEvent::Fork> CLASS
315
316This module exports nothing, and only implements a single class -
317C<AnyEvent::Fork>.
318
319There are two class constructors that both create new processes - C<new>
320and C<new_exec>. The C<fork> method creates a new process by forking an
321existing one and could be considered a third constructor.
322
323Most of the remaining methods deal with preparing the new process, by
324loading code, evaluating code and sending data to the new process. They
325usually return the process object, so you can chain method calls.
326
327If a process object is destroyed before calling its C<run> method, then
328the process simply exits. After C<run> is called, all responsibility is
329passed to the specified function.
237 330
238=over 4 331=over 4
239 332
240=cut 333=cut
241 334
242package AnyEvent::Fork; 335package AnyEvent::Fork;
243 336
244use common::sense; 337use common::sense;
245 338
246use Socket (); 339use Errno ();
247 340
248use AnyEvent; 341use AnyEvent;
249use AnyEvent::Fork::Util;
250use AnyEvent::Util (); 342use AnyEvent::Util ();
251 343
344use IO::FDPass;
345
346our $VERSION = 0.5;
347
252our $PERL; # the path to the perl interpreter, deduces with various forms of magic 348our $PERL; # the path to the perl interpreter, deduces with various forms of magic
253
254=item my $pool = new AnyEvent::Fork key => value...
255
256Create a new process pool. The following named parameters are supported:
257 349
258=over 4 350=over 4
259 351
260=back 352=back
261 353
… …
268our $TEMPLATE; 360our $TEMPLATE;
269 361
270sub _cmd { 362sub _cmd {
271 my $self = shift; 363 my $self = shift;
272 364
273 #TODO: maybe append the packet to any existing string command already in the queue
274
275 # ideally, we would want to use "a (w/a)*" as format string, but perl versions 365 # ideally, we would want to use "a (w/a)*" as format string, but perl
276 # from at least 5.8.9 to 5.16.3 are all buggy and can't unpack it. 366 # versions from at least 5.8.9 to 5.16.3 are all buggy and can't unpack
277 push @{ $self->[2] }, pack "N/a", pack "(w/a)*", @_; 367 # it.
368 push @{ $self->[2] }, pack "a L/a*", $_[0], $_[1];
278 369
279 $self->[3] ||= AE::io $self->[1], 1, sub { 370 $self->[3] ||= AE::io $self->[1], 1, sub {
371 do {
280 # send the next "thing" in the queue - either a reference to an fh, 372 # send the next "thing" in the queue - either a reference to an fh,
281 # or a plain string. 373 # or a plain string.
282 374
283 if (ref $self->[2][0]) { 375 if (ref $self->[2][0]) {
284 # send fh 376 # send fh
285 AnyEvent::Fork::Util::fd_send fileno $self->[1], fileno ${ $self->[2][0] } 377 unless (IO::FDPass::send fileno $self->[1], fileno ${ $self->[2][0] }) {
378 return if $! == Errno::EAGAIN || $! == Errno::EWOULDBLOCK;
379 undef $self->[3];
380 die "AnyEvent::Fork: file descriptor send failure: $!";
381 }
382
286 and shift @{ $self->[2] }; 383 shift @{ $self->[2] };
287 384
288 } else { 385 } else {
289 # send string 386 # send string
290 my $len = syswrite $self->[1], $self->[2][0] 387 my $len = syswrite $self->[1], $self->[2][0];
388
389 unless ($len) {
390 return if $! == Errno::EAGAIN || $! == Errno::EWOULDBLOCK;
391 undef $self->[3];
291 or do { undef $self->[3]; die "AnyEvent::Fork: command write failure: $!" }; 392 die "AnyEvent::Fork: command write failure: $!";
393 }
292 394
293 substr $self->[2][0], 0, $len, ""; 395 substr $self->[2][0], 0, $len, "";
294 shift @{ $self->[2] } unless length $self->[2][0]; 396 shift @{ $self->[2] } unless length $self->[2][0];
295 } 397 }
398 } while @{ $self->[2] };
296 399
297 unless (@{ $self->[2] }) { 400 # everything written
298 undef $self->[3]; 401 undef $self->[3];
402
299 # invoke run callback 403 # invoke run callback, if any
300 $self->[0]->($self->[1]) if $self->[0]; 404 $self->[4]->($self->[1]) if $self->[4];
301 }
302 }; 405 };
406
407 () # make sure we don't leak the watcher
303} 408}
304 409
305sub _new { 410sub _new {
306 my ($self, $fh) = @_; 411 my ($self, $fh, $pid) = @_;
307 412
308 AnyEvent::Util::fh_nonblocking $fh, 1; 413 AnyEvent::Util::fh_nonblocking $fh, 1;
309 414
310 $self = bless [ 415 $self = bless [
311 undef, # run callback 416 $pid,
312 $fh, 417 $fh,
313 [], # write queue - strings or fd's 418 [], # write queue - strings or fd's
314 undef, # AE watcher 419 undef, # AE watcher
315 ], $self; 420 ], $self;
316 421
… …
327 if ($pid eq 0) { 432 if ($pid eq 0) {
328 require AnyEvent::Fork::Serve; 433 require AnyEvent::Fork::Serve;
329 $AnyEvent::Fork::Serve::OWNER = $parent; 434 $AnyEvent::Fork::Serve::OWNER = $parent;
330 close $fh; 435 close $fh;
331 $0 = "$_[1] of $parent"; 436 $0 = "$_[1] of $parent";
437 $SIG{CHLD} = 'IGNORE';
332 AnyEvent::Fork::Serve::serve ($slave); 438 AnyEvent::Fork::Serve::serve ($slave);
333 AnyEvent::Fork::Util::_exit 0; 439 exit 0;
334 } elsif (!$pid) { 440 } elsif (!$pid) {
335 die "AnyEvent::Fork::Early/Template: unable to fork template process: $!"; 441 die "AnyEvent::Fork::Early/Template: unable to fork template process: $!";
336 } 442 }
337 443
338 AnyEvent::Fork->_new ($fh) 444 AnyEvent::Fork->_new ($fh, $pid)
339} 445}
340 446
341=item my $proc = new AnyEvent::Fork 447=item my $proc = new AnyEvent::Fork
342 448
343Create a new "empty" perl interpreter process and returns its process 449Create a new "empty" perl interpreter process and returns its process
… …
394reduces the amount of memory sharing that is possible, and is also slower. 500reduces the amount of memory sharing that is possible, and is also slower.
395 501
396You should use C<new> whenever possible, except when having a template 502You should use C<new> whenever possible, except when having a template
397process around is unacceptable. 503process around is unacceptable.
398 504
399The path to the perl interpreter is divined usign various methods - first 505The path to the perl interpreter is divined using various methods - first
400C<$^X> is investigated to see if the path ends with something that sounds 506C<$^X> is investigated to see if the path ends with something that sounds
401as if it were the perl interpreter. Failing this, the module falls back to 507as if it were the perl interpreter. Failing this, the module falls back to
402using C<$Config::Config{perlpath}>. 508using C<$Config::Config{perlpath}>.
403 509
404=cut 510=cut
… …
413 my $perl = $; 519 my $perl = $;
414 520
415 # first we try $^X, but the path must be absolute (always on win32), and end in sth. 521 # first we try $^X, but the path must be absolute (always on win32), and end in sth.
416 # that looks like perl. this obviously only works for posix and win32 522 # that looks like perl. this obviously only works for posix and win32
417 unless ( 523 unless (
418 (AnyEvent::Fork::Util::WIN32 || $perl =~ m%^/%) 524 ($^O eq "MSWin32" || $perl =~ m%^/%)
419 && $perl =~ m%[/\\]perl(?:[0-9]+(\.[0-9]+)+)?(\.exe)?$%i 525 && $perl =~ m%[/\\]perl(?:[0-9]+(\.[0-9]+)+)?(\.exe)?$%i
420 ) { 526 ) {
421 # if it doesn't look perlish enough, try Config 527 # if it doesn't look perlish enough, try Config
422 require Config; 528 require Config;
423 $perl = $Config::Config{perlpath}; 529 $perl = $Config::Config{perlpath};
… …
434 Proc::FastSpawn::fd_inherit (fileno $fh, 0); 540 Proc::FastSpawn::fd_inherit (fileno $fh, 0);
435 541
436 # quick. also doesn't work in win32. of course. what did you expect 542 # quick. also doesn't work in win32. of course. what did you expect
437 #local $ENV{PERL5LIB} = join ":", grep !ref, @INC; 543 #local $ENV{PERL5LIB} = join ":", grep !ref, @INC;
438 my %env = %ENV; 544 my %env = %ENV;
439 $env{PERL5LIB} = join +(AnyEvent::Fork::Util::WIN32 ? ";" : ":"), grep !ref, @INC; 545 $env{PERL5LIB} = join +($^O eq "MSWin32" ? ";" : ":"), grep !ref, @INC;
440 546
441 Proc::FastSpawn::spawn ( 547 my $pid = Proc::FastSpawn::spawn (
442 $perl, 548 $perl,
443 ["perl", "-MAnyEvent::Fork::Serve", "-e", "AnyEvent::Fork::Serve::me", fileno $slave, $$], 549 ["perl", "-MAnyEvent::Fork::Serve", "-e", "AnyEvent::Fork::Serve::me", fileno $slave, $$],
444 [map "$_=$env{$_}", keys %env], 550 [map "$_=$env{$_}", keys %env],
445 ) or die "unable to spawn AnyEvent::Fork server: $!"; 551 ) or die "unable to spawn AnyEvent::Fork server: $!";
446 552
447 $self->_new ($fh) 553 $self->_new ($fh, $pid)
554}
555
556=item $pid = $proc->pid
557
558Returns the process id of the process I<iff it is a direct child of the
559process> running AnyEvent::Fork, and C<undef> otherwise.
560
561Normally, only processes created via C<< AnyEvent::Fork->new_exec >> and
562L<AnyEvent::Fork::Template> are direct children, and you are responsible
563to clean up their zombies when they die.
564
565All other processes are not direct children, and will be cleaned up by
566AnyEvent::Fork.
567
568=cut
569
570sub pid {
571 $_[0][0]
448} 572}
449 573
450=item $proc = $proc->eval ($perlcode, @args) 574=item $proc = $proc->eval ($perlcode, @args)
451 575
452Evaluates the given C<$perlcode> as ... perl code, while setting C<@_> to 576Evaluates the given C<$perlcode> as ... perl code, while setting C<@_> to
453the strings specified by C<@args>. 577the strings specified by C<@args>, in the "main" package.
454 578
455This call is meant to do any custom initialisation that might be required 579This call is meant to do any custom initialisation that might be required
456(for example, the C<require> method uses it). It's not supposed to be used 580(for example, the C<require> method uses it). It's not supposed to be used
457to completely take over the process, use C<run> for that. 581to completely take over the process, use C<run> for that.
458 582
459The code will usually be executed after this call returns, and there is no 583The code will usually be executed after this call returns, and there is no
460way to pass anything back to the calling process. Any evaluation errors 584way to pass anything back to the calling process. Any evaluation errors
461will be reported to stderr and cause the process to exit. 585will be reported to stderr and cause the process to exit.
462 586
587If you want to execute some code to take over the process (see the
588"fork+exec" example in the SYNOPSIS), you should compile a function via
589C<eval> first, and then call it via C<run>. This also gives you access to
590any arguments passed via the C<send_xxx> methods, such as file handles.
591
463Returns the process object for easy chaining of method calls. 592Returns the process object for easy chaining of method calls.
464 593
465=cut 594=cut
466 595
467sub eval { 596sub eval {
468 my ($self, $code, @args) = @_; 597 my ($self, $code, @args) = @_;
469 598
470 $self->_cmd (e => $code, @args); 599 $self->_cmd (e => pack "(w/a*)*", $code, @args);
471 600
472 $self 601 $self
473} 602}
474 603
475=item $proc = $proc->require ($module, ...) 604=item $proc = $proc->require ($module, ...)
… …
499accomplished by simply not storing the file handles anywhere after passing 628accomplished by simply not storing the file handles anywhere after passing
500them to this method. 629them to this method.
501 630
502Returns the process object for easy chaining of method calls. 631Returns the process object for easy chaining of method calls.
503 632
504Example: pass an fh to a process, and release it without closing. it will 633Example: pass a file handle to a process, and release it without
505be closed automatically when it is no longer used. 634closing. It will be closed automatically when it is no longer used.
506 635
507 $proc->send_fh ($my_fh); 636 $proc->send_fh ($my_fh);
508 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT 637 undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT
509 638
510=cut 639=cut
… …
523=item $proc = $proc->send_arg ($string, ...) 652=item $proc = $proc->send_arg ($string, ...)
524 653
525Send one or more argument strings to the process, to prepare a call to 654Send one or more argument strings to the process, to prepare a call to
526C<run>. The strings can be any octet string. 655C<run>. The strings can be any octet string.
527 656
657The protocol is optimised to pass a moderate number of relatively short
658strings - while you can pass up to 4GB of data in one go, this is more
659meant to pass some ID information or other startup info, not big chunks of
660data.
661
528Returns the process object for easy chaining of emthod calls. 662Returns the process object for easy chaining of method calls.
529 663
530=cut 664=cut
531 665
532sub send_arg { 666sub send_arg {
533 my ($self, @arg) = @_; 667 my ($self, @arg) = @_;
534 668
535 $self->_cmd (a => @arg); 669 $self->_cmd (a => pack "(w/a*)*", @arg);
536 670
537 $self 671 $self
538} 672}
539 673
540=item $proc->run ($func, $cb->($fh)) 674=item $proc->run ($func, $cb->($fh))
541 675
542Enter the function specified by the fully qualified name in C<$func> in 676Enter the function specified by the function name in C<$func> in the
543the process. The function is called with the communication socket as first 677process. The function is called with the communication socket as first
544argument, followed by all file handles and string arguments sent earlier 678argument, followed by all file handles and string arguments sent earlier
545via C<send_fh> and C<send_arg> methods, in the order they were called. 679via C<send_fh> and C<send_arg> methods, in the order they were called.
546 680
547If the called function returns, the process exits. 681The function name should be fully qualified, but if it isn't, it will be
682looked up in the main package.
548 683
549Preparing the process can take time - when the process is ready, the 684If the called function returns, doesn't exist, or any error occurs, the
685process exits.
686
687Preparing the process is done in the background - when all commands have
550callback is invoked with the local communications socket as argument. 688been sent, the callback is invoked with the local communications socket
689as argument. At this point you can start using the socket in any way you
690like.
551 691
552The process object becomes unusable on return from this function. 692The process object becomes unusable on return from this function - any
693further method calls result in undefined behaviour.
553 694
554If the communication socket isn't used, it should be closed on both sides, 695If the communication socket isn't used, it should be closed on both sides,
555to save on kernel memory. 696to save on kernel memory.
556 697
557The socket is non-blocking in the parent, and blocking in the newly 698The socket is non-blocking in the parent, and blocking in the newly
558created process. The close-on-exec flag is set on both. Even if not used 699created process. The close-on-exec flag is set in both.
700
559otherwise, the socket can be a good indicator for the existance of the 701Even if not used otherwise, the socket can be a good indicator for the
560process - if the other process exits, you get a readable event on it, 702existence of the process - if the other process exits, you get a readable
561because exiting the process closes the socket (if it didn't create any 703event on it, because exiting the process closes the socket (if it didn't
562children using fork). 704create any children using fork).
563 705
564Example: create a template for a process pool, pass a few strings, some 706Example: create a template for a process pool, pass a few strings, some
565file handles, then fork, pass one more string, and run some code. 707file handles, then fork, pass one more string, and run some code.
566 708
567 my $pool = AnyEvent::Fork 709 my $pool = AnyEvent::Fork
… …
575 ->send_arg ("str3") 717 ->send_arg ("str3")
576 ->run ("Some::function", sub { 718 ->run ("Some::function", sub {
577 my ($fh) = @_; 719 my ($fh) = @_;
578 720
579 # fh is nonblocking, but we trust that the OS can accept these 721 # fh is nonblocking, but we trust that the OS can accept these
580 # extra 3 octets anyway. 722 # few octets anyway.
581 syswrite $fh, "hi #$_\n"; 723 syswrite $fh, "hi #$_\n";
582 724
583 # $fh is being closed here, as we don't store it anywhere 725 # $fh is being closed here, as we don't store it anywhere
584 }); 726 });
585 } 727 }
… …
587 # Some::function might look like this - all parameters passed before fork 729 # Some::function might look like this - all parameters passed before fork
588 # and after will be passed, in order, after the communications socket. 730 # and after will be passed, in order, after the communications socket.
589 sub Some::function { 731 sub Some::function {
590 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; 732 my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_;
591 733
592 print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" 734 print scalar <$fh>; # prints "hi #1\n" and "hi #2\n" in any order
593 } 735 }
594 736
595=cut 737=cut
596 738
597sub run { 739sub run {
598 my ($self, $func, $cb) = @_; 740 my ($self, $func, $cb) = @_;
599 741
600 $self->[0] = $cb; 742 $self->[4] = $cb;
601 $self->_cmd (r => $func); 743 $self->_cmd (r => $func);
602} 744}
745
746=back
747
748=head1 PERFORMANCE
749
750Now for some unscientific benchmark numbers (all done on an amd64
751GNU/Linux box). These are intended to give you an idea of the relative
752performance you can expect, they are not meant to be absolute performance
753numbers.
754
755OK, so, I ran a simple benchmark that creates a socket pair, forks, calls
756exit in the child and waits for the socket to close in the parent. I did
757load AnyEvent, EV and AnyEvent::Fork, for a total process size of 5100kB.
758
759 2079 new processes per second, using manual socketpair + fork
760
761Then I did the same thing, but instead of calling fork, I called
762AnyEvent::Fork->new->run ("CORE::exit") and then again waited for the
763socket form the child to close on exit. This does the same thing as manual
764socket pair + fork, except that what is forked is the template process
765(2440kB), and the socket needs to be passed to the server at the other end
766of the socket first.
767
768 2307 new processes per second, using AnyEvent::Fork->new
769
770And finally, using C<new_exec> instead C<new>, using vforks+execs to exec
771a new perl interpreter and compile the small server each time, I get:
772
773 479 vfork+execs per second, using AnyEvent::Fork->new_exec
774
775So how can C<< AnyEvent->new >> be faster than a standard fork, even
776though it uses the same operations, but adds a lot of overhead?
777
778The difference is simply the process size: forking the 6MB process takes
779so much longer than forking the 2.5MB template process that the overhead
780introduced is canceled out.
781
782If the benchmark process grows, the normal fork becomes even slower:
783
784 1340 new processes, manual fork in a 20MB process
785 731 new processes, manual fork in a 200MB process
786 235 new processes, manual fork in a 2000MB process
787
788What that means (to me) is that I can use this module without having a
789very bad conscience because of the extra overhead required to start new
790processes.
791
792=head1 TYPICAL PROBLEMS
793
794This section lists typical problems that remain. I hope by recognising
795them, most can be avoided.
796
797=over 4
798
799=item "leaked" file descriptors for exec'ed processes
800
801POSIX systems inherit file descriptors by default when exec'ing a new
802process. While perl itself laudably sets the close-on-exec flags on new
803file handles, most C libraries don't care, and even if all cared, it's
804often not possible to set the flag in a race-free manner.
805
806That means some file descriptors can leak through. And since it isn't
807possible to know which file descriptors are "good" and "necessary" (or
808even to know which file descriptors are open), there is no good way to
809close the ones that might harm.
810
811As an example of what "harm" can be done consider a web server that
812accepts connections and afterwards some module uses AnyEvent::Fork for the
813first time, causing it to fork and exec a new process, which might inherit
814the network socket. When the server closes the socket, it is still open
815in the child (which doesn't even know that) and the client might conclude
816that the connection is still fine.
817
818For the main program, there are multiple remedies available -
819L<AnyEvent::Fork::Early> is one, creating a process early and not using
820C<new_exec> is another, as in both cases, the first process can be exec'ed
821well before many random file descriptors are open.
822
823In general, the solution for these kind of problems is to fix the
824libraries or the code that leaks those file descriptors.
825
826Fortunately, most of these leaked descriptors do no harm, other than
827sitting on some resources.
828
829=item "leaked" file descriptors for fork'ed processes
830
831Normally, L<AnyEvent::Fork> does start new processes by exec'ing them,
832which closes file descriptors not marked for being inherited.
833
834However, L<AnyEvent::Fork::Early> and L<AnyEvent::Fork::Template> offer
835a way to create these processes by forking, and this leaks more file
836descriptors than exec'ing them, as there is no way to mark descriptors as
837"close on fork".
838
839An example would be modules like L<EV>, L<IO::AIO> or L<Gtk2>. Both create
840pipes for internal uses, and L<Gtk2> might open a connection to the X
841server. L<EV> and L<IO::AIO> can deal with fork, but Gtk2 might have
842trouble with a fork.
843
844The solution is to either not load these modules before use'ing
845L<AnyEvent::Fork::Early> or L<AnyEvent::Fork::Template>, or to delay
846initialising them, for example, by calling C<init Gtk2> manually.
847
848=item exit runs destructors
849
850This only applies to users of Lc<AnyEvent::Fork:Early> and
851L<AnyEvent::Fork::Template>.
852
853When a process created by AnyEvent::Fork exits, it might do so by calling
854exit, or simply letting perl reach the end of the program. At which point
855Perl runs all destructors.
856
857Not all destructors are fork-safe - for example, an object that represents
858the connection to an X display might tell the X server to free resources,
859which is inconvenient when the "real" object in the parent still needs to
860use them.
861
862This is obviously not a problem for L<AnyEvent::Fork::Early>, as you used
863it as the very first thing, right?
864
865It is a problem for L<AnyEvent::Fork::Template> though - and the solution
866is to not create objects with nontrivial destructors that might have an
867effect outside of Perl.
603 868
604=back 869=back
605 870
606=head1 PORTABILITY NOTES 871=head1 PORTABILITY NOTES
607 872
608Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a nop, 873Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a nop,
609and ::Template is not going to work), and it cost a lot of blood and sweat 874and ::Template is not going to work), and it cost a lot of blood and sweat
610to make it so, mostly due to the bloody broken perl that nobody seems to 875to make it so, mostly due to the bloody broken perl that nobody seems to
611care about. The fork emulation is a bad joke - I have yet to see something 876care about. The fork emulation is a bad joke - I have yet to see something
612useful that you cna do with it without running into memory corruption 877useful that you can do with it without running into memory corruption
613issues or other braindamage. Hrrrr. 878issues or other braindamage. Hrrrr.
614 879
615Cygwin perl is not supported at the moment, as it should implement fd 880Cygwin perl is not supported at the moment, as it should implement fd
616passing, but doesn't, and rolling my own is hard, as cygwin doesn't 881passing, but doesn't, and rolling my own is hard, as cygwin doesn't
617support enough functionality to do it. 882support enough functionality to do it.
618 883
884=head1 SEE ALSO
885
886L<AnyEvent::Fork::Early> (to avoid executing a perl interpreter),
887L<AnyEvent::Fork::Template> (to create a process by forking the main
888program at a convenient time).
889
619=head1 AUTHOR 890=head1 AUTHOR
620 891
621 Marc Lehmann <schmorp@schmorp.de> 892 Marc Lehmann <schmorp@schmorp.de>
622 http://home.schmorp.de/ 893 http://home.schmorp.de/
623 894

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