1Multicore(3)          User Contributed Perl Documentation         Multicore(3)
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NAME

6       Coro::Multicore - make coro threads on multiple cores with specially
7       supported modules
8

SYNOPSIS

10        # when you DO control the main event loop, e.g. in the main program
11
12        use Coro::Multicore; # enable by default
13
14        Coro::Multicore::scoped_disable;
15        AE::cv->recv; # or EV::run, AnyEvent::Loop::run, Event::loop, ...
16
17        # when you DO NOT control the event loop, e.g. in a module on CPAN
18        # do nothing (see HOW TO USE IT) or something like this:
19
20        use Coro::Multicore (); # disable by default
21
22        async {
23           Coro::Multicore::scoped_enable;
24
25           # blocking is safe in your own threads
26           ...
27        };
28

DESCRIPTION

30       While Coro threads (unlike ithreads) provide real threads similar to
31       pthreads, python threads and so on, they do not run in parallel to each
32       other even on machines with multiple CPUs or multiple CPU cores.
33
34       This module lifts this restriction under two very specific but useful
35       conditions: firstly, the coro thread executes in XS code and does not
36       touch any perl data structures, and secondly, the XS code is specially
37       prepared to allow this.
38
39       This means that, when you call an XS function of a module prepared for
40       it, this XS function can execute in parallel to any other Coro threads.
41       This is useful for both CPU bound tasks (such as cryptography) as well
42       as I/O bound tasks (such as loading an image from disk). It can also be
43       used to do stuff in parallel via APIs that were not meant for this,
44       such as database accesses via DBI.
45
46       The mechanism to support this is easily added to existing modules and
47       is independent of Coro or Coro::Multicore, and therefore could be used,
48       without changes, with other, similar, modules, or even the perl core,
49       should it gain real thread support anytime soon. See
50       <http://perlmulticore.schmorp.de/> for more info on how to prepare a
51       module to allow parallel execution. Preparing an existing module is
52       easy, doesn't add much overhead and no dependencies.
53
54       This module is an AnyEvent user (and also, if not obvious, uses Coro).
55

HOW TO USE IT

57       Quick explanation: decide whether you control the main program/the
58       event 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
66       normally block and wait for something in an event loop callbacks. Now
67       imagine somebody patches your favourite module (e.g. Digest::MD5) to
68       take 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 "Coro::Multicore" - it would
72       simply calculate the MD5 digest and block execution of anything else.
73       But with "Coro::Multicore" enabled, the same operation would try to run
74       other threads. And when those wait for events, there is no event loop
75       anymore, as the event loop thread is busy doing the MD5 calculation,
76       leading to a deadlock.
77
78   USE IT IN THE MAIN PROGRAM
79       One way to avoid this is to not run perlmulticore enabled functions in
80       any callbacks. A simpler way to ensure it works is to disable
81       "Coro::Multicore" thread switching in event loop callbacks, and enable
82       it everywhere else.
83
84       Therefore, if you control the event loop, as is usually the case when
85       you write program and not a module, then you can enable
86       "Coro::Multicore" by default, and disable it in your event loop thread:
87
88          # example 1, separate thread for event loop
89
90          use EV;
91          use Coro;
92          use Coro::Multicore;
93
94          async {
95             Coro::Multicore::scoped_disable;
96             EV::run;
97          };
98
99          # do something else
100
101          # example 2, run event loop as main program
102
103          use EV;
104          use Coro;
105          use Coro::Multicore;
106
107          Coro::Multicore::scoped_disable;
108
109          ... initialisation
110
111          EV::run;
112
113       The latter form is usually better and more idiomatic - the main thread
114       is the best place to run the event loop.
115
116       Often you want to do some initialisation before running the event loop.
117       The most efficient way to do that is to put your intialisation code
118       (and main program) into its own thread and run the event loop in your
119       main program:
120
121          use AnyEvent::Loop;
122          use Coro::Multicore; # enable by default
123
124          async {
125             load_data;
126             do_other_init;
127             bind_socket;
128             ...
129          };
130
131          Coro::Multicore::scoped_disable;
132          AnyEvent::Loop::run;
133
134       This has the effect of running the event loop first, so the
135       initialisation code can block if it wants to.
136
137       If this is too cumbersome but you still want to make sure you can call
138       blocking functions before entering the event loop, you can keep
139       "Coro::Multicore" disabled till you cna run the event loop:
140
141          use AnyEvent::Loop;
142          use Coro::Multicore (); # disable by default
143
144          load_data;
145          do_other_init;
146          bind_socket;
147          ...
148
149          Coro::Multicore::scoped_disable; # disable for event loop
150          Coro::Multicore::enable 1; # enable for the rest of the program
151          AnyEvent::Loop::run;
152
153   USE IT IN A MODULE
154       When you do not control the event loop, for example, because you want
155       to use this from a module you published on CPAN, then the previous
156       method doesn't work.
157
158       However, this is not normally a problem in practise - most modules only
159       do work at request of the caller. In that case, you might not care
160       whether it does block other threads or not, as this would be the
161       callers responsibility (or decision), and by extension, a decision for
162       the main program.
163
164       So unless you use XS and want your XS functions to run asynchronously,
165       you don't have to worry about "Coro::Multicore" at all - if you happen
166       to call XS functions that are multicore-enabled and your caller has
167       configured things correctly, they will automatically run
168       asynchronously. Or in other words: nothing needs to be done at all,
169       which also means that this method works fine for existing pure-perl
170       modules, without having to change them at all.
171
172       Only if your module runs it's own Coro threads could it be an issue -
173       maybe your module implements some kind of job pool and relies on
174       certain operations to run asynchronously. Then you can still use
175       "Coro::Multicore" by not enabling it be default and only enabling it in
176       your own threads:
177
178          use Coro;
179          use Coro::Multicore (); # note the () to disable by default
180
181          async {
182             Coro::Multicore::scoped_enable;
183
184             # do things asynchronously by calling perlmulticore-enabled functions
185          };
186
187   EXPORTS
188       This module does not (at the moment) export any symbols. It does,
189       however, export "behaviour" - if you use the default import, then
190       Coro::Multicore will be enabled for all threads and all callers in the
191       whole program:
192
193          use Coro::Multicore;
194
195       In a module where you don't control what else might be loaded and run,
196       you might want to be more conservative, and not import anything. This
197       has the effect of not enabling the functionality by default, so you
198       have to enable it per scope:
199
200          use Coro::Multicore ();
201
202          sub myfunc {
203             Coro::Multicore::scoped_enable;
204
205             # from here to the end of this function, and in any functions
206             # called from this function, tasks will be executed asynchronously.
207          }
208

API FUNCTIONS

210       $previous = Coro::Multicore::enable [$enable]
211           This function enables (if $enable is true) or disables (if $enable
212           is false) the multicore functionality globally. By default, it is
213           enabled.
214
215           This can be used to effectively disable this module's functionality
216           by default, and enable it only for selected threads or scopes, by
217           calling "Coro::Multicore::scoped_enable".
218
219           The function returns the previous value of the enable flag.
220
221       Coro::Multicore::scoped_enable
222           This function instructs Coro::Multicore to handle all requests
223           executed in the current coro thread, from the call to the end of
224           the current scope.
225
226           Calls to "scoped_enable" and "scoped_disable" don't nest very well
227           at the moment, so don't nest them.
228
229       Coro::Multicore::scoped_disable
230           The opposite of "Coro::Multicore::scope_disable": instructs
231           Coro::Multicore to not handle the next multicore-enabled request.
232

THREAD SAFETY OF SUPPORTING XS MODULES

234       Just because an XS module supports perlmulticore might not immediately
235       make it reentrant. For example, while you can (try to) call "execute"
236       on the same database handle for the patched "DBD::mysql" (see the
237       registry <http://perlmulticore.schmorp.de/registry>), this will almost
238       certainly not work, despite "DBD::mysql" and "libmysqlclient" being
239       thread safe and reentrant - just not on the same database handle.
240
241       Many modules have limitations such as these - some can only be called
242       concurrently from a single thread as they use global variables, some
243       can only be called concurrently on different handles (e.g. database
244       connections for DBD modules, or digest objects for Digest modules), and
245       some can be called at any time (such as the "md5" function in
246       "Digest::MD5").
247
248       Generally, you only have to be careful with the very few modules that
249       use global variables or rely on C libraries that aren't thread-safe,
250       which should be documented clearly in the module documentation.
251
252       Most modules are either perfectly reentrant, or at least reentrant as
253       long as you give every thread it's own handle object.
254

EXCEPTIONS AND THREAD CANCELLATION

256       Coro allows you to cancel threads even when they execute within an XS
257       function ("cancel" vs. "cancel" methods). Similarly, Coro allows you to
258       send exceptions (e.g. via the "throw" method) to threads executing
259       inside an XS function.
260
261       While doing this is questionable and dangerous with normal Coro threads
262       already, they are both supported in this module, although with
263       potentially unwanted effects. The following describes the current
264       implementation and is subject to change. It is described primarily so
265       you can understand what went wrong, if things go wrong.
266
267       EXCEPTIONS
268           When a thread that has currently released the perl interpreter
269           (e.g.  because it is executing a perlmulticore enabled XS function)
270           receives an exception, it will at first continue normally.
271
272           After acquiring the perl interpreter again, it will throw the
273           exception it previously received. More specifically, when a thread
274           calls "perlinterp_acquire ()" and has received an exception, then
275           "perlinterp_acquire ()" will not return but instead "die".
276
277           Most code that has been updated for perlmulticore support will not
278           expect this, and might leave internal state corrupted to some
279           extent.
280
281       CANCELLATION
282           Unsafe cancellation on a thread that has released the perl
283           interpreter frees its resources, but let's the XS code continue at
284           first. This should not lead to corruption on the perl level, as the
285           code isn't allowed to touch perl data structures until it
286           reacquires the interpreter.
287
288           The call to "perlinterp_acquire ()" will then block indefinitely,
289           leaking the (OS level) thread.
290
291           Safe cancellation will simply fail in this case, so is still "safe"
292           to call.
293

INTERACTION WITH OTHER SOFTWARE

295       This module is very similar to other environments where perl
296       interpreters are moved between threads, such as mod_perl2, and the same
297       caveats apply.
298
299       I want to spell out the most important ones:
300
301       pthreads usage
302           Any creation of pthreads make it impossible to fork portably from a
303           perl program, as forking from within a threaded program will leave
304           the program in a state similar to a signal handler. While it might
305           work on some platforms (as an extension), this might also result in
306           silent data corruption. It also seems to work most of the time, so
307           it's hard to test for this.
308
309           I recommend using something like AnyEvent::Fork, which can create
310           subprocesses safely (via Proc::FastSpawn).
311
312           Similar issues exist for signal handlers, although this module
313           works hard to keep safe perl signals safe.
314
315       module support
316           This module moves the same perl interpreter between different
317           threads. Some modules might get confused by that (although this can
318           usually be considered a bug). This is a rare case though.
319
320       event loop reliance
321           To be able to wake up programs waiting for results, this module
322           relies on an active event loop (via AnyEvent). This is used to
323           notify the perl interpreter when the asynchronous task is done.
324
325           Since event loops typically fail to work properly after a fork,
326           this means that some operations that were formerly working will now
327           hang after fork.
328
329           A workaround is to call "Coro::Multicore::enable 0" after a fork to
330           disable the module.
331
332           Future versions of this module might do this automatically.
333

BUGS

335       (OS-) threads are never released
336           At the moment, threads that were created once will never be freed.
337           They will be reused for asynchronous requests, though, so as long
338           as you limit the maximum number of concurrent asynchronous tasks,
339           this will also limit the maximum number of threads created.
340
341           The idle threads are not necessarily using a lot of resources: on
342           GNU/Linux + glibc, each thread takes about 8KiB of userspace memory
343           + whatever the kernel needs (probably less than 8KiB).
344
345           Future versions will likely lift this limitation.
346
347       AnyEvent is initalised at module load time
348           AnyEvent is initialised on module load, as opposed to at a later
349           time.
350
351           Future versions will likely change this.
352

AUTHOR

354        Marc Lehmann <schmorp@schmorp.de>
355        http://software.schmorp.de/pkg/AnyEvent-XSThreadPool.html
356
357       Additional thanks to Zsbán Ambrus, who gave considerable desing input
358       for this module and the perl multicore specification.
359
360
361
362perl v5.28.0                      2018-08-15                      Multicore(3)
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