[ViewVC] Diff of: cvs/AnyEvent-Fork/Fork.pm

Comparing AnyEvent-Fork/Fork.pm (file contents):
Revision 1.24 by root, Sat Apr 6 08:32:23 2013 UTC vs.
Revision 1.25 by root, Sat Apr 6 08:55:16 2013 UTC

…		…
39	If you need some form of RPC, you can either implement it yourself	39	If you need some form of RPC, you can either implement it yourself
40	in whatever way you like, use some message-passing module such	40	in whatever way you like, use some message-passing module such
41	as L<AnyEvent::MP>, some pipe such as L<AnyEvent::ZeroMQ>, use	41	as L<AnyEvent::MP>, some pipe such as L<AnyEvent::ZeroMQ>, use
42	L<AnyEvent::Handle> on both sides to send e.g. JSON or Storable messages,	42	L<AnyEvent::Handle> on both sides to send e.g. JSON or Storable messages,
43	and so on.	43	and so on.
		44
		45	=head1 PROBLEM STATEMENT
		46
		47	There are two traditional ways to implement parallel processing on UNIX
		48	like operating systems - fork and process, and fork+exec and process. They
		49	have different advantages and disadvantages that I describe below,
		50	together 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
		56	A 5GB process needs 0.05s to fork on my 3.6GHz amd64 GNU/Linux box. This
		57	overhead is often shared with exec (because you have to fork first), but
		58	in some circumstances (e.g. when vfork is used), fork+exec can be much
		59	faster.
		60
		61	This module can help here by telling a small(er) helper process to fork,
		62	which is faster then forking the main process, and also uses vfork where
		63	possible. 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
		66	process.
		67
		68	For example, modules or data files that are loaded will not use additional
		69	memory after a fork. When exec'ing a new process, modules and data files
		70	might need to be loaded again, at extra CPU and memory cost. But when
		71	forking, literally all data structures are copied - if the program frees
		72	them and replaces them by new data, the child processes will retain the
		73	old version even if it isn't used, which can suddenly and unexpectedly
		74	increase memory usage when freeing memory.
		75
		76	The trade-off is between more sharing with fork (which can be good or
		77	bad), and no sharing with exec.
		78
		79	This module allows the main program to do a controlled fork, and allows
		80	modules to exec processes safely at any time. When creating a custom
		81	process pool you can take advantage of data sharing via fork without
		82	risking to share large dynamic data structures that will blow up child
		83	memory usage.
		84
		85	In other words, this module puts you into control over what is being
		86	shared and what isn't, at all times.
		87
		88	=item Exec'ing a new perl process might be difficult.
		89
		90	For example, it is not easy to find the correct path to the perl
		91	interpreter - C<$^X> might not be a perl interpreter at all.
		92
		93	This module tries hard to identify the correct path to the perl
		94	interpreter. With a cooperative main program, exec'ing the interpreter
		95	might not even be necessary, but even without help from the main program,
		96	it will still work when used from a module.
		97
		98	=item Exec'ing a new perl process might be slow, as all necessary modules
		99	have to be loaded from disk again, with no guarantees of success.
		100
		101	Long running processes might run into problems when perl is upgraded
		102	and modules are no longer loadable because they refer to a different
		103	perl version, or parts of a distribution are newer than the ones already
		104	loaded.
		105
		106	This module supports creating pre-initialised perl processes to be used as
		107	a template for new processes.
		108
		109	=item Forking might be impossible when a program is running.
		110
		111	For example, POSIX makes it almost impossible to fork from a
		112	multi-threaded program while doing anything useful in the child - in
		113	fact, if your perl program uses POSIX threads (even indirectly via
		114	e.g. L<IO::AIO> or L<threads>), you cannot call fork on the perl level
		115	anymore without risking corruption issues on a number of operating
		116	systems.
		117
		118	This module can safely fork helper processes at any time, by calling
		119	fork+exec in C, in a POSIX-compatible way (via L<Proc::FastSpawn>).
		120
		121	=item Parallel processing with fork might be inconvenient or difficult
		122	to implement. Modules might not work in both parent and child.
		123
		124	For example, when a program uses an event loop and creates watchers it
		125	becomes very hard to use the event loop from a child program, as the
		126	watchers already exist but are only meaningful in the parent. Worse, a
		127	module might want to use such a module, not knowing whether another module
		128	or the main program also does, leading to problems.
		129
		130	With this module only the main program is allowed to create new processes
		131	by forking (because only the main program can know when it is still safe
		132	to do so) - all other processes are created via fork+exec, which makes it
		133	possible to use modules such as event loops or window interfaces safely.
		134
		135	=back
44		136
45	=head1 EXAMPLES	137	=head1 EXAMPLES
46		138
47	=head2 Create a single new process, tell it to run your worker function.	139	=head2 Create a single new process, tell it to run your worker function.
48		140
…		…
123	->send_fh ($output)	215	->send_fh ($output)
124	->send_arg ("/bin/echo", "hi")	216	->send_arg ("/bin/echo", "hi")
125	->run ("run", my $cv = AE::cv);	217	->run ("run", my $cv = AE::cv);
126		218
127	my $stderr = $cv->recv;	219	my $stderr = $cv->recv;
128
129	=head1 PROBLEM STATEMENT
130
131	There are two ways to implement parallel processing on UNIX like operating
132	systems - fork and process, and fork+exec and process. They have different
133	advantages and disadvantages that I describe below, together with how this
134	module tries to mitigate the disadvantages.
135
136	=over 4
137
138	=item Forking from a big process can be very slow (a 5GB process needs
139	0.05s to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead
140	is often shared with exec (because you have to fork first), but in some
141	circumstances (e.g. when vfork is used), fork+exec can be much faster.
142
143	This module can help here by telling a small(er) helper process to fork,
144	or fork+exec instead.
145
146	=item Forking usually creates a copy-on-write copy of the parent
147	process. Memory (for example, modules or data files that have been
148	will not take additional memory). When exec'ing a new process, modules
149	and data files might need to be loaded again, at extra CPU and memory
150	cost. Likewise when forking, all data structures are copied as well - if
151	the program frees them and replaces them by new data, the child processes
152	will retain the memory even if it isn't used.
153
154	This module allows the main program to do a controlled fork, and allows
155	modules to exec processes safely at any time. When creating a custom
156	process pool you can take advantage of data sharing via fork without
157	risking to share large dynamic data structures that will blow up child
158	memory usage.
159
160	=item Exec'ing a new perl process might be difficult and slow. For
161	example, it is not easy to find the correct path to the perl interpreter,
162	and all modules have to be loaded from disk again. Long running processes
163	might run into problems when perl is upgraded for example.
164
165	This module supports creating pre-initialised perl processes to be used
166	as template, and also tries hard to identify the correct path to the perl
167	interpreter. With a cooperative main program, exec'ing the interpreter
168	might not even be necessary.
169
170	=item Forking might be impossible when a program is running. For example,
171	POSIX makes it almost impossible to fork from a multi-threaded program and
172	do anything useful in the child - strictly speaking, if your perl program
173	uses posix threads (even indirectly via e.g. L<IO::AIO> or L<threads>),
174	you cannot call fork on the perl level anymore, at all.
175
176	This module can safely fork helper processes at any time, by calling
177	fork+exec in C, in a POSIX-compatible way.
178
179	=item Parallel processing with fork might be inconvenient or difficult
180	to implement. For example, when a program uses an event loop and creates
181	watchers it becomes very hard to use the event loop from a child
182	program, as the watchers already exist but are only meaningful in the
183	parent. Worse, a module might want to use such a system, not knowing
184	whether another module or the main program also does, leading to problems.
185
186	This module only lets the main program create pools by forking (because
187	only the main program can know when it is still safe to do so) - all other
188	pools are created by fork+exec, after which such modules can again be
189	loaded.
190
191	=back
192		220
193	=head1 CONCEPTS	221	=head1 CONCEPTS
194		222
195	This module can create new processes either by executing a new perl	223	This module can create new processes either by executing a new perl
196	process, or by forking from an existing "template" process.	224	process, or by forking from an existing "template" process.

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Comparing AnyEvent-Fork/Fork.pm (file contents): Revision 1.24 by root, Sat Apr 6 08:32:23 2013 UTC vs. Revision 1.25 by root, Sat Apr 6 08:55:16 2013 UTC

Diff Legend

Comparing AnyEvent-Fork/Fork.pm (file contents):
Revision 1.24 by root, Sat Apr 6 08:32:23 2013 UTC vs.
Revision 1.25 by root, Sat Apr 6 08:55:16 2013 UTC