Coro-Multicore/Multicore.pm

=head1 NAME

Coro::Multicore - make coro threads on multiple cores with specially supported modules

=head1 SYNOPSIS

 # when you DO control the main event loop, e.g. in the main program

 use Coro::Multicore; # enable by default

 Coro::Multicore::scoped_disable;
 AE::cv->recv; # or EV::run, AnyEvent::Loop::run, Event::loop, ...

 # when you DO NOT control the event loop, e.g. in a module on CPAN
 # do nothing (see HOW TO USE IT) or something like this:

 use Coro::Multicore (); # disable by default

 async {
    Coro::Multicore::scoped_enable;

    # blocking is safe in your own threads
    ...
 };

=head1 DESCRIPTION

While L<Coro> threads (unlike ithreads) provide real threads similar to
pthreads, python threads and so on, they do not run in parallel to each
other even on machines with multiple CPUs or multiple CPU cores.

This module lifts this restriction under two very specific but useful
conditions: firstly, the coro thread executes in XS code and does not
touch any perl data structures, and secondly, the XS code is specially
prepared to allow this.

This means that, when you call an XS function of a module prepared for it,
this XS function can execute in parallel to any other Coro threads. This
is useful for both CPU bound tasks (such as cryptography) as well as I/O
bound tasks (such as loading an image from disk). It can also be used
to do stuff in parallel via APIs that were not meant for this, such as
database accesses via DBI.

The mechanism to support this is easily added to existing modules
and is independent of L<Coro> or L<Coro::Multicore>, and therefore
could be used, without changes, with other, similar, modules, or even
the perl core, should it gain real thread support anytime soon. See
L<http://perlmulticore.schmorp.de/> for more info on how to prepare a
module to allow parallel execution. Preparing an existing module is easy,
doesn't add much overhead and no dependencies.

This module is an L<AnyEvent> user (and also, if not obvious, uses
L<Coro>).

=head1 HOW TO USE IT

Quick explanation: decide whether you control the main program/the event
loop and choose one of the two styles from the SYNOPSIS.

Longer explanation: There are two major modes this module can used in -
supported operations run asynchronously either by default, or only when
requested. The reason you might not want to enable this module for all
operations by default is compatibility with existing code:

Since this module integrates into an event loop and you must not normally
block and wait for something in an event loop callbacks. Now imagine
somebody patches your favourite module (e.g. Digest::MD5) to take
advantage of of the Perl Multicore API.

Then code that runs in an event loop callback and executes
Digest::MD5::md5 would work fine without C<Coro::Multicore> - it would
simply calculate the MD5 digest and block execution of anything else. But
with C<Coro::Multicore> enabled, the same operation would try to run other
threads. And when those wait for events, there is no event loop anymore,
as the event loop thread is busy doing the MD5 calculation, leading to a
deadlock.

=head2 USE IT IN THE MAIN PROGRAM

One way to avoid this is to not run perlmulticore enabled functions
in any callbacks. A simpler way to ensure it works is to disable
C<Coro::Multicore> thread switching in event loop callbacks, and enable it
everywhere else.

Therefore, if you control the event loop, as is usually the case when
you write I<program> and not a I<module>, then you can enable C<Coro::Multicore>
by default, and disable it in your event loop thread:

   # example 1, separate thread for event loop

   use EV;
   use Coro;
   use Coro::Multicore;

   async {
      Coro::Multicore::scoped_disable;
      EV::run;
   };

   # do something else

   # example 2, run event loop as main program

   use EV;
   use Coro;
   use Coro::Multicore;

   Coro::Multicore::scoped_disable;

   ... initialisation

   EV::run;

The latter form is usually better and more idiomatic - the main thread is
the best place to run the event loop.

Often you want to do some initialisation before running the event
loop. The most efficient way to do that is to put your intialisation code
(and main program) into its own thread and run the event loop in your main
program:

   use AnyEvent::Loop;
   use Coro::Multicore; # enable by default

   async {
      load_data;
      do_other_init;
      bind_socket;
      ...
   };

   Coro::Multicore::scoped_disable;
   AnyEvent::Loop::run;

This has the effect of running the event loop first, so the initialisation
code can block if it wants to.

If this is too cumbersome but you still want to make sure you can
call blocking functions before entering the event loop, you can keep
C<Coro::Multicore> disabled till you cna run the event loop:

   use AnyEvent::Loop;
   use Coro::Multicore (); # disable by default

   load_data;
   do_other_init;
   bind_socket;
   ...

   Coro::Multicore::scoped_disable; # disable for event loop
   Coro::Multicore::enable 1; # enable for the rest of the program
   AnyEvent::Loop::run;

=head2 USE IT IN A MODULE

When you I<do not> control the event loop, for example, because you want
to use this from a module you published on CPAN, then the previous method
doesn't work.

However, this is not normally a problem in practise - most modules only
do work at request of the caller. In that case, you might not care
whether it does block other threads or not, as this would be the callers
responsibility (or decision), and by extension, a decision for the main
program.

So unless you use XS and want your XS functions to run asynchronously,
you don't have to worry about C<Coro::Multicore> at all - if you
happen to call XS functions that are multicore-enabled and your
caller has configured things correctly, they will automatically run
asynchronously. Or in other words: nothing needs to be done at all, which
also means that this method works fine for existing pure-perl modules,
without having to change them at all.

Only if your module runs it's own L<Coro> threads could it be an
issue - maybe your module implements some kind of job pool and relies
on certain operations to run asynchronously. Then you can still use
C<Coro::Multicore> by not enabling it be default and only enabling it in
your own threads:

   use Coro;
   use Coro::Multicore (); # note the () to disable by default

   async {
      Coro::Multicore::scoped_enable;

      # do things asynchronously by calling perlmulticore-enabled functions
   };

=head2 EXPORTS

This module does not (at the moment) export any symbols. It does, however,
export "behaviour" - if you use the default import, then Coro::Multicore
will be enabled for all threads and all callers in the whole program:

   use Coro::Multicore;

In a module where you don't control what else might be loaded and run, you
might want to be more conservative, and not import anything. This has the
effect of not enabling the functionality by default, so you have to enable
it per scope:

   use Coro::Multicore ();

   sub myfunc {
      Coro::Multicore::scoped_enable;

      # from here to the end of this function, and in any functions
      # called from this function, tasks will be executed asynchronously.
   }

=head1 API FUNCTIONS

=over 4

=item $previous = Coro::Multicore::enable [$enable]

This function enables (if C<$enable> is true) or disables (if C<$enable>
is false) the multicore functionality globally. By default, it is enabled.

This can be used to effectively disable this module's functionality by
default, and enable it only for selected threads or scopes, by calling
C<Coro::Multicore::scoped_enable>.

The function returns the previous value of the enable flag.

=item Coro::Multicore::scoped_enable

This function instructs Coro::Multicore to handle all requests executed
in the current coro thread, from the call to the end of the current scope.

Calls to C<scoped_enable> and C<scoped_disable> don't nest very well at
the moment, so don't nest them.

=item Coro::Multicore::scoped_disable

The opposite of C<Coro::Multicore::scope_disable>: instructs Coro::Multicore to
I<not> handle the next multicore-enabled request.

=back

=cut

package Coro::Multicore;

use Coro ();
use AnyEvent ();

BEGIN {
   our $VERSION = '1.02';

   use XSLoader;
   XSLoader::load __PACKAGE__, $VERSION;
}


sub import {
   if (@_ > 1) {
      require Carp;
      Carp::croak ("Coro::Multicore does not export any symbols");
   }

   enable 1;
}

our $WATCHER = AE::io fd, 0, \&poll;

=head1 THREAD SAFETY OF SUPPORTING XS MODULES

Just because an XS module supports perlmulticore might not immediately
make it reentrant. For example, while you can (try to) call C<execute>
on the same database handle for the patched C<DBD::mysql> (see the
L<registry|http://perlmulticore.schmorp.de/registry>), this will almost
certainly not work, despite C<DBD::mysql> and C<libmysqlclient> being
thread safe and reentrant - just not on the same database handle.

Many modules have limitations such as these - some can only be called
concurrently from a single thread as they use global variables, some
can only be called concurrently on different I<handles> (e.g. database
connections for DBD modules, or digest objects for Digest modules),
and some can be called at any time (such as the C<md5> function in
C<Digest::MD5>).

Generally, you only have to be careful with the very few modules that use
global variables or rely on C libraries that aren't thread-safe, which
should be documented clearly in the module documentation.

Most modules are either perfectly reentrant, or at least reentrant as long
as you give every thread it's own I<handle> object.

=head1 EXCEPTIONS AND THREAD CANCELLATION

L<Coro> allows you to cancel threads even when they execute within an XS
function (C<cancel> vs. C<cancel> methods). Similarly, L<Coro> allows you
to send exceptions (e.g. via the C<throw> method) to threads executing
inside an XS function.

While doing this is questionable and dangerous with normal Coro threads
already, they are both supported in this module, although with potentially
unwanted effects. The following describes the current implementation and
is subject to change. It is described primarily so you can understand what
went wrong, if things go wrong.

=over 4

=item EXCEPTIONS

When a thread that has currently released the perl interpreter (e.g.
because it is executing a perlmulticore enabled XS function) receives an exception, it will
at first continue normally.

After acquiring the perl interpreter again, it will throw the
exception it previously received. More specifically, when a thread
calls C<perlinterp_acquire ()> and has received an exception, then
C<perlinterp_acquire ()> will not return but instead C<die>.

Most code that has been updated for perlmulticore support will not expect
this, and might leave internal state corrupted to some extent.

=item CANCELLATION

Unsafe cancellation on a thread that has released the perl interpreter
frees its resources, but let's the XS code continue at first. This should
not lead to corruption on the perl level, as the code isn't allowed to
touch perl data structures until it reacquires the interpreter.

The call to C<perlinterp_acquire ()> will then block indefinitely, leaking
the (OS level) thread.

Safe cancellation will simply fail in this case, so is still "safe" to
call.

=back

=head1 INTERACTION WITH OTHER SOFTWARE

This module is very similar to other environments where perl interpreters
are moved between threads, such as mod_perl2, and the same caveats apply.

I want to spell out the most important ones:

=over 4

=item pthreads usage

Any creation of pthreads make it impossible to fork portably from a
perl program, as forking from within a threaded program will leave the
program in a state similar to a signal handler. While it might work on
some platforms (as an extension), this might also result in silent data
corruption. It also seems to work most of the time, so it's hard to test
for this.

I recommend using something like L<AnyEvent::Fork>, which can create
subprocesses safely (via L<Proc::FastSpawn>).

Similar issues exist for signal handlers, although this module works hard
to keep safe perl signals safe.

=item module support

This module moves the same perl interpreter between different
threads. Some modules might get confused by that (although this can
usually be considered a bug). This is a rare case though.

=item event loop reliance

To be able to wake up programs waiting for results, this module relies on
an active event loop (via L<AnyEvent>). This is used to notify the perl
interpreter when the asynchronous task is done.

Since event loops typically fail to work properly after a fork, this means
that some operations that were formerly working will now hang after fork.

A workaround is to call C<Coro::Multicore::enable 0> after a fork to
disable the module.

Future versions of this module might do this automatically.

=back

=head1 BUGS

=over 4

=item (OS-) threads are never released

At the moment, threads that were created once will never be freed. They
will be reused for asynchronous requests, though, so as long as you limit
the maximum number of concurrent asynchronous tasks, this will also limit
the maximum number of threads created.

The idle threads are not necessarily using a lot of resources: on
GNU/Linux + glibc, each thread takes about 8KiB of userspace memory +
whatever the kernel needs (probably less than 8KiB).

Future versions will likely lift this limitation.

=item AnyEvent is initalised at module load time

AnyEvent is initialised on module load, as opposed to at a later time.

Future versions will likely change this.

=back

=head1 AUTHOR

 Marc Lehmann <schmorp@schmorp.de>
 http://software.schmorp.de/pkg/AnyEvent-XSThreadPool.html

Additional thanks to Zsbán Ambrus, who gave considerable desing input for
this module and the perl multicore specification.

=cut

1

Revision:	1.15
Committed:	Tue Mar 5 12:31:27 2019 UTC (5 years, 2 months ago) by root
Branch:	MAIN
CVS Tags:	rel-1_02
Changes since 1.14:	+1 -1 lines
Log Message:	1.02
#	Content
1	=head1 NAME
2
3	Coro::Multicore - make coro threads on multiple cores with specially supported modules
4
5	=head1 SYNOPSIS
6
7	# when you DO control the main event loop, e.g. in the main program
8
9	use Coro::Multicore; # enable by default
10
11	Coro::Multicore::scoped_disable;
12	AE::cv->recv; # or EV::run, AnyEvent::Loop::run, Event::loop, ...
13
14	# when you DO NOT control the event loop, e.g. in a module on CPAN
15	# do nothing (see HOW TO USE IT) or something like this:
16
17	use Coro::Multicore (); # disable by default
18
19	async {
20	Coro::Multicore::scoped_enable;
21
22	# blocking is safe in your own threads
23	...
24	};
25
26	=head1 DESCRIPTION
27
28	While L<Coro> threads (unlike ithreads) provide real threads similar to
29	pthreads, python threads and so on, they do not run in parallel to each
30	other even on machines with multiple CPUs or multiple CPU cores.
31
32	This module lifts this restriction under two very specific but useful
33	conditions: firstly, the coro thread executes in XS code and does not
34	touch any perl data structures, and secondly, the XS code is specially
35	prepared to allow this.
36
37	This means that, when you call an XS function of a module prepared for it,
38	this XS function can execute in parallel to any other Coro threads. This
39	is useful for both CPU bound tasks (such as cryptography) as well as I/O
40	bound tasks (such as loading an image from disk). It can also be used
41	to do stuff in parallel via APIs that were not meant for this, such as
42	database accesses via DBI.
43
44	The mechanism to support this is easily added to existing modules
45	and is independent of L<Coro> or L<Coro::Multicore>, and therefore
46	could be used, without changes, with other, similar, modules, or even
47	the perl core, should it gain real thread support anytime soon. See
48	L<http://perlmulticore.schmorp.de/> for more info on how to prepare a
49	module to allow parallel execution. Preparing an existing module is easy,
50	doesn't add much overhead and no dependencies.
51
52	This module is an L<AnyEvent> user (and also, if not obvious, uses
53	L<Coro>).
54
55	=head1 HOW TO USE IT
56
57	Quick explanation: decide whether you control the main program/the event
58	loop and choose one of the two styles from the SYNOPSIS.
59
60	Longer explanation: There are two major modes this module can used in -
61	supported operations run asynchronously either by default, or only when
62	requested. The reason you might not want to enable this module for all
63	operations by default is compatibility with existing code:
64
65	Since this module integrates into an event loop and you must not normally
66	block and wait for something in an event loop callbacks. Now imagine
67	somebody patches your favourite module (e.g. Digest::MD5) to take
68	advantage of of the Perl Multicore API.
69
70	Then code that runs in an event loop callback and executes
71	Digest::MD5::md5 would work fine without C<Coro::Multicore> - it would
72	simply calculate the MD5 digest and block execution of anything else. But
73	with C<Coro::Multicore> enabled, the same operation would try to run other
74	threads. And when those wait for events, there is no event loop anymore,
75	as the event loop thread is busy doing the MD5 calculation, leading to a
76	deadlock.
77
78	=head2 USE IT IN THE MAIN PROGRAM
79
80	One way to avoid this is to not run perlmulticore enabled functions
81	in any callbacks. A simpler way to ensure it works is to disable
82	C<Coro::Multicore> thread switching in event loop callbacks, and enable it
83	everywhere else.
84
85	Therefore, if you control the event loop, as is usually the case when
86	you write I<program> and not a I<module>, then you can enable C<Coro::Multicore>
87	by default, and disable it in your event loop thread:
88
89	# example 1, separate thread for event loop
90
91	use EV;
92	use Coro;
93	use Coro::Multicore;
94
95	async {
96	Coro::Multicore::scoped_disable;
97	EV::run;
98	};
99
100	# do something else
101
102	# example 2, run event loop as main program
103
104	use EV;
105	use Coro;
106	use Coro::Multicore;
107
108	Coro::Multicore::scoped_disable;
109
110	... initialisation
111
112	EV::run;
113
114	The latter form is usually better and more idiomatic - the main thread is
115	the best place to run the event loop.
116
117	Often you want to do some initialisation before running the event
118	loop. The most efficient way to do that is to put your intialisation code
119	(and main program) into its own thread and run the event loop in your main
120	program:
121
122	use AnyEvent::Loop;
123	use Coro::Multicore; # enable by default
124
125	async {
126	load_data;
127	do_other_init;
128	bind_socket;
129	...
130	};
131
132	Coro::Multicore::scoped_disable;
133	AnyEvent::Loop::run;
134
135	This has the effect of running the event loop first, so the initialisation
136	code can block if it wants to.
137
138	If this is too cumbersome but you still want to make sure you can
139	call blocking functions before entering the event loop, you can keep
140	C<Coro::Multicore> disabled till you cna run the event loop:
141
142	use AnyEvent::Loop;
143	use Coro::Multicore (); # disable by default
144
145	load_data;
146	do_other_init;
147	bind_socket;
148	...
149
150	Coro::Multicore::scoped_disable; # disable for event loop
151	Coro::Multicore::enable 1; # enable for the rest of the program
152	AnyEvent::Loop::run;
153
154	=head2 USE IT IN A MODULE
155
156	When you I<do not> control the event loop, for example, because you want
157	to use this from a module you published on CPAN, then the previous method
158	doesn't work.
159
160	However, this is not normally a problem in practise - most modules only
161	do work at request of the caller. In that case, you might not care
162	whether it does block other threads or not, as this would be the callers
163	responsibility (or decision), and by extension, a decision for the main
164	program.
165
166	So unless you use XS and want your XS functions to run asynchronously,
167	you don't have to worry about C<Coro::Multicore> at all - if you
168	happen to call XS functions that are multicore-enabled and your
169	caller has configured things correctly, they will automatically run
170	asynchronously. Or in other words: nothing needs to be done at all, which
171	also means that this method works fine for existing pure-perl modules,
172	without having to change them at all.
173
174	Only if your module runs it's own L<Coro> threads could it be an
175	issue - maybe your module implements some kind of job pool and relies
176	on certain operations to run asynchronously. Then you can still use
177	C<Coro::Multicore> by not enabling it be default and only enabling it in
178	your own threads:
179
180	use Coro;
181	use Coro::Multicore (); # note the () to disable by default
182
183	async {
184	Coro::Multicore::scoped_enable;
185
186	# do things asynchronously by calling perlmulticore-enabled functions
187	};
188
189	=head2 EXPORTS
190
191	This module does not (at the moment) export any symbols. It does, however,
192	export "behaviour" - if you use the default import, then Coro::Multicore
193	will be enabled for all threads and all callers in the whole program:
194
195	use Coro::Multicore;
196
197	In a module where you don't control what else might be loaded and run, you
198	might want to be more conservative, and not import anything. This has the
199	effect of not enabling the functionality by default, so you have to enable
200	it per scope:
201
202	use Coro::Multicore ();
203
204	sub myfunc {
205	Coro::Multicore::scoped_enable;
206
207	# from here to the end of this function, and in any functions
208	# called from this function, tasks will be executed asynchronously.
209	}
210
211	=head1 API FUNCTIONS
212
213	=over 4
214
215	=item $previous = Coro::Multicore::enable [$enable]
216
217	This function enables (if C<$enable> is true) or disables (if C<$enable>
218	is false) the multicore functionality globally. By default, it is enabled.
219
220	This can be used to effectively disable this module's functionality by
221	default, and enable it only for selected threads or scopes, by calling
222	C<Coro::Multicore::scoped_enable>.
223
224	The function returns the previous value of the enable flag.
225
226	=item Coro::Multicore::scoped_enable
227
228	This function instructs Coro::Multicore to handle all requests executed
229	in the current coro thread, from the call to the end of the current scope.
230
231	Calls to C<scoped_enable> and C<scoped_disable> don't nest very well at
232	the moment, so don't nest them.
233
234	=item Coro::Multicore::scoped_disable
235
236	The opposite of C<Coro::Multicore::scope_disable>: instructs Coro::Multicore to
237	I<not> handle the next multicore-enabled request.
238
239	=back
240
241	=cut
242
243	package Coro::Multicore;
244
245	use Coro ();
246	use AnyEvent ();
247
248	BEGIN {
249	our $VERSION = '1.02';
250
251	use XSLoader;
252	XSLoader::load __PACKAGE__, $VERSION;
253	}
254
255
256	sub import {
257	if (@_ > 1) {
258	require Carp;
259	Carp::croak ("Coro::Multicore does not export any symbols");
260	}
261
262	enable 1;
263	}
264
265	our $WATCHER = AE::io fd, 0, \&poll;
266
267	=head1 THREAD SAFETY OF SUPPORTING XS MODULES
268
269	Just because an XS module supports perlmulticore might not immediately
270	make it reentrant. For example, while you can (try to) call C<execute>
271	on the same database handle for the patched C<DBD::mysql> (see the
272	L<registry\|http://perlmulticore.schmorp.de/registry>), this will almost
273	certainly not work, despite C<DBD::mysql> and C<libmysqlclient> being
274	thread safe and reentrant - just not on the same database handle.
275
276	Many modules have limitations such as these - some can only be called
277	concurrently from a single thread as they use global variables, some
278	can only be called concurrently on different I<handles> (e.g. database
279	connections for DBD modules, or digest objects for Digest modules),
280	and some can be called at any time (such as the C<md5> function in
281	C<Digest::MD5>).
282
283	Generally, you only have to be careful with the very few modules that use
284	global variables or rely on C libraries that aren't thread-safe, which
285	should be documented clearly in the module documentation.
286
287	Most modules are either perfectly reentrant, or at least reentrant as long
288	as you give every thread it's own I<handle> object.
289
290	=head1 EXCEPTIONS AND THREAD CANCELLATION
291
292	L<Coro> allows you to cancel threads even when they execute within an XS
293	function (C<cancel> vs. C<cancel> methods). Similarly, L<Coro> allows you
294	to send exceptions (e.g. via the C<throw> method) to threads executing
295	inside an XS function.
296
297	While doing this is questionable and dangerous with normal Coro threads
298	already, they are both supported in this module, although with potentially
299	unwanted effects. The following describes the current implementation and
300	is subject to change. It is described primarily so you can understand what
301	went wrong, if things go wrong.
302
303	=over 4
304
305	=item EXCEPTIONS
306
307	When a thread that has currently released the perl interpreter (e.g.
308	because it is executing a perlmulticore enabled XS function) receives an exception, it will
309	at first continue normally.
310
311	After acquiring the perl interpreter again, it will throw the
312	exception it previously received. More specifically, when a thread
313	calls C<perlinterp_acquire ()> and has received an exception, then
314	C<perlinterp_acquire ()> will not return but instead C<die>.
315
316	Most code that has been updated for perlmulticore support will not expect
317	this, and might leave internal state corrupted to some extent.
318
319	=item CANCELLATION
320
321	Unsafe cancellation on a thread that has released the perl interpreter
322	frees its resources, but let's the XS code continue at first. This should
323	not lead to corruption on the perl level, as the code isn't allowed to
324	touch perl data structures until it reacquires the interpreter.
325
326	The call to C<perlinterp_acquire ()> will then block indefinitely, leaking
327	the (OS level) thread.
328
329	Safe cancellation will simply fail in this case, so is still "safe" to
330	call.
331
332	=back
333
334	=head1 INTERACTION WITH OTHER SOFTWARE
335
336	This module is very similar to other environments where perl interpreters
337	are moved between threads, such as mod_perl2, and the same caveats apply.
338
339	I want to spell out the most important ones:
340
341	=over 4
342
343	=item pthreads usage
344
345	Any creation of pthreads make it impossible to fork portably from a
346	perl program, as forking from within a threaded program will leave the
347	program in a state similar to a signal handler. While it might work on
348	some platforms (as an extension), this might also result in silent data
349	corruption. It also seems to work most of the time, so it's hard to test
350	for this.
351
352	I recommend using something like L<AnyEvent::Fork>, which can create
353	subprocesses safely (via L<Proc::FastSpawn>).
354
355	Similar issues exist for signal handlers, although this module works hard
356	to keep safe perl signals safe.
357
358	=item module support
359
360	This module moves the same perl interpreter between different
361	threads. Some modules might get confused by that (although this can
362	usually be considered a bug). This is a rare case though.
363
364	=item event loop reliance
365
366	To be able to wake up programs waiting for results, this module relies on
367	an active event loop (via L<AnyEvent>). This is used to notify the perl
368	interpreter when the asynchronous task is done.
369
370	Since event loops typically fail to work properly after a fork, this means
371	that some operations that were formerly working will now hang after fork.
372
373	A workaround is to call C<Coro::Multicore::enable 0> after a fork to
374	disable the module.
375
376	Future versions of this module might do this automatically.
377
378	=back
379
380	=head1 BUGS
381
382	=over 4
383
384	=item (OS-) threads are never released
385
386	At the moment, threads that were created once will never be freed. They
387	will be reused for asynchronous requests, though, so as long as you limit
388	the maximum number of concurrent asynchronous tasks, this will also limit
389	the maximum number of threads created.
390
391	The idle threads are not necessarily using a lot of resources: on
392	GNU/Linux + glibc, each thread takes about 8KiB of userspace memory +
393	whatever the kernel needs (probably less than 8KiB).
394
395	Future versions will likely lift this limitation.
396
397	=item AnyEvent is initalised at module load time
398
399	AnyEvent is initialised on module load, as opposed to at a later time.
400
401	Future versions will likely change this.
402
403	=back
404
405	=head1 AUTHOR
406
407	Marc Lehmann <schmorp@schmorp.de>
408	http://software.schmorp.de/pkg/AnyEvent-XSThreadPool.html
409
410	Additional thanks to Zsbán Ambrus, who gave considerable desing input for
411	this module and the perl multicore specification.
412
413	=cut
414
415	1
416