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Revision 1.4 by root, Thu Jan 18 16:46:22 2018 UTC

1NAME 1NAME
2 Coro::Multicore - make coro threads on multiple cores with specially 2 Coro::Multicore - make coro threads on multiple cores with specially
3 supported modules 3 supported modules
4 4
5SYNOPSIS 5SYNOPSIS
6 use Coro::Multicore; 6 # when you DO control the main event loop, e.g. in the main program
7 7
8 # or, if you want it disabled by default (e.g. to use it from a module) 8 use Coro::Multicore; # enable by default
9 use Coro::Multicore (); 9
10 Coro::Multicore::scoped_disable;
11 AE::cv->recv; # or EV::run, AnyEvent::Loop::run, Event::loop, ...
12
13 # when you DO NOT control the event loop, e.g. in a module on CPAN
14 # do nothing (see HOW TO USE IT) or something like this:
15
16 use Coro::Multicore (); # disable by default
17
18 async {
19 Coro::Multicore::scoped_enable;
20
21 # blocking is safe in your own threads
22 ...
23 };
10 24
11DESCRIPTION 25DESCRIPTION
12 EXPERIMENTAL WARNING: This module is in its early stages of development.
13 It's fine to try out, but it didn't receive the normal amount of testing
14 and real-world usage that my other modules have gone through.
15
16 While Coro threads (unlike ithreads) provide real threads similar to 26 While Coro threads (unlike ithreads) provide real threads similar to
17 pthreads, python threads and so on, they do not run in parallel to each 27 pthreads, python threads and so on, they do not run in parallel to each
18 other even on machines with multiple CPUs or multiple CPU cores. 28 other even on machines with multiple CPUs or multiple CPU cores.
19 29
20 This module lifts this restriction under two very specific but useful 30 This module lifts this restriction under two very specific but useful
… …
22 touch any perl data structures, and secondly, the XS code is specially 32 touch any perl data structures, and secondly, the XS code is specially
23 prepared to allow this. 33 prepared to allow this.
24 34
25 This means that, when you call an XS function of a module prepared for 35 This means that, when you call an XS function of a module prepared for
26 it, this XS function can execute in parallel to any other Coro threads. 36 it, this XS function can execute in parallel to any other Coro threads.
37 This is useful for both CPU bound tasks (such as cryptography) as well
38 as I/O bound tasks (such as loading an image from disk). It can also be
39 used to do stuff in parallel via APIs that were not meant for this, such
40 as database accesses via DBI.
27 41
28 The mechanism to support this is easily added to existing modules and is 42 The mechanism to support this is easily added to existing modules and is
29 independent of Coro or Coro::Multicore, and therefore could be used, 43 independent of Coro or Coro::Multicore, and therefore could be used,
30 without changes, with other, similar, modules, or even the perl core, 44 without changes, with other, similar, modules, or even the perl core,
31 should it gain real thread support anytime soon. See 45 should it gain real thread support anytime soon. See
… …
34 easy, doesn't add much overhead and no dependencies. 48 easy, doesn't add much overhead and no dependencies.
35 49
36 This module is an AnyEvent user (and also, if not obvious, uses Coro). 50 This module is an AnyEvent user (and also, if not obvious, uses Coro).
37 51
38HOW TO USE IT 52HOW TO USE IT
39 It could hardly be simpler - if you use coro threads, and before you 53 Quick explanation: decide whether you control the main program/the event
40 call a supported lengthy operation implemented in XS, use this module 54 loop and choose one of the two styles from the SYNOPSIS.
41 and other coro threads can run in parallel:
42 55
56 Longer explanation: There are two major modes this module can used in -
57 supported operations run asynchronously either by default, or only when
58 requested. The reason you might not want to enable this module for all
59 operations by default is compatibility with existing code:
60
61 Since this module integrates into an event loop and you must not
62 normally block and wait for something in an event loop callbacks. Now
63 imagine somebody patches your favourite module (e.g. Digest::MD5) to
64 take advantage of of the Perl Multicore API.
65
66 Then code that runs in an event loop callback and executes
67 Digest::MD5::md5 would work fine without "Coro::Multicore" - it would
68 simply calculate the MD5 digest and block execution of anything else.
69 But with "Coro::Multicore" enabled, the same operation would try to run
70 other threads. And when those wait for events, there is no event loop
71 anymore, as the event loop thread is busy doing the MD5 calculation,
72 leading to a deadlock.
73
74 USE IT IN THE MAIN PROGRAM
75 One way to avoid this is to not run perlmulticore enabled functions in
76 any callbacks. A simpler way to ensure it works is to disable
77 "Coro::Multicore" thread switching in event loop callbacks, and enable
78 it everywhere else.
79
80 Therefore, if you control the event loop, as is usually the case when
81 you write *program* and not a *module*, then you can enable
82 "Coro::Multicore" by default, and disable it in your event loop thread:
83
84 # example 1, separate thread for event loop
85
86 use EV;
87 use Coro;
43 use Coro::Multicore; 88 use Coro::Multicore;
44 89
45 This module has no important API functions to learn or remember. All you 90 async {
46 need to do is *load* it before you can take advantage of it. 91 Coro::Multicore::scoped_disable;
92 EV::run;
93 };
94
95 # do something else
96
97 # example 2, run event loop as main program
98
99 use EV;
100 use Coro;
101 use Coro::Multicore;
102
103 Coro::Multicore::scoped_disable;
104
105 ... initialisation
106
107 EV::run;
108
109 The latter form is usually better and more idiomatic - the main thread
110 is the best place to run the event loop.
111
112 Often you want to do some initialisation before running the event loop.
113 The most efficient way to do that is to put your intialisation code (and
114 main program) into its own thread and run the event loop in your main
115 program:
116
117 use AnyEvent::Loop;
118 use Coro::Multicore; # enable by default
119
120 async {
121 load_data;
122 do_other_init;
123 bind_socket;
124 ...
125 };
126
127 Coro::Multicore::scoped_disable;
128 AnyEvent::Loop::run;
129
130 This has the effect of running the event loop first, so the
131 initialisation code can block if it wants to.
132
133 If this is too cumbersome but you still want to make sure you can call
134 blocking functions before entering the event loop, you can keep
135 "Coro::Multicore" disabled till you cna run the event loop:
136
137 use AnyEvent::Loop;
138 use Coro::Multicore (); # disable by default
139
140 load_data;
141 do_other_init;
142 bind_socket;
143 ...
144
145 Coro::Multicore::scoped_disable; # disable for event loop
146 Coro::Multicore::enable 1; # enable for the rest of the program
147 AnyEvent::Loop::run;
148
149 USE IT IN A MODULE
150 When you *do not* control the event loop, for example, because you want
151 to use this from a module you published on CPAN, then the previous
152 method doesn't work.
153
154 However, this is not normally a problem in practise - most modules only
155 do work at request of the caller. In that case, you might not care
156 whether it does block other threads or not, as this would be the callers
157 responsibility (or decision), and by extension, a decision for the main
158 program.
159
160 So unless you use XS and want your XS functions to run asynchronously,
161 you don't have to worry about "Coro::Multicore" at all - if you happen
162 to call XS functions that are multicore-enabled and your caller has
163 configured things correctly, they will automatically run asynchronously.
164 Or in other words: nothing needs to be done at all, which also means
165 that this method works fine for existing pure-perl modules, without
166 having to change them at all.
167
168 Only if your module runs it's own Coro threads could it be an issue -
169 maybe your module implements some kind of job pool and relies on certain
170 operations to run asynchronously. Then you can still use
171 "Coro::Multicore" by not enabling it be default and only enabling it in
172 your own threads:
173
174 use Coro;
175 use Coro::Multicore (); # note the () to disable by default
176
177 async {
178 Coro::Multicore::scoped_enable;
179
180 # do things asynchronously by calling perlmulticore-enabled functions
181 };
47 182
48 EXPORTS 183 EXPORTS
49 This module does not (at the moment) export any symbols. It does, 184 This module does not (at the moment) export any symbols. It does,
50 however, export "behaviour" - if you use the default import, then 185 however, export "behaviour" - if you use the default import, then
51 Coro::Multicore will be enabled for all threads and all callers in the 186 Coro::Multicore will be enabled for all threads and all callers in the
… …
73 is false) the multicore functionality globally. By default, it is 208 is false) the multicore functionality globally. By default, it is
74 enabled. 209 enabled.
75 210
76 This can be used to effectively disable this module's functionality 211 This can be used to effectively disable this module's functionality
77 by default, and enable it only for selected threads or scopes, by 212 by default, and enable it only for selected threads or scopes, by
78 calling "Coro::Multicore::scope_enable". 213 calling "Coro::Multicore::scoped_enable".
79 214
80 The function returns the previous value of the enable flag. 215 The function returns the previous value of the enable flag.
81 216
82 Coro::Multicore::scoped_enable 217 Coro::Multicore::scoped_enable
83 This function instructs Coro::Multicore to handle all requests 218 This function instructs Coro::Multicore to handle all requests
… …
88 at the moment, so don't nest them. 223 at the moment, so don't nest them.
89 224
90 Coro::Multicore::scoped_disable 225 Coro::Multicore::scoped_disable
91 The opposite of "Coro::Multicore::scope_disable": instructs 226 The opposite of "Coro::Multicore::scope_disable": instructs
92 Coro::Multicore to *not* handle the next multicore-enabled request. 227 Coro::Multicore to *not* handle the next multicore-enabled request.
228
229THREAD SAFETY OF SUPPORTING XS MODULES
230 Just because an XS module supports perlmulticore might not immediately
231 make it reentrant. For example, while you can (try to) call "execute" on
232 the same database handle for the patched "DBD::mysql" (see the registry
233 <http://perlmulticore.schmorp.de/registry>), this will almost certainly
234 not work, despite "DBD::mysql" and "libmysqlclient" being thread safe
235 and reentrant - just not on the same database handle.
236
237 Many modules have limitations such as these - some can only be called
238 concurrently from a single thread as they use global variables, some can
239 only be called concurrently on different *handles* (e.g. database
240 connections for DBD modules, or digest objects for Digest modules), and
241 some can be called at any time (such as the "md5" function in
242 "Digest::MD5").
243
244 Generally, you only have to be careful with the very few modules that
245 use global variables or rely on C libraries that aren't thread-safe,
246 which should be documented clearly in the module documentation.
247
248 Most modules are either perfectly reentrant, or at least reentrant as
249 long as you give every thread it's own *handle* object.
250
251EXCEPTIONS AND THREAD CANCELLATION
252 Coro allows you to cancel threads even when they execute within an XS
253 function ("cancel" vs. "cancel" methods). Similarly, Coro allows you to
254 send exceptions (e.g. via the "throw" method) to threads executing
255 inside an XS function.
256
257 While doing this is questionable and dangerous with normal Coro threads
258 already, they are both supported in this module, although with
259 potentially unwanted effects. The following describes the current
260 implementation and is subject to change. It is described primarily so
261 you can understand what went wrong, if things go wrong.
262
263 EXCEPTIONS
264 When a thread that has currently released the perl interpreter (e.g.
265 because it is executing a perlmulticore enabled XS function)
266 receives an exception, it will at first continue normally.
267
268 After acquiring the perl interpreter again, it will throw the
269 exception it previously received. More specifically, when a thread
270 calls "perlinterp_acquire ()" and has received an exception, then
271 "perlinterp_acquire ()" will not return but instead "die".
272
273 Most code that has been updated for perlmulticore support will not
274 expect this, and might leave internal state corrupted to some
275 extent.
276
277 CANCELLATION
278 Unsafe cancellation on a thread that has released the perl
279 interpreter frees its resources, but let's the XS code continue at
280 first. This should not lead to corruption on the perl level, as the
281 code isn't allowed to touch perl data structures until it reacquires
282 the interpreter.
283
284 The call to "perlinterp_acquire ()" will then block indefinitely,
285 leaking the (OS level) thread.
286
287 Safe cancellation will simply fail in this case, so is still "safe"
288 to call.
93 289
94INTERACTION WITH OTHER SOFTWARE 290INTERACTION WITH OTHER SOFTWARE
95 This module is very similar to other environments where perl 291 This module is very similar to other environments where perl
96 interpreters are moved between threads, such as mod_perl2, and the same 292 interpreters are moved between threads, such as mod_perl2, and the same
97 caveats apply. 293 caveats apply.
… …
132 Future versions of this module might do this automatically. 328 Future versions of this module might do this automatically.
133 329
134BUGS 330BUGS
135 (OS-) threads are never released 331 (OS-) threads are never released
136 At the moment, threads that were created once will never be freed. 332 At the moment, threads that were created once will never be freed.
137 They will be reused for asynchronous requests, though, so a slong as 333 They will be reused for asynchronous requests, though, so as long as
138 you limit the maximum number of concurrent asynchronous tasks, this 334 you limit the maximum number of concurrent asynchronous tasks, this
139 will also limit the maximum number of threads created. 335 will also limit the maximum number of threads created.
336
337 The idle threads are not necessarily using a lot of resources: on
338 GNU/Linux + glibc, each thread takes about 8KiB of userspace memory
339 + whatever the kernel needs (probably less than 8KiB).
140 340
141 Future versions will likely lift this limitation. 341 Future versions will likely lift this limitation.
142 342
143 AnyEvent is initalised at module load time 343 AnyEvent is initalised at module load time
144 AnyEvent is initialised on module load, as opposed to at a later 344 AnyEvent is initialised on module load, as opposed to at a later

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