Commit | Line | Data |
2605996a |
1 | =head1 NAME |
2 | |
3 | perlthrtut - tutorial on threads in Perl |
4 | |
5 | =head1 DESCRIPTION |
6 | |
53d7eaa8 |
7 | B<NOTE>: this tutorial describes the new Perl threading flavour |
9316ed2f |
8 | introduced in Perl 5.6.0 called interpreter threads, or B<ithreads> |
9 | for short. In this model each thread runs in its own Perl interpreter, |
10 | and any data sharing between threads must be explicit. |
11 | |
12 | There is another older Perl threading flavour called the 5.005 model, |
13 | unsurprisingly for 5.005 versions of Perl. The old model is known to |
14 | have problems, deprecated, and will probably be removed around release |
15 | 5.10. You are strongly encouraged to migrate any existing 5.005 |
16 | threads code to the new model as soon as possible. |
2a4bf773 |
17 | |
53d7eaa8 |
18 | You can see which (or neither) threading flavour you have by |
6eded8f3 |
19 | running C<perl -V> and looking at the C<Platform> section. |
53d7eaa8 |
20 | If you have C<useithreads=define> you have ithreads, if you |
21 | have C<use5005threads=define> you have 5.005 threads. |
22 | If you have neither, you don't have any thread support built in. |
23 | If you have both, you are in trouble. |
2605996a |
24 | |
bfce6503 |
25 | The user-level interface to the 5.005 threads was via the L<Threads> |
26 | class, while ithreads uses the L<threads> class. Note the change in case. |
27 | |
28 | =head1 Status |
29 | |
30 | The ithreads code has been available since Perl 5.6.0, and is considered |
31 | stable. The user-level interface to ithreads (the L<threads> classes) |
9e75ef81 |
32 | appeared in the 5.8.0 release, and as of this time is considered stable |
33 | although it should be treated with caution as with all new features. |
2605996a |
34 | |
c975c451 |
35 | =head1 What Is A Thread Anyway? |
36 | |
37 | A thread is a flow of control through a program with a single |
38 | execution point. |
39 | |
40 | Sounds an awful lot like a process, doesn't it? Well, it should. |
41 | Threads are one of the pieces of a process. Every process has at least |
42 | one thread and, up until now, every process running Perl had only one |
43 | thread. With 5.8, though, you can create extra threads. We're going |
44 | to show you how, when, and why. |
45 | |
46 | =head1 Threaded Program Models |
47 | |
48 | There are three basic ways that you can structure a threaded |
49 | program. Which model you choose depends on what you need your program |
50 | to do. For many non-trivial threaded programs you'll need to choose |
51 | different models for different pieces of your program. |
52 | |
53 | =head2 Boss/Worker |
54 | |
55 | The boss/worker model usually has one `boss' thread and one or more |
56 | `worker' threads. The boss thread gathers or generates tasks that need |
57 | to be done, then parcels those tasks out to the appropriate worker |
58 | thread. |
59 | |
60 | This model is common in GUI and server programs, where a main thread |
61 | waits for some event and then passes that event to the appropriate |
62 | worker threads for processing. Once the event has been passed on, the |
63 | boss thread goes back to waiting for another event. |
64 | |
65 | The boss thread does relatively little work. While tasks aren't |
66 | necessarily performed faster than with any other method, it tends to |
67 | have the best user-response times. |
68 | |
69 | =head2 Work Crew |
70 | |
71 | In the work crew model, several threads are created that do |
72 | essentially the same thing to different pieces of data. It closely |
73 | mirrors classical parallel processing and vector processors, where a |
74 | large array of processors do the exact same thing to many pieces of |
75 | data. |
76 | |
77 | This model is particularly useful if the system running the program |
78 | will distribute multiple threads across different processors. It can |
79 | also be useful in ray tracing or rendering engines, where the |
80 | individual threads can pass on interim results to give the user visual |
81 | feedback. |
82 | |
83 | =head2 Pipeline |
84 | |
85 | The pipeline model divides up a task into a series of steps, and |
86 | passes the results of one step on to the thread processing the |
87 | next. Each thread does one thing to each piece of data and passes the |
88 | results to the next thread in line. |
89 | |
90 | This model makes the most sense if you have multiple processors so two |
91 | or more threads will be executing in parallel, though it can often |
92 | make sense in other contexts as well. It tends to keep the individual |
93 | tasks small and simple, as well as allowing some parts of the pipeline |
94 | to block (on I/O or system calls, for example) while other parts keep |
95 | going. If you're running different parts of the pipeline on different |
96 | processors you may also take advantage of the caches on each |
97 | processor. |
98 | |
99 | This model is also handy for a form of recursive programming where, |
100 | rather than having a subroutine call itself, it instead creates |
101 | another thread. Prime and Fibonacci generators both map well to this |
102 | form of the pipeline model. (A version of a prime number generator is |
103 | presented later on.) |
104 | |
bfce6503 |
105 | =head1 What kind of threads are Perl threads? |
c975c451 |
106 | |
107 | If you have experience with other thread implementations, you might |
108 | find that things aren't quite what you expect. It's very important to |
109 | remember when dealing with Perl threads that Perl Threads Are Not X |
110 | Threads, for all values of X. They aren't POSIX threads, or |
111 | DecThreads, or Java's Green threads, or Win32 threads. There are |
112 | similarities, and the broad concepts are the same, but if you start |
113 | looking for implementation details you're going to be either |
114 | disappointed or confused. Possibly both. |
115 | |
116 | This is not to say that Perl threads are completely different from |
117 | everything that's ever come before--they're not. Perl's threading |
118 | model owes a lot to other thread models, especially POSIX. Just as |
119 | Perl is not C, though, Perl threads are not POSIX threads. So if you |
120 | find yourself looking for mutexes, or thread priorities, it's time to |
121 | step back a bit and think about what you want to do and how Perl can |
122 | do it. |
123 | |
6eded8f3 |
124 | However it is important to remember that Perl threads cannot magically |
c975c451 |
125 | do things unless your operating systems threads allows it. So if your |
bfce6503 |
126 | system blocks the entire process on sleep(), Perl usually will as well. |
c975c451 |
127 | |
9316ed2f |
128 | Perl Threads Are Different. |
129 | |
cf5baa48 |
130 | =head1 Thread-Safe Modules |
c975c451 |
131 | |
cf5baa48 |
132 | The addition of threads has changed Perl's internals |
c975c451 |
133 | substantially. There are implications for people who write |
cf5baa48 |
134 | modules with XS code or external libraries. However, since perl data is |
135 | not shared among threads by default, Perl modules stand a high chance of |
136 | being thread-safe or can be made thread-safe easily. Modules that are not |
137 | tagged as thread-safe should be tested or code reviewed before being used |
138 | in production code. |
c975c451 |
139 | |
140 | Not all modules that you might use are thread-safe, and you should |
141 | always assume a module is unsafe unless the documentation says |
142 | otherwise. This includes modules that are distributed as part of the |
143 | core. Threads are a new feature, and even some of the standard |
bfce6503 |
144 | modules aren't thread-safe. |
c975c451 |
145 | |
cf5baa48 |
146 | Even if a module is thread-safe, it doesn't mean that the module is optimized |
6eded8f3 |
147 | to work well with threads. A module could possibly be rewritten to utilize |
148 | the new features in threaded Perl to increase performance in a threaded |
149 | environment. |
c975c451 |
150 | |
151 | If you're using a module that's not thread-safe for some reason, you |
cf5baa48 |
152 | can protect yourself by using it from one, and only one thread at all. |
153 | If you need multiple threads to access such a module, you can use semaphores and |
154 | lots of programming discipline to control access to it. Semaphores |
155 | are covered in L</"Basic semaphores">. |
9316ed2f |
156 | |
cf5baa48 |
157 | See also L</"Thread-Safety of System Libraries">. |
c975c451 |
158 | |
159 | =head1 Thread Basics |
160 | |
161 | The core L<threads> module provides the basic functions you need to write |
162 | threaded programs. In the following sections we'll cover the basics, |
163 | showing you what you need to do to create a threaded program. After |
164 | that, we'll go over some of the features of the L<threads> module that |
165 | make threaded programming easier. |
166 | |
167 | =head2 Basic Thread Support |
168 | |
6eded8f3 |
169 | Thread support is a Perl compile-time option - it's something that's |
c975c451 |
170 | turned on or off when Perl is built at your site, rather than when |
171 | your programs are compiled. If your Perl wasn't compiled with thread |
172 | support enabled, then any attempt to use threads will fail. |
173 | |
c975c451 |
174 | Your programs can use the Config module to check whether threads are |
175 | enabled. If your program can't run without them, you can say something |
176 | like: |
177 | |
9316ed2f |
178 | $Config{useithreads} or die "Recompile Perl with threads to run this program."; |
c975c451 |
179 | |
180 | A possibly-threaded program using a possibly-threaded module might |
181 | have code like this: |
182 | |
cf5baa48 |
183 | use Config; |
184 | use MyMod; |
c975c451 |
185 | |
9316ed2f |
186 | BEGIN { |
cf5baa48 |
187 | if ($Config{useithreads}) { |
188 | # We have threads |
189 | require MyMod_threaded; |
190 | import MyMod_threaded; |
191 | } else { |
192 | require MyMod_unthreaded; |
193 | import MyMod_unthreaded; |
9316ed2f |
194 | } |
cf5baa48 |
195 | } |
c975c451 |
196 | |
197 | Since code that runs both with and without threads is usually pretty |
198 | messy, it's best to isolate the thread-specific code in its own |
199 | module. In our example above, that's what MyMod_threaded is, and it's |
200 | only imported if we're running on a threaded Perl. |
201 | |
8f95bfb9 |
202 | =head2 A Note about the Examples |
203 | |
204 | Although thread support is considered to be stable, there are still a number |
205 | of quirks that may startle you when you try out any of the examples below. |
206 | In a real situation, care should be taken that all threads are finished |
207 | executing before the program exits. That care has B<not> been taken in these |
208 | examples in the interest of simplicity. Running these examples "as is" will |
209 | produce error messages, usually caused by the fact that there are still |
210 | threads running when the program exits. You should not be alarmed by this. |
211 | Future versions of Perl may fix this problem. |
212 | |
c975c451 |
213 | =head2 Creating Threads |
214 | |
215 | The L<threads> package provides the tools you need to create new |
9e75ef81 |
216 | threads. Like any other module, you need to tell Perl that you want to use |
c975c451 |
217 | it; C<use threads> imports all the pieces you need to create basic |
218 | threads. |
219 | |
9e75ef81 |
220 | The simplest, most straightforward way to create a thread is with new(): |
c975c451 |
221 | |
222 | use threads; |
223 | |
224 | $thr = threads->new(\&sub1); |
225 | |
226 | sub sub1 { |
227 | print "In the thread\n"; |
228 | } |
229 | |
230 | The new() method takes a reference to a subroutine and creates a new |
231 | thread, which starts executing in the referenced subroutine. Control |
232 | then passes both to the subroutine and the caller. |
233 | |
234 | If you need to, your program can pass parameters to the subroutine as |
235 | part of the thread startup. Just include the list of parameters as |
236 | part of the C<threads::new> call, like this: |
237 | |
238 | use threads; |
bfce6503 |
239 | |
c975c451 |
240 | $Param3 = "foo"; |
241 | $thr = threads->new(\&sub1, "Param 1", "Param 2", $Param3); |
242 | $thr = threads->new(\&sub1, @ParamList); |
8f95bfb9 |
243 | $thr = threads->new(\&sub1, qw(Param1 Param2 Param3)); |
c975c451 |
244 | |
245 | sub sub1 { |
246 | my @InboundParameters = @_; |
247 | print "In the thread\n"; |
248 | print "got parameters >", join("<>", @InboundParameters), "<\n"; |
249 | } |
250 | |
251 | |
252 | The last example illustrates another feature of threads. You can spawn |
253 | off several threads using the same subroutine. Each thread executes |
254 | the same subroutine, but in a separate thread with a separate |
255 | environment and potentially separate arguments. |
256 | |
9316ed2f |
257 | C<create()> is a synonym for C<new()>. |
bfce6503 |
258 | |
c975c451 |
259 | =head2 Waiting For A Thread To Exit |
260 | |
261 | Since threads are also subroutines, they can return values. To wait |
6eded8f3 |
262 | for a thread to exit and extract any values it might return, you can |
263 | use the join() method: |
c975c451 |
264 | |
265 | use threads; |
bfce6503 |
266 | |
c975c451 |
267 | $thr = threads->new(\&sub1); |
268 | |
269 | @ReturnData = $thr->join; |
270 | print "Thread returned @ReturnData"; |
271 | |
272 | sub sub1 { return "Fifty-six", "foo", 2; } |
273 | |
274 | In the example above, the join() method returns as soon as the thread |
275 | ends. In addition to waiting for a thread to finish and gathering up |
276 | any values that the thread might have returned, join() also performs |
277 | any OS cleanup necessary for the thread. That cleanup might be |
278 | important, especially for long-running programs that spawn lots of |
279 | threads. If you don't want the return values and don't want to wait |
280 | for the thread to finish, you should call the detach() method |
bfce6503 |
281 | instead, as described next. |
c975c451 |
282 | |
283 | =head2 Ignoring A Thread |
284 | |
285 | join() does three things: it waits for a thread to exit, cleans up |
286 | after it, and returns any data the thread may have produced. But what |
287 | if you're not interested in the thread's return values, and you don't |
288 | really care when the thread finishes? All you want is for the thread |
289 | to get cleaned up after when it's done. |
290 | |
291 | In this case, you use the detach() method. Once a thread is detached, |
292 | it'll run until it's finished, then Perl will clean up after it |
293 | automatically. |
294 | |
295 | use threads; |
bfce6503 |
296 | |
6eded8f3 |
297 | $thr = threads->new(\&sub1); # Spawn the thread |
c975c451 |
298 | |
299 | $thr->detach; # Now we officially don't care any more |
300 | |
cf5baa48 |
301 | sub sub1 { |
c975c451 |
302 | $a = 0; |
303 | while (1) { |
304 | $a++; |
305 | print "\$a is $a\n"; |
306 | sleep 1; |
307 | } |
308 | } |
309 | |
bfce6503 |
310 | Once a thread is detached, it may not be joined, and any return data |
311 | that it might have produced (if it was done and waiting for a join) is |
c975c451 |
312 | lost. |
313 | |
314 | =head1 Threads And Data |
315 | |
316 | Now that we've covered the basics of threads, it's time for our next |
317 | topic: data. Threading introduces a couple of complications to data |
318 | access that non-threaded programs never need to worry about. |
319 | |
320 | =head2 Shared And Unshared Data |
321 | |
bfce6503 |
322 | The biggest difference between Perl ithreads and the old 5.005 style |
323 | threading, or for that matter, to most other threading systems out there, |
324 | is that by default, no data is shared. When a new perl thread is created, |
325 | all the data associated with the current thread is copied to the new |
326 | thread, and is subsequently private to that new thread! |
327 | This is similar in feel to what happens when a UNIX process forks, |
328 | except that in this case, the data is just copied to a different part of |
329 | memory within the same process rather than a real fork taking place. |
c975c451 |
330 | |
9e75ef81 |
331 | To make use of threading however, one usually wants the threads to share |
bfce6503 |
332 | at least some data between themselves. This is done with the |
333 | L<threads::shared> module and the C< : shared> attribute: |
334 | |
335 | use threads; |
336 | use threads::shared; |
337 | |
338 | my $foo : shared = 1; |
339 | my $bar = 1; |
340 | threads->new(sub { $foo++; $bar++ })->join; |
818c4caa |
341 | |
bfce6503 |
342 | print "$foo\n"; #prints 2 since $foo is shared |
343 | print "$bar\n"; #prints 1 since $bar is not shared |
344 | |
345 | In the case of a shared array, all the array's elements are shared, and for |
346 | a shared hash, all the keys and values are shared. This places |
347 | restrictions on what may be assigned to shared array and hash elements: only |
348 | simple values or references to shared variables are allowed - this is |
f3278b06 |
349 | so that a private variable can't accidentally become shared. A bad |
bfce6503 |
350 | assignment will cause the thread to die. For example: |
351 | |
352 | use threads; |
353 | use threads::shared; |
354 | |
355 | my $var = 1; |
356 | my $svar : shared = 2; |
357 | my %hash : shared; |
358 | |
359 | ... create some threads ... |
360 | |
361 | $hash{a} = 1; # all threads see exists($hash{a}) and $hash{a} == 1 |
56ca1794 |
362 | $hash{a} = $var # okay - copy-by-value: same effect as previous |
363 | $hash{a} = $svar # okay - copy-by-value: same effect as previous |
bfce6503 |
364 | $hash{a} = \$svar # okay - a reference to a shared variable |
365 | $hash{a} = \$var # This will die |
366 | delete $hash{a} # okay - all threads will see !exists($hash{a}) |
367 | |
368 | Note that a shared variable guarantees that if two or more threads try to |
369 | modify it at the same time, the internal state of the variable will not |
370 | become corrupted. However, there are no guarantees beyond this, as |
371 | explained in the next section. |
c975c451 |
372 | |
6eded8f3 |
373 | =head2 Thread Pitfalls: Races |
c975c451 |
374 | |
375 | While threads bring a new set of useful tools, they also bring a |
376 | number of pitfalls. One pitfall is the race condition: |
377 | |
378 | use threads; |
379 | use threads::shared; |
bfce6503 |
380 | |
c975c451 |
381 | my $a : shared = 1; |
382 | $thr1 = threads->new(\&sub1); |
383 | $thr2 = threads->new(\&sub2); |
384 | |
385 | $thr1->join; |
386 | $thr2->join; |
387 | print "$a\n"; |
388 | |
bfce6503 |
389 | sub sub1 { my $foo = $a; $a = $foo + 1; } |
390 | sub sub2 { my $bar = $a; $a = $bar + 1; } |
c975c451 |
391 | |
392 | What do you think $a will be? The answer, unfortunately, is "it |
393 | depends." Both sub1() and sub2() access the global variable $a, once |
394 | to read and once to write. Depending on factors ranging from your |
395 | thread implementation's scheduling algorithm to the phase of the moon, |
396 | $a can be 2 or 3. |
397 | |
398 | Race conditions are caused by unsynchronized access to shared |
399 | data. Without explicit synchronization, there's no way to be sure that |
400 | nothing has happened to the shared data between the time you access it |
401 | and the time you update it. Even this simple code fragment has the |
402 | possibility of error: |
403 | |
404 | use threads; |
405 | my $a : shared = 2; |
406 | my $b : shared; |
407 | my $c : shared; |
408 | my $thr1 = threads->create(sub { $b = $a; $a = $b + 1; }); |
409 | my $thr2 = threads->create(sub { $c = $a; $a = $c + 1; }); |
8f95bfb9 |
410 | $thr1->join; |
411 | $thr2->join; |
c975c451 |
412 | |
413 | Two threads both access $a. Each thread can potentially be interrupted |
414 | at any point, or be executed in any order. At the end, $a could be 3 |
415 | or 4, and both $b and $c could be 2 or 3. |
416 | |
bfce6503 |
417 | Even C<$a += 5> or C<$a++> are not guaranteed to be atomic. |
418 | |
c975c451 |
419 | Whenever your program accesses data or resources that can be accessed |
420 | by other threads, you must take steps to coordinate access or risk |
bfce6503 |
421 | data inconsistency and race conditions. Note that Perl will protect its |
422 | internals from your race conditions, but it won't protect you from you. |
423 | |
f3278b06 |
424 | =head1 Synchronization and control |
bfce6503 |
425 | |
426 | Perl provides a number of mechanisms to coordinate the interactions |
427 | between themselves and their data, to avoid race conditions and the like. |
428 | Some of these are designed to resemble the common techniques used in thread |
429 | libraries such as C<pthreads>; others are Perl-specific. Often, the |
9e75ef81 |
430 | standard techniques are clumsy and difficult to get right (such as |
bfce6503 |
431 | condition waits). Where possible, it is usually easier to use Perlish |
432 | techniques such as queues, which remove some of the hard work involved. |
c975c451 |
433 | |
434 | =head2 Controlling access: lock() |
435 | |
436 | The lock() function takes a shared variable and puts a lock on it. |
a6d05634 |
437 | No other thread may lock the variable until the variable is unlocked |
bfce6503 |
438 | by the thread holding the lock. Unlocking happens automatically |
8f95bfb9 |
439 | when the locking thread exits the outermost block that contains |
bfce6503 |
440 | C<lock()> function. Using lock() is straightforward: this example has |
f3278b06 |
441 | several threads doing some calculations in parallel, and occasionally |
bfce6503 |
442 | updating a running total: |
443 | |
444 | use threads; |
445 | use threads::shared; |
446 | |
447 | my $total : shared = 0; |
448 | |
449 | sub calc { |
450 | for (;;) { |
451 | my $result; |
452 | # (... do some calculations and set $result ...) |
453 | { |
454 | lock($total); # block until we obtain the lock |
8f95bfb9 |
455 | $total += $result; |
f3278b06 |
456 | } # lock implicitly released at end of scope |
bfce6503 |
457 | last if $result == 0; |
458 | } |
459 | } |
460 | |
461 | my $thr1 = threads->new(\&calc); |
462 | my $thr2 = threads->new(\&calc); |
463 | my $thr3 = threads->new(\&calc); |
464 | $thr1->join; |
465 | $thr2->join; |
466 | $thr3->join; |
467 | print "total=$total\n"; |
c975c451 |
468 | |
c975c451 |
469 | |
470 | lock() blocks the thread until the variable being locked is |
471 | available. When lock() returns, your thread can be sure that no other |
bfce6503 |
472 | thread can lock that variable until the outermost block containing the |
c975c451 |
473 | lock exits. |
474 | |
475 | It's important to note that locks don't prevent access to the variable |
476 | in question, only lock attempts. This is in keeping with Perl's |
477 | longstanding tradition of courteous programming, and the advisory file |
478 | locking that flock() gives you. |
479 | |
480 | You may lock arrays and hashes as well as scalars. Locking an array, |
481 | though, will not block subsequent locks on array elements, just lock |
482 | attempts on the array itself. |
483 | |
bfce6503 |
484 | Locks are recursive, which means it's okay for a thread to |
c975c451 |
485 | lock a variable more than once. The lock will last until the outermost |
bfce6503 |
486 | lock() on the variable goes out of scope. For example: |
487 | |
488 | my $x : shared; |
489 | doit(); |
490 | |
491 | sub doit { |
492 | { |
493 | { |
494 | lock($x); # wait for lock |
8f95bfb9 |
495 | lock($x); # NOOP - we already have the lock |
bfce6503 |
496 | { |
497 | lock($x); # NOOP |
498 | { |
499 | lock($x); # NOOP |
500 | lockit_some_more(); |
501 | } |
502 | } |
503 | } # *** implicit unlock here *** |
504 | } |
505 | } |
506 | |
507 | sub lockit_some_more { |
508 | lock($x); # NOOP |
509 | } # nothing happens here |
510 | |
511 | Note that there is no unlock() function - the only way to unlock a |
512 | variable is to allow it to go out of scope. |
513 | |
514 | A lock can either be used to guard the data contained within the variable |
515 | being locked, or it can be used to guard something else, like a section |
516 | of code. In this latter case, the variable in question does not hold any |
517 | useful data, and exists only for the purpose of being locked. In this |
518 | respect, the variable behaves like the mutexes and basic semaphores of |
519 | traditional thread libraries. |
c975c451 |
520 | |
bfce6503 |
521 | =head2 A Thread Pitfall: Deadlocks |
c975c451 |
522 | |
bfce6503 |
523 | Locks are a handy tool to synchronize access to data, and using them |
c975c451 |
524 | properly is the key to safe shared data. Unfortunately, locks aren't |
f3278b06 |
525 | without their dangers, especially when multiple locks are involved. |
bfce6503 |
526 | Consider the following code: |
c975c451 |
527 | |
528 | use threads; |
bfce6503 |
529 | |
c975c451 |
530 | my $a : shared = 4; |
531 | my $b : shared = "foo"; |
532 | my $thr1 = threads->new(sub { |
533 | lock($a); |
c975c451 |
534 | sleep 20; |
bfce6503 |
535 | lock($b); |
c975c451 |
536 | }); |
537 | my $thr2 = threads->new(sub { |
538 | lock($b); |
c975c451 |
539 | sleep 20; |
bfce6503 |
540 | lock($a); |
c975c451 |
541 | }); |
542 | |
543 | This program will probably hang until you kill it. The only way it |
bfce6503 |
544 | won't hang is if one of the two threads acquires both locks |
c975c451 |
545 | first. A guaranteed-to-hang version is more complicated, but the |
546 | principle is the same. |
547 | |
bfce6503 |
548 | The first thread will grab a lock on $a, then, after a pause during which |
549 | the second thread has probably had time to do some work, try to grab a |
550 | lock on $b. Meanwhile, the second thread grabs a lock on $b, then later |
551 | tries to grab a lock on $a. The second lock attempt for both threads will |
552 | block, each waiting for the other to release its lock. |
c975c451 |
553 | |
554 | This condition is called a deadlock, and it occurs whenever two or |
555 | more threads are trying to get locks on resources that the others |
556 | own. Each thread will block, waiting for the other to release a lock |
557 | on a resource. That never happens, though, since the thread with the |
558 | resource is itself waiting for a lock to be released. |
559 | |
560 | There are a number of ways to handle this sort of problem. The best |
561 | way is to always have all threads acquire locks in the exact same |
562 | order. If, for example, you lock variables $a, $b, and $c, always lock |
563 | $a before $b, and $b before $c. It's also best to hold on to locks for |
564 | as short a period of time to minimize the risks of deadlock. |
565 | |
48b96218 |
566 | The other synchronization primitives described below can suffer from |
bfce6503 |
567 | similar problems. |
568 | |
c975c451 |
569 | =head2 Queues: Passing Data Around |
570 | |
571 | A queue is a special thread-safe object that lets you put data in one |
572 | end and take it out the other without having to worry about |
573 | synchronization issues. They're pretty straightforward, and look like |
574 | this: |
575 | |
576 | use threads; |
83272a45 |
577 | use Thread::Queue; |
c975c451 |
578 | |
83272a45 |
579 | my $DataQueue = Thread::Queue->new; |
c975c451 |
580 | $thr = threads->new(sub { |
581 | while ($DataElement = $DataQueue->dequeue) { |
582 | print "Popped $DataElement off the queue\n"; |
583 | } |
584 | }); |
585 | |
586 | $DataQueue->enqueue(12); |
587 | $DataQueue->enqueue("A", "B", "C"); |
588 | $DataQueue->enqueue(\$thr); |
589 | sleep 10; |
590 | $DataQueue->enqueue(undef); |
8f95bfb9 |
591 | $thr->join; |
c975c451 |
592 | |
83272a45 |
593 | You create the queue with C<new Thread::Queue>. Then you can |
6eded8f3 |
594 | add lists of scalars onto the end with enqueue(), and pop scalars off |
595 | the front of it with dequeue(). A queue has no fixed size, and can grow |
596 | as needed to hold everything pushed on to it. |
c975c451 |
597 | |
598 | If a queue is empty, dequeue() blocks until another thread enqueues |
599 | something. This makes queues ideal for event loops and other |
600 | communications between threads. |
601 | |
c975c451 |
602 | =head2 Semaphores: Synchronizing Data Access |
603 | |
bfce6503 |
604 | Semaphores are a kind of generic locking mechanism. In their most basic |
fa11829f |
605 | form, they behave very much like lockable scalars, except that they |
bfce6503 |
606 | can't hold data, and that they must be explicitly unlocked. In their |
607 | advanced form, they act like a kind of counter, and can allow multiple |
608 | threads to have the 'lock' at any one time. |
2605996a |
609 | |
bfce6503 |
610 | =head2 Basic semaphores |
2605996a |
611 | |
bfce6503 |
612 | Semaphores have two methods, down() and up(): down() decrements the resource |
613 | count, while up increments it. Calls to down() will block if the |
c975c451 |
614 | semaphore's current count would decrement below zero. This program |
615 | gives a quick demonstration: |
616 | |
536bca94 |
617 | use threads; |
83272a45 |
618 | use Thread::Semaphore; |
bfce6503 |
619 | |
83272a45 |
620 | my $semaphore = new Thread::Semaphore; |
bfce6503 |
621 | my $GlobalVariable : shared = 0; |
2605996a |
622 | |
c975c451 |
623 | $thr1 = new threads \&sample_sub, 1; |
624 | $thr2 = new threads \&sample_sub, 2; |
625 | $thr3 = new threads \&sample_sub, 3; |
2605996a |
626 | |
c975c451 |
627 | sub sample_sub { |
628 | my $SubNumber = shift @_; |
629 | my $TryCount = 10; |
630 | my $LocalCopy; |
631 | sleep 1; |
632 | while ($TryCount--) { |
633 | $semaphore->down; |
634 | $LocalCopy = $GlobalVariable; |
635 | print "$TryCount tries left for sub $SubNumber (\$GlobalVariable is $GlobalVariable)\n"; |
c975c451 |
636 | sleep 2; |
637 | $LocalCopy++; |
638 | $GlobalVariable = $LocalCopy; |
639 | $semaphore->up; |
640 | } |
641 | } |
6eded8f3 |
642 | |
8f95bfb9 |
643 | $thr1->join; |
644 | $thr2->join; |
645 | $thr3->join; |
2605996a |
646 | |
c975c451 |
647 | The three invocations of the subroutine all operate in sync. The |
648 | semaphore, though, makes sure that only one thread is accessing the |
649 | global variable at once. |
2605996a |
650 | |
bfce6503 |
651 | =head2 Advanced Semaphores |
2605996a |
652 | |
c975c451 |
653 | By default, semaphores behave like locks, letting only one thread |
654 | down() them at a time. However, there are other uses for semaphores. |
2605996a |
655 | |
6eded8f3 |
656 | Each semaphore has a counter attached to it. By default, semaphores are |
657 | created with the counter set to one, down() decrements the counter by |
658 | one, and up() increments by one. However, we can override any or all |
659 | of these defaults simply by passing in different values: |
660 | |
661 | use threads; |
83272a45 |
662 | use Thread::Semaphore; |
663 | my $semaphore = Thread::Semaphore->new(5); |
6eded8f3 |
664 | # Creates a semaphore with the counter set to five |
665 | |
666 | $thr1 = threads->new(\&sub1); |
667 | $thr2 = threads->new(\&sub1); |
668 | |
669 | sub sub1 { |
670 | $semaphore->down(5); # Decrements the counter by five |
671 | # Do stuff here |
672 | $semaphore->up(5); # Increment the counter by five |
673 | } |
674 | |
8f95bfb9 |
675 | $thr1->detach; |
676 | $thr2->detach; |
6eded8f3 |
677 | |
678 | If down() attempts to decrement the counter below zero, it blocks until |
679 | the counter is large enough. Note that while a semaphore can be created |
680 | with a starting count of zero, any up() or down() always changes the |
681 | counter by at least one, and so $semaphore->down(0) is the same as |
682 | $semaphore->down(1). |
2605996a |
683 | |
c975c451 |
684 | The question, of course, is why would you do something like this? Why |
685 | create a semaphore with a starting count that's not one, or why |
686 | decrement/increment it by more than one? The answer is resource |
687 | availability. Many resources that you want to manage access for can be |
688 | safely used by more than one thread at once. |
2605996a |
689 | |
c975c451 |
690 | For example, let's take a GUI driven program. It has a semaphore that |
691 | it uses to synchronize access to the display, so only one thread is |
692 | ever drawing at once. Handy, but of course you don't want any thread |
693 | to start drawing until things are properly set up. In this case, you |
694 | can create a semaphore with a counter set to zero, and up it when |
695 | things are ready for drawing. |
2605996a |
696 | |
c975c451 |
697 | Semaphores with counters greater than one are also useful for |
698 | establishing quotas. Say, for example, that you have a number of |
699 | threads that can do I/O at once. You don't want all the threads |
700 | reading or writing at once though, since that can potentially swamp |
701 | your I/O channels, or deplete your process' quota of filehandles. You |
702 | can use a semaphore initialized to the number of concurrent I/O |
703 | requests (or open files) that you want at any one time, and have your |
704 | threads quietly block and unblock themselves. |
2605996a |
705 | |
c975c451 |
706 | Larger increments or decrements are handy in those cases where a |
707 | thread needs to check out or return a number of resources at once. |
2605996a |
708 | |
bfce6503 |
709 | =head2 cond_wait() and cond_signal() |
710 | |
711 | These two functions can be used in conjunction with locks to notify |
712 | co-operating threads that a resource has become available. They are |
713 | very similar in use to the functions found in C<pthreads>. However |
714 | for most purposes, queues are simpler to use and more intuitive. See |
715 | L<threads::shared> for more details. |
2605996a |
716 | |
536bca94 |
717 | =head2 Giving up control |
718 | |
719 | There are times when you may find it useful to have a thread |
720 | explicitly give up the CPU to another thread. You may be doing something |
721 | processor-intensive and want to make sure that the user-interface thread |
722 | gets called frequently. Regardless, there are times that you might want |
723 | a thread to give up the processor. |
724 | |
725 | Perl's threading package provides the yield() function that does |
726 | this. yield() is pretty straightforward, and works like this: |
727 | |
728 | use threads; |
729 | |
730 | sub loop { |
731 | my $thread = shift; |
732 | my $foo = 50; |
733 | while($foo--) { print "in thread $thread\n" } |
734 | threads->yield; |
735 | $foo = 50; |
736 | while($foo--) { print "in thread $thread\n" } |
737 | } |
738 | |
739 | my $thread1 = threads->new(\&loop, 'first'); |
740 | my $thread2 = threads->new(\&loop, 'second'); |
741 | my $thread3 = threads->new(\&loop, 'third'); |
742 | |
743 | It is important to remember that yield() is only a hint to give up the CPU, |
744 | it depends on your hardware, OS and threading libraries what actually happens. |
745 | B<On many operating systems, yield() is a no-op.> Therefore it is important |
746 | to note that one should not build the scheduling of the threads around |
747 | yield() calls. It might work on your platform but it won't work on another |
748 | platform. |
749 | |
c975c451 |
750 | =head1 General Thread Utility Routines |
751 | |
752 | We've covered the workhorse parts of Perl's threading package, and |
753 | with these tools you should be well on your way to writing threaded |
754 | code and packages. There are a few useful little pieces that didn't |
755 | really fit in anyplace else. |
756 | |
757 | =head2 What Thread Am I In? |
758 | |
bfce6503 |
759 | The C<< threads->self >> class method provides your program with a way to |
760 | get an object representing the thread it's currently in. You can use this |
6eded8f3 |
761 | object in the same way as the ones returned from thread creation. |
c975c451 |
762 | |
763 | =head2 Thread IDs |
764 | |
765 | tid() is a thread object method that returns the thread ID of the |
766 | thread the object represents. Thread IDs are integers, with the main |
767 | thread in a program being 0. Currently Perl assigns a unique tid to |
768 | every thread ever created in your program, assigning the first thread |
769 | to be created a tid of 1, and increasing the tid by 1 for each new |
770 | thread that's created. |
771 | |
772 | =head2 Are These Threads The Same? |
773 | |
774 | The equal() method takes two thread objects and returns true |
775 | if the objects represent the same thread, and false if they don't. |
776 | |
777 | Thread objects also have an overloaded == comparison so that you can do |
778 | comparison on them as you would with normal objects. |
779 | |
780 | =head2 What Threads Are Running? |
781 | |
bfce6503 |
782 | C<< threads->list >> returns a list of thread objects, one for each thread |
c975c451 |
783 | that's currently running and not detached. Handy for a number of things, |
784 | including cleaning up at the end of your program: |
785 | |
786 | # Loop through all the threads |
787 | foreach $thr (threads->list) { |
788 | # Don't join the main thread or ourselves |
789 | if ($thr->tid && !threads::equal($thr, threads->self)) { |
790 | $thr->join; |
791 | } |
792 | } |
793 | |
bfce6503 |
794 | If some threads have not finished running when the main Perl thread |
795 | ends, Perl will warn you about it and die, since it is impossible for Perl |
6eded8f3 |
796 | to clean up itself while other threads are running |
c975c451 |
797 | |
798 | =head1 A Complete Example |
799 | |
800 | Confused yet? It's time for an example program to show some of the |
801 | things we've covered. This program finds prime numbers using threads. |
802 | |
803 | 1 #!/usr/bin/perl -w |
804 | 2 # prime-pthread, courtesy of Tom Christiansen |
805 | 3 |
806 | 4 use strict; |
807 | 5 |
808 | 6 use threads; |
83272a45 |
809 | 7 use Thread::Queue; |
c975c451 |
810 | 8 |
83272a45 |
811 | 9 my $stream = new Thread::Queue; |
c975c451 |
812 | 10 my $kid = new threads(\&check_num, $stream, 2); |
813 | 11 |
814 | 12 for my $i ( 3 .. 1000 ) { |
815 | 13 $stream->enqueue($i); |
816 | 14 } |
817 | 15 |
818 | 16 $stream->enqueue(undef); |
8f95bfb9 |
819 | 17 $kid->join; |
c975c451 |
820 | 18 |
821 | 19 sub check_num { |
822 | 20 my ($upstream, $cur_prime) = @_; |
823 | 21 my $kid; |
83272a45 |
824 | 22 my $downstream = new Thread::Queue; |
c975c451 |
825 | 23 while (my $num = $upstream->dequeue) { |
826 | 24 next unless $num % $cur_prime; |
827 | 25 if ($kid) { |
828 | 26 $downstream->enqueue($num); |
829 | 27 } else { |
830 | 28 print "Found prime $num\n"; |
831 | 29 $kid = new threads(\&check_num, $downstream, $num); |
832 | 30 } |
833 | 31 } |
834 | 32 $downstream->enqueue(undef) if $kid; |
8f95bfb9 |
835 | 33 $kid->join if $kid; |
c975c451 |
836 | 34 } |
837 | |
838 | This program uses the pipeline model to generate prime numbers. Each |
839 | thread in the pipeline has an input queue that feeds numbers to be |
840 | checked, a prime number that it's responsible for, and an output queue |
9e75ef81 |
841 | into which it funnels numbers that have failed the check. If the thread |
c975c451 |
842 | has a number that's failed its check and there's no child thread, then |
843 | the thread must have found a new prime number. In that case, a new |
844 | child thread is created for that prime and stuck on the end of the |
845 | pipeline. |
846 | |
6eded8f3 |
847 | This probably sounds a bit more confusing than it really is, so let's |
c975c451 |
848 | go through this program piece by piece and see what it does. (For |
849 | those of you who might be trying to remember exactly what a prime |
850 | number is, it's a number that's only evenly divisible by itself and 1) |
851 | |
852 | The bulk of the work is done by the check_num() subroutine, which |
853 | takes a reference to its input queue and a prime number that it's |
854 | responsible for. After pulling in the input queue and the prime that |
855 | the subroutine's checking (line 20), we create a new queue (line 22) |
856 | and reserve a scalar for the thread that we're likely to create later |
857 | (line 21). |
858 | |
859 | The while loop from lines 23 to line 31 grabs a scalar off the input |
860 | queue and checks against the prime this thread is responsible |
861 | for. Line 24 checks to see if there's a remainder when we modulo the |
862 | number to be checked against our prime. If there is one, the number |
863 | must not be evenly divisible by our prime, so we need to either pass |
864 | it on to the next thread if we've created one (line 26) or create a |
865 | new thread if we haven't. |
866 | |
867 | The new thread creation is line 29. We pass on to it a reference to |
868 | the queue we've created, and the prime number we've found. |
869 | |
870 | Finally, once the loop terminates (because we got a 0 or undef in the |
871 | queue, which serves as a note to die), we pass on the notice to our |
6eded8f3 |
872 | child and wait for it to exit if we've created a child (lines 32 and |
c975c451 |
873 | 37). |
874 | |
875 | Meanwhile, back in the main thread, we create a queue (line 9) and the |
876 | initial child thread (line 10), and pre-seed it with the first prime: |
877 | 2. Then we queue all the numbers from 3 to 1000 for checking (lines |
878 | 12-14), then queue a die notice (line 16) and wait for the first child |
879 | thread to terminate (line 17). Because a child won't die until its |
880 | child has died, we know that we're done once we return from the join. |
881 | |
882 | That's how it works. It's pretty simple; as with many Perl programs, |
883 | the explanation is much longer than the program. |
884 | |
536bca94 |
885 | =head1 Different implementations of threads |
886 | |
887 | Some background on thread implementations from the operating system |
888 | viewpoint. There are three basic categories of threads: user-mode threads, |
889 | kernel threads, and multiprocessor kernel threads. |
890 | |
891 | User-mode threads are threads that live entirely within a program and |
892 | its libraries. In this model, the OS knows nothing about threads. As |
893 | far as it's concerned, your process is just a process. |
894 | |
895 | This is the easiest way to implement threads, and the way most OSes |
896 | start. The big disadvantage is that, since the OS knows nothing about |
897 | threads, if one thread blocks they all do. Typical blocking activities |
898 | include most system calls, most I/O, and things like sleep(). |
899 | |
900 | Kernel threads are the next step in thread evolution. The OS knows |
901 | about kernel threads, and makes allowances for them. The main |
902 | difference between a kernel thread and a user-mode thread is |
903 | blocking. With kernel threads, things that block a single thread don't |
904 | block other threads. This is not the case with user-mode threads, |
905 | where the kernel blocks at the process level and not the thread level. |
906 | |
907 | This is a big step forward, and can give a threaded program quite a |
908 | performance boost over non-threaded programs. Threads that block |
909 | performing I/O, for example, won't block threads that are doing other |
910 | things. Each process still has only one thread running at once, |
911 | though, regardless of how many CPUs a system might have. |
912 | |
913 | Since kernel threading can interrupt a thread at any time, they will |
914 | uncover some of the implicit locking assumptions you may make in your |
915 | program. For example, something as simple as C<$a = $a + 2> can behave |
916 | unpredictably with kernel threads if $a is visible to other |
917 | threads, as another thread may have changed $a between the time it |
918 | was fetched on the right hand side and the time the new value is |
919 | stored. |
920 | |
921 | Multiprocessor kernel threads are the final step in thread |
922 | support. With multiprocessor kernel threads on a machine with multiple |
923 | CPUs, the OS may schedule two or more threads to run simultaneously on |
924 | different CPUs. |
925 | |
926 | This can give a serious performance boost to your threaded program, |
927 | since more than one thread will be executing at the same time. As a |
928 | tradeoff, though, any of those nagging synchronization issues that |
929 | might not have shown with basic kernel threads will appear with a |
930 | vengeance. |
931 | |
932 | In addition to the different levels of OS involvement in threads, |
933 | different OSes (and different thread implementations for a particular |
934 | OS) allocate CPU cycles to threads in different ways. |
935 | |
936 | Cooperative multitasking systems have running threads give up control |
937 | if one of two things happen. If a thread calls a yield function, it |
938 | gives up control. It also gives up control if the thread does |
939 | something that would cause it to block, such as perform I/O. In a |
940 | cooperative multitasking implementation, one thread can starve all the |
941 | others for CPU time if it so chooses. |
942 | |
943 | Preemptive multitasking systems interrupt threads at regular intervals |
944 | while the system decides which thread should run next. In a preemptive |
945 | multitasking system, one thread usually won't monopolize the CPU. |
946 | |
947 | On some systems, there can be cooperative and preemptive threads |
948 | running simultaneously. (Threads running with realtime priorities |
949 | often behave cooperatively, for example, while threads running at |
950 | normal priorities behave preemptively.) |
951 | |
952 | Most modern operating systems support preemptive multitasking nowadays. |
953 | |
bfce6503 |
954 | =head1 Performance considerations |
955 | |
956 | The main thing to bear in mind when comparing ithreads to other threading |
957 | models is the fact that for each new thread created, a complete copy of |
958 | all the variables and data of the parent thread has to be taken. Thus |
959 | thread creation can be quite expensive, both in terms of memory usage and |
960 | time spent in creation. The ideal way to reduce these costs is to have a |
961 | relatively short number of long-lived threads, all created fairly early |
962 | on - before the base thread has accumulated too much data. Of course, this |
963 | may not always be possible, so compromises have to be made. However, after |
964 | a thread has been created, its performance and extra memory usage should |
965 | be little different than ordinary code. |
966 | |
967 | Also note that under the current implementation, shared variables |
968 | use a little more memory and are a little slower than ordinary variables. |
969 | |
cf5baa48 |
970 | =head1 Process-scope Changes |
971 | |
972 | Note that while threads themselves are separate execution threads and |
973 | Perl data is thread-private unless explicitly shared, the threads can |
974 | affect process-scope state, affecting all the threads. |
975 | |
976 | The most common example of this is changing the current working |
977 | directory using chdir(). One thread calls chdir(), and the working |
978 | directory of all the threads changes. |
bdcfa4c7 |
979 | |
cf5baa48 |
980 | Even more drastic example of a process-scope change is chroot(): |
981 | the root directory of all the threads changes, and no thread can |
982 | undo it (as opposed to chdir()). |
983 | |
984 | Further examples of process-scope changes include umask() and |
985 | changing uids/gids. |
986 | |
987 | Thinking of mixing fork() and threads? Please lie down and wait |
a95a5f75 |
988 | until the feeling passes. Be aware that the semantics of fork() vary |
989 | between platforms. For example, some UNIX systems copy all the current |
990 | threads into the child process, while others only copy the thread that |
991 | called fork(). You have been warned! |
cf5baa48 |
992 | |
b03ad8f6 |
993 | Similarly, mixing signals and threads should not be attempted. |
994 | Implementations are platform-dependent, and even the POSIX |
995 | semantics may not be what you expect (and Perl doesn't even |
996 | give you the full POSIX API). |
997 | |
cf5baa48 |
998 | =head1 Thread-Safety of System Libraries |
999 | |
1000 | Whether various library calls are thread-safe is outside the control |
1001 | of Perl. Calls often suffering from not being thread-safe include: |
bdcfa4c7 |
1002 | localtime(), gmtime(), get{gr,host,net,proto,serv,pw}*(), readdir(), |
cf5baa48 |
1003 | rand(), and srand() -- in general, calls that depend on some global |
1004 | external state. |
80bbcbc4 |
1005 | |
cf5baa48 |
1006 | If the system Perl is compiled in has thread-safe variants of such |
80bbcbc4 |
1007 | calls, they will be used. Beyond that, Perl is at the mercy of |
cf5baa48 |
1008 | the thread-safety or -unsafety of the calls. Please consult your |
80bbcbc4 |
1009 | C library call documentation. |
1010 | |
af685957 |
1011 | On some platforms the thread-safe library interfaces may fail if the |
1012 | result buffer is too small (for example the user group databases may |
1013 | be rather large, and the reentrant interfaces may have to carry around |
1014 | a full snapshot of those databases). Perl will start with a small |
1015 | buffer, but keep retrying and growing the result buffer |
1016 | until the result fits. If this limitless growing sounds bad for |
1017 | security or memory consumption reasons you can recompile Perl with |
1018 | PERL_REENTRANT_MAXSIZE defined to the maximum number of bytes you will |
1019 | allow. |
bdcfa4c7 |
1020 | |
c975c451 |
1021 | =head1 Conclusion |
1022 | |
1023 | A complete thread tutorial could fill a book (and has, many times), |
6eded8f3 |
1024 | but with what we've covered in this introduction, you should be well |
1025 | on your way to becoming a threaded Perl expert. |
c975c451 |
1026 | |
1027 | =head1 Bibliography |
1028 | |
1029 | Here's a short bibliography courtesy of Jürgen Christoffel: |
1030 | |
1031 | =head2 Introductory Texts |
1032 | |
1033 | Birrell, Andrew D. An Introduction to Programming with |
1034 | Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report |
1035 | #35 online as |
6eded8f3 |
1036 | http://gatekeeper.dec.com/pub/DEC/SRC/research-reports/abstracts/src-rr-035.html |
1037 | (highly recommended) |
c975c451 |
1038 | |
1039 | Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A |
1040 | Guide to Concurrency, Communication, and |
1041 | Multithreading. Prentice-Hall, 1996. |
1042 | |
1043 | Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with |
1044 | Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written |
1045 | introduction to threads). |
1046 | |
1047 | Nelson, Greg (editor). Systems Programming with Modula-3. Prentice |
1048 | Hall, 1991, ISBN 0-13-590464-1. |
1049 | |
1050 | Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell. |
1051 | Pthreads Programming. O'Reilly & Associates, 1996, ISBN 156592-115-1 |
1052 | (covers POSIX threads). |
1053 | |
1054 | =head2 OS-Related References |
1055 | |
1056 | Boykin, Joseph, David Kirschen, Alan Langerman, and Susan |
1057 | LoVerso. Programming under Mach. Addison-Wesley, 1994, ISBN |
1058 | 0-201-52739-1. |
1059 | |
1060 | Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall, |
1061 | 1995, ISBN 0-13-219908-4 (great textbook). |
1062 | |
1063 | Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts, |
1064 | 4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4 |
1065 | |
1066 | =head2 Other References |
1067 | |
1068 | Arnold, Ken and James Gosling. The Java Programming Language, 2nd |
1069 | ed. Addison-Wesley, 1998, ISBN 0-201-31006-6. |
1070 | |
b03ad8f6 |
1071 | comp.programming.threads FAQ, |
1072 | L<http://www.serpentine.com/~bos/threads-faq/> |
1073 | |
c975c451 |
1074 | Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage |
1075 | Collection on Virtually Shared Memory Architectures" in Memory |
1076 | Management: Proc. of the International Workshop IWMM 92, St. Malo, |
1077 | France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer, |
1078 | 1992, ISBN 3540-55940-X (real-life thread applications). |
1079 | |
5e549d84 |
1080 | Artur Bergman, "Where Wizards Fear To Tread", June 11, 2002, |
1081 | L<http://www.perl.com/pub/a/2002/06/11/threads.html> |
1082 | |
c975c451 |
1083 | =head1 Acknowledgements |
1084 | |
1085 | Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy |
1086 | Sarathy, Ilya Zakharevich, Benjamin Sugars, Jürgen Christoffel, Joshua |
1087 | Pritikin, and Alan Burlison, for their help in reality-checking and |
1088 | polishing this article. Big thanks to Tom Christiansen for his rewrite |
1089 | of the prime number generator. |
1090 | |
1091 | =head1 AUTHOR |
1092 | |
9316ed2f |
1093 | Dan Sugalski E<lt>dan@sidhe.org<gt> |
c975c451 |
1094 | |
1095 | Slightly modified by Arthur Bergman to fit the new thread model/module. |
1096 | |
cf5baa48 |
1097 | Reworked slightly by Jörg Walter E<lt>jwalt@cpan.org<gt> to be more concise |
1098 | about thread-safety of perl code. |
1099 | |
536bca94 |
1100 | Rearranged slightly by Elizabeth Mattijsen E<lt>liz@dijkmat.nl<gt> to put |
1101 | less emphasis on yield(). |
1102 | |
c975c451 |
1103 | =head1 Copyrights |
1104 | |
bfce6503 |
1105 | The original version of this article originally appeared in The Perl |
1106 | Journal #10, and is copyright 1998 The Perl Journal. It appears courtesy |
1107 | of Jon Orwant and The Perl Journal. This document may be distributed |
1108 | under the same terms as Perl itself. |
2605996a |
1109 | |
53d7eaa8 |
1110 | For more information please see L<threads> and L<threads::shared>. |