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1=head1 NAME
2
3perlthrtut - tutorial on threads in Perl
4
5=head1 DESCRIPTION
6
53d7eaa8 7B<NOTE>: this tutorial describes the new Perl threading flavour
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8introduced in Perl 5.6.0 called interpreter threads, or B<ithreads>
9for short. In this model each thread runs in its own Perl interpreter,
10and any data sharing between threads must be explicit.
11
12There is another older Perl threading flavour called the 5.005 model,
13unsurprisingly for 5.005 versions of Perl. The old model is known to
14have problems, deprecated, and will probably be removed around release
155.10. You are strongly encouraged to migrate any existing 5.005
16threads code to the new model as soon as possible.
2a4bf773 17
53d7eaa8 18You can see which (or neither) threading flavour you have by
6eded8f3 19running C<perl -V> and looking at the C<Platform> section.
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20If you have C<useithreads=define> you have ithreads, if you
21have C<use5005threads=define> you have 5.005 threads.
22If you have neither, you don't have any thread support built in.
23If you have both, you are in trouble.
2605996a 24
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25The user-level interface to the 5.005 threads was via the L<Threads>
26class, while ithreads uses the L<threads> class. Note the change in case.
27
28=head1 Status
29
30The ithreads code has been available since Perl 5.6.0, and is considered
31stable. The user-level interface to ithreads (the L<threads> classes)
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32appeared in the 5.8.0 release, and as of this time is considered stable
33although it should be treated with caution as with all new features.
2605996a 34
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35=head1 What Is A Thread Anyway?
36
37A thread is a flow of control through a program with a single
38execution point.
39
40Sounds an awful lot like a process, doesn't it? Well, it should.
41Threads are one of the pieces of a process. Every process has at least
42one thread and, up until now, every process running Perl had only one
43thread. With 5.8, though, you can create extra threads. We're going
44to show you how, when, and why.
45
46=head1 Threaded Program Models
47
48There are three basic ways that you can structure a threaded
49program. Which model you choose depends on what you need your program
50to do. For many non-trivial threaded programs you'll need to choose
51different models for different pieces of your program.
52
53=head2 Boss/Worker
54
55The boss/worker model usually has one `boss' thread and one or more
56`worker' threads. The boss thread gathers or generates tasks that need
57to be done, then parcels those tasks out to the appropriate worker
58thread.
59
60This model is common in GUI and server programs, where a main thread
61waits for some event and then passes that event to the appropriate
62worker threads for processing. Once the event has been passed on, the
63boss thread goes back to waiting for another event.
64
65The boss thread does relatively little work. While tasks aren't
66necessarily performed faster than with any other method, it tends to
67have the best user-response times.
68
69=head2 Work Crew
70
71In the work crew model, several threads are created that do
72essentially the same thing to different pieces of data. It closely
73mirrors classical parallel processing and vector processors, where a
74large array of processors do the exact same thing to many pieces of
75data.
76
77This model is particularly useful if the system running the program
78will distribute multiple threads across different processors. It can
79also be useful in ray tracing or rendering engines, where the
80individual threads can pass on interim results to give the user visual
81feedback.
82
83=head2 Pipeline
84
85The pipeline model divides up a task into a series of steps, and
86passes the results of one step on to the thread processing the
87next. Each thread does one thing to each piece of data and passes the
88results to the next thread in line.
89
90This model makes the most sense if you have multiple processors so two
91or more threads will be executing in parallel, though it can often
92make sense in other contexts as well. It tends to keep the individual
93tasks small and simple, as well as allowing some parts of the pipeline
94to block (on I/O or system calls, for example) while other parts keep
95going. If you're running different parts of the pipeline on different
96processors you may also take advantage of the caches on each
97processor.
98
99This model is also handy for a form of recursive programming where,
100rather than having a subroutine call itself, it instead creates
101another thread. Prime and Fibonacci generators both map well to this
102form of the pipeline model. (A version of a prime number generator is
103presented later on.)
104
105=head1 Native threads
106
107There are several different ways to implement threads on a system. How
108threads are implemented depends both on the vendor and, in some cases,
109the version of the operating system. Often the first implementation
110will be relatively simple, but later versions of the OS will be more
111sophisticated.
112
113While the information in this section is useful, it's not necessary,
114so you can skip it if you don't feel up to it.
115
6eded8f3 116There are three basic categories of threads: user-mode threads, kernel
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117threads, and multiprocessor kernel threads.
118
119User-mode threads are threads that live entirely within a program and
120its libraries. In this model, the OS knows nothing about threads. As
121far as it's concerned, your process is just a process.
122
123This is the easiest way to implement threads, and the way most OSes
124start. The big disadvantage is that, since the OS knows nothing about
125threads, if one thread blocks they all do. Typical blocking activities
126include most system calls, most I/O, and things like sleep().
127
128Kernel threads are the next step in thread evolution. The OS knows
129about kernel threads, and makes allowances for them. The main
130difference between a kernel thread and a user-mode thread is
131blocking. With kernel threads, things that block a single thread don't
132block other threads. This is not the case with user-mode threads,
133where the kernel blocks at the process level and not the thread level.
134
135This is a big step forward, and can give a threaded program quite a
136performance boost over non-threaded programs. Threads that block
137performing I/O, for example, won't block threads that are doing other
138things. Each process still has only one thread running at once,
139though, regardless of how many CPUs a system might have.
140
141Since kernel threading can interrupt a thread at any time, they will
142uncover some of the implicit locking assumptions you may make in your
143program. For example, something as simple as C<$a = $a + 2> can behave
144unpredictably with kernel threads if $a is visible to other
145threads, as another thread may have changed $a between the time it
146was fetched on the right hand side and the time the new value is
147stored.
148
6eded8f3 149Multiprocessor kernel threads are the final step in thread
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150support. With multiprocessor kernel threads on a machine with multiple
151CPUs, the OS may schedule two or more threads to run simultaneously on
152different CPUs.
153
154This can give a serious performance boost to your threaded program,
155since more than one thread will be executing at the same time. As a
156tradeoff, though, any of those nagging synchronization issues that
157might not have shown with basic kernel threads will appear with a
158vengeance.
159
160In addition to the different levels of OS involvement in threads,
161different OSes (and different thread implementations for a particular
162OS) allocate CPU cycles to threads in different ways.
163
164Cooperative multitasking systems have running threads give up control
165if one of two things happen. If a thread calls a yield function, it
166gives up control. It also gives up control if the thread does
167something that would cause it to block, such as perform I/O. In a
168cooperative multitasking implementation, one thread can starve all the
169others for CPU time if it so chooses.
170
171Preemptive multitasking systems interrupt threads at regular intervals
172while the system decides which thread should run next. In a preemptive
173multitasking system, one thread usually won't monopolize the CPU.
174
175On some systems, there can be cooperative and preemptive threads
176running simultaneously. (Threads running with realtime priorities
177often behave cooperatively, for example, while threads running at
178normal priorities behave preemptively.)
179
bfce6503 180=head1 What kind of threads are Perl threads?
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181
182If you have experience with other thread implementations, you might
183find that things aren't quite what you expect. It's very important to
184remember when dealing with Perl threads that Perl Threads Are Not X
185Threads, for all values of X. They aren't POSIX threads, or
186DecThreads, or Java's Green threads, or Win32 threads. There are
187similarities, and the broad concepts are the same, but if you start
188looking for implementation details you're going to be either
189disappointed or confused. Possibly both.
190
191This is not to say that Perl threads are completely different from
192everything that's ever come before--they're not. Perl's threading
193model owes a lot to other thread models, especially POSIX. Just as
194Perl is not C, though, Perl threads are not POSIX threads. So if you
195find yourself looking for mutexes, or thread priorities, it's time to
196step back a bit and think about what you want to do and how Perl can
197do it.
198
6eded8f3 199However it is important to remember that Perl threads cannot magically
c975c451 200do things unless your operating systems threads allows it. So if your
bfce6503 201system blocks the entire process on sleep(), Perl usually will as well.
c975c451 202
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203Perl Threads Are Different.
204
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205=head1 Threadsafe Modules
206
207The addition of threads has changed Perl's internals
208substantially. There are implications for people who write
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209modules with XS code or external libraries. However, since the threads
210do not share data, pure Perl modules that don't interact with external
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211systems should be safe. Modules that are not tagged as thread-safe should
212be tested or code reviewed before being used in production code.
213
214Not all modules that you might use are thread-safe, and you should
215always assume a module is unsafe unless the documentation says
216otherwise. This includes modules that are distributed as part of the
217core. Threads are a new feature, and even some of the standard
bfce6503 218modules aren't thread-safe.
c975c451 219
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220Even if a module is threadsafe, it doesn't mean that the module is optimized
221to work well with threads. A module could possibly be rewritten to utilize
222the new features in threaded Perl to increase performance in a threaded
223environment.
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224
225If you're using a module that's not thread-safe for some reason, you
226can protect yourself by using semaphores and lots of programming
227discipline to control access to the module. Semaphores are covered
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228later in the article.
229
230See also L</"Threadsafety of System Libraries">.
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231
232=head1 Thread Basics
233
234The core L<threads> module provides the basic functions you need to write
235threaded programs. In the following sections we'll cover the basics,
236showing you what you need to do to create a threaded program. After
237that, we'll go over some of the features of the L<threads> module that
238make threaded programming easier.
239
240=head2 Basic Thread Support
241
6eded8f3 242Thread support is a Perl compile-time option - it's something that's
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243turned on or off when Perl is built at your site, rather than when
244your programs are compiled. If your Perl wasn't compiled with thread
245support enabled, then any attempt to use threads will fail.
246
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247Your programs can use the Config module to check whether threads are
248enabled. If your program can't run without them, you can say something
249like:
250
9316ed2f 251 $Config{useithreads} or die "Recompile Perl with threads to run this program.";
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252
253A possibly-threaded program using a possibly-threaded module might
254have code like this:
255
256 use Config;
257 use MyMod;
258
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259 BEGIN {
260 if ($Config{useithreads}) {
261 # We have threads
262 require MyMod_threaded;
263 import MyMod_threaded;
264 } else {
265 require MyMod_unthreaded;
266 import MyMod_unthreaded;
267 }
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268 }
269
270Since code that runs both with and without threads is usually pretty
271messy, it's best to isolate the thread-specific code in its own
272module. In our example above, that's what MyMod_threaded is, and it's
273only imported if we're running on a threaded Perl.
274
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275=head2 A Note about the Examples
276
277Although thread support is considered to be stable, there are still a number
278of quirks that may startle you when you try out any of the examples below.
279In a real situation, care should be taken that all threads are finished
280executing before the program exits. That care has B<not> been taken in these
281examples in the interest of simplicity. Running these examples "as is" will
282produce error messages, usually caused by the fact that there are still
283threads running when the program exits. You should not be alarmed by this.
284Future versions of Perl may fix this problem.
285
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286=head2 Creating Threads
287
288The L<threads> package provides the tools you need to create new
9e75ef81 289threads. Like any other module, you need to tell Perl that you want to use
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290it; C<use threads> imports all the pieces you need to create basic
291threads.
292
9e75ef81 293The simplest, most straightforward way to create a thread is with new():
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294
295 use threads;
296
297 $thr = threads->new(\&sub1);
298
299 sub sub1 {
300 print "In the thread\n";
301 }
302
303The new() method takes a reference to a subroutine and creates a new
304thread, which starts executing in the referenced subroutine. Control
305then passes both to the subroutine and the caller.
306
307If you need to, your program can pass parameters to the subroutine as
308part of the thread startup. Just include the list of parameters as
309part of the C<threads::new> call, like this:
310
311 use threads;
bfce6503 312
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313 $Param3 = "foo";
314 $thr = threads->new(\&sub1, "Param 1", "Param 2", $Param3);
315 $thr = threads->new(\&sub1, @ParamList);
8f95bfb9 316 $thr = threads->new(\&sub1, qw(Param1 Param2 Param3));
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317
318 sub sub1 {
319 my @InboundParameters = @_;
320 print "In the thread\n";
321 print "got parameters >", join("<>", @InboundParameters), "<\n";
322 }
323
324
325The last example illustrates another feature of threads. You can spawn
326off several threads using the same subroutine. Each thread executes
327the same subroutine, but in a separate thread with a separate
328environment and potentially separate arguments.
329
9316ed2f 330C<create()> is a synonym for C<new()>.
bfce6503 331
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332=head2 Giving up control
333
334There are times when you may find it useful to have a thread
335explicitly give up the CPU to another thread. Your threading package
336might not support preemptive multitasking for threads, for example, or
9e75ef81 337you may be doing something processor-intensive and want to make sure
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338that the user-interface thread gets called frequently. Regardless,
339there are times that you might want a thread to give up the processor.
340
341Perl's threading package provides the yield() function that does
342this. yield() is pretty straightforward, and works like this:
343
344 use threads;
818c4caa 345
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346 sub loop {
347 my $thread = shift;
348 my $foo = 50;
349 while($foo--) { print "in thread $thread\n" }
8f95bfb9 350 threads->yield;
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351 $foo = 50;
352 while($foo--) { print "in thread $thread\n" }
353 }
c975c451 354
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355 my $thread1 = threads->new(\&loop, 'first');
356 my $thread2 = threads->new(\&loop, 'second');
357 my $thread3 = threads->new(\&loop, 'third');
818c4caa 358
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359It is important to remember that yield() is only a hint to give up the CPU,
360it depends on your hardware, OS and threading libraries what actually happens.
361Therefore it is important to note that one should not build the scheduling of
362the threads around yield() calls. It might work on your platform but it won't
363work on another platform.
364
365=head2 Waiting For A Thread To Exit
366
367Since threads are also subroutines, they can return values. To wait
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368for a thread to exit and extract any values it might return, you can
369use the join() method:
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370
371 use threads;
bfce6503 372
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373 $thr = threads->new(\&sub1);
374
375 @ReturnData = $thr->join;
376 print "Thread returned @ReturnData";
377
378 sub sub1 { return "Fifty-six", "foo", 2; }
379
380In the example above, the join() method returns as soon as the thread
381ends. In addition to waiting for a thread to finish and gathering up
382any values that the thread might have returned, join() also performs
383any OS cleanup necessary for the thread. That cleanup might be
384important, especially for long-running programs that spawn lots of
385threads. If you don't want the return values and don't want to wait
386for the thread to finish, you should call the detach() method
bfce6503 387instead, as described next.
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388
389=head2 Ignoring A Thread
390
391join() does three things: it waits for a thread to exit, cleans up
392after it, and returns any data the thread may have produced. But what
393if you're not interested in the thread's return values, and you don't
394really care when the thread finishes? All you want is for the thread
395to get cleaned up after when it's done.
396
397In this case, you use the detach() method. Once a thread is detached,
398it'll run until it's finished, then Perl will clean up after it
399automatically.
400
401 use threads;
bfce6503 402
6eded8f3 403 $thr = threads->new(\&sub1); # Spawn the thread
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404
405 $thr->detach; # Now we officially don't care any more
406
407 sub sub1 {
408 $a = 0;
409 while (1) {
410 $a++;
411 print "\$a is $a\n";
412 sleep 1;
413 }
414 }
415
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416Once a thread is detached, it may not be joined, and any return data
417that it might have produced (if it was done and waiting for a join) is
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418lost.
419
420=head1 Threads And Data
421
422Now that we've covered the basics of threads, it's time for our next
423topic: data. Threading introduces a couple of complications to data
424access that non-threaded programs never need to worry about.
425
426=head2 Shared And Unshared Data
427
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428The biggest difference between Perl ithreads and the old 5.005 style
429threading, or for that matter, to most other threading systems out there,
430is that by default, no data is shared. When a new perl thread is created,
431all the data associated with the current thread is copied to the new
432thread, and is subsequently private to that new thread!
433This is similar in feel to what happens when a UNIX process forks,
434except that in this case, the data is just copied to a different part of
435memory within the same process rather than a real fork taking place.
c975c451 436
9e75ef81 437To make use of threading however, one usually wants the threads to share
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438at least some data between themselves. This is done with the
439L<threads::shared> module and the C< : shared> attribute:
440
441 use threads;
442 use threads::shared;
443
444 my $foo : shared = 1;
445 my $bar = 1;
446 threads->new(sub { $foo++; $bar++ })->join;
818c4caa 447
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448 print "$foo\n"; #prints 2 since $foo is shared
449 print "$bar\n"; #prints 1 since $bar is not shared
450
451In the case of a shared array, all the array's elements are shared, and for
452a shared hash, all the keys and values are shared. This places
453restrictions on what may be assigned to shared array and hash elements: only
454simple values or references to shared variables are allowed - this is
f3278b06 455so that a private variable can't accidentally become shared. A bad
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456assignment will cause the thread to die. For example:
457
458 use threads;
459 use threads::shared;
460
461 my $var = 1;
462 my $svar : shared = 2;
463 my %hash : shared;
464
465 ... create some threads ...
466
467 $hash{a} = 1; # all threads see exists($hash{a}) and $hash{a} == 1
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468 $hash{a} = $var # okay - copy-by-value: same effect as previous
469 $hash{a} = $svar # okay - copy-by-value: same effect as previous
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470 $hash{a} = \$svar # okay - a reference to a shared variable
471 $hash{a} = \$var # This will die
472 delete $hash{a} # okay - all threads will see !exists($hash{a})
473
474Note that a shared variable guarantees that if two or more threads try to
475modify it at the same time, the internal state of the variable will not
476become corrupted. However, there are no guarantees beyond this, as
477explained in the next section.
c975c451 478
6eded8f3 479=head2 Thread Pitfalls: Races
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480
481While threads bring a new set of useful tools, they also bring a
482number of pitfalls. One pitfall is the race condition:
483
484 use threads;
485 use threads::shared;
bfce6503 486
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487 my $a : shared = 1;
488 $thr1 = threads->new(\&sub1);
489 $thr2 = threads->new(\&sub2);
490
491 $thr1->join;
492 $thr2->join;
493 print "$a\n";
494
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495 sub sub1 { my $foo = $a; $a = $foo + 1; }
496 sub sub2 { my $bar = $a; $a = $bar + 1; }
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497
498What do you think $a will be? The answer, unfortunately, is "it
499depends." Both sub1() and sub2() access the global variable $a, once
500to read and once to write. Depending on factors ranging from your
501thread implementation's scheduling algorithm to the phase of the moon,
502$a can be 2 or 3.
503
504Race conditions are caused by unsynchronized access to shared
505data. Without explicit synchronization, there's no way to be sure that
506nothing has happened to the shared data between the time you access it
507and the time you update it. Even this simple code fragment has the
508possibility of error:
509
510 use threads;
511 my $a : shared = 2;
512 my $b : shared;
513 my $c : shared;
514 my $thr1 = threads->create(sub { $b = $a; $a = $b + 1; });
515 my $thr2 = threads->create(sub { $c = $a; $a = $c + 1; });
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516 $thr1->join;
517 $thr2->join;
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518
519Two threads both access $a. Each thread can potentially be interrupted
520at any point, or be executed in any order. At the end, $a could be 3
521or 4, and both $b and $c could be 2 or 3.
522
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523Even C<$a += 5> or C<$a++> are not guaranteed to be atomic.
524
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525Whenever your program accesses data or resources that can be accessed
526by other threads, you must take steps to coordinate access or risk
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527data inconsistency and race conditions. Note that Perl will protect its
528internals from your race conditions, but it won't protect you from you.
529
f3278b06 530=head1 Synchronization and control
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531
532Perl provides a number of mechanisms to coordinate the interactions
533between themselves and their data, to avoid race conditions and the like.
534Some of these are designed to resemble the common techniques used in thread
535libraries such as C<pthreads>; others are Perl-specific. Often, the
9e75ef81 536standard techniques are clumsy and difficult to get right (such as
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537condition waits). Where possible, it is usually easier to use Perlish
538techniques such as queues, which remove some of the hard work involved.
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539
540=head2 Controlling access: lock()
541
542The lock() function takes a shared variable and puts a lock on it.
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543No other thread may lock the variable until the the variable is unlocked
544by the thread holding the lock. Unlocking happens automatically
8f95bfb9 545when the locking thread exits the outermost block that contains
bfce6503 546C<lock()> function. Using lock() is straightforward: this example has
f3278b06 547several threads doing some calculations in parallel, and occasionally
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548updating a running total:
549
550 use threads;
551 use threads::shared;
552
553 my $total : shared = 0;
554
555 sub calc {
556 for (;;) {
557 my $result;
558 # (... do some calculations and set $result ...)
559 {
560 lock($total); # block until we obtain the lock
8f95bfb9 561 $total += $result;
f3278b06 562 } # lock implicitly released at end of scope
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563 last if $result == 0;
564 }
565 }
566
567 my $thr1 = threads->new(\&calc);
568 my $thr2 = threads->new(\&calc);
569 my $thr3 = threads->new(\&calc);
570 $thr1->join;
571 $thr2->join;
572 $thr3->join;
573 print "total=$total\n";
c975c451 574
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575
576lock() blocks the thread until the variable being locked is
577available. When lock() returns, your thread can be sure that no other
bfce6503 578thread can lock that variable until the outermost block containing the
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579lock exits.
580
581It's important to note that locks don't prevent access to the variable
582in question, only lock attempts. This is in keeping with Perl's
583longstanding tradition of courteous programming, and the advisory file
584locking that flock() gives you.
585
586You may lock arrays and hashes as well as scalars. Locking an array,
587though, will not block subsequent locks on array elements, just lock
588attempts on the array itself.
589
bfce6503 590Locks are recursive, which means it's okay for a thread to
c975c451 591lock a variable more than once. The lock will last until the outermost
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592lock() on the variable goes out of scope. For example:
593
594 my $x : shared;
595 doit();
596
597 sub doit {
598 {
599 {
600 lock($x); # wait for lock
8f95bfb9 601 lock($x); # NOOP - we already have the lock
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602 {
603 lock($x); # NOOP
604 {
605 lock($x); # NOOP
606 lockit_some_more();
607 }
608 }
609 } # *** implicit unlock here ***
610 }
611 }
612
613 sub lockit_some_more {
614 lock($x); # NOOP
615 } # nothing happens here
616
617Note that there is no unlock() function - the only way to unlock a
618variable is to allow it to go out of scope.
619
620A lock can either be used to guard the data contained within the variable
621being locked, or it can be used to guard something else, like a section
622of code. In this latter case, the variable in question does not hold any
623useful data, and exists only for the purpose of being locked. In this
624respect, the variable behaves like the mutexes and basic semaphores of
625traditional thread libraries.
c975c451 626
bfce6503 627=head2 A Thread Pitfall: Deadlocks
c975c451 628
bfce6503 629Locks are a handy tool to synchronize access to data, and using them
c975c451 630properly is the key to safe shared data. Unfortunately, locks aren't
f3278b06 631without their dangers, especially when multiple locks are involved.
bfce6503 632Consider the following code:
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633
634 use threads;
bfce6503 635
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636 my $a : shared = 4;
637 my $b : shared = "foo";
638 my $thr1 = threads->new(sub {
639 lock($a);
bfce6503 640 threads->yield;
c975c451 641 sleep 20;
bfce6503 642 lock($b);
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643 });
644 my $thr2 = threads->new(sub {
645 lock($b);
bfce6503 646 threads->yield;
c975c451 647 sleep 20;
bfce6503 648 lock($a);
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649 });
650
651This program will probably hang until you kill it. The only way it
bfce6503 652won't hang is if one of the two threads acquires both locks
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653first. A guaranteed-to-hang version is more complicated, but the
654principle is the same.
655
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656The first thread will grab a lock on $a, then, after a pause during which
657the second thread has probably had time to do some work, try to grab a
658lock on $b. Meanwhile, the second thread grabs a lock on $b, then later
659tries to grab a lock on $a. The second lock attempt for both threads will
660block, each waiting for the other to release its lock.
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661
662This condition is called a deadlock, and it occurs whenever two or
663more threads are trying to get locks on resources that the others
664own. Each thread will block, waiting for the other to release a lock
665on a resource. That never happens, though, since the thread with the
666resource is itself waiting for a lock to be released.
667
668There are a number of ways to handle this sort of problem. The best
669way is to always have all threads acquire locks in the exact same
670order. If, for example, you lock variables $a, $b, and $c, always lock
671$a before $b, and $b before $c. It's also best to hold on to locks for
672as short a period of time to minimize the risks of deadlock.
673
48b96218 674The other synchronization primitives described below can suffer from
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675similar problems.
676
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677=head2 Queues: Passing Data Around
678
679A queue is a special thread-safe object that lets you put data in one
680end and take it out the other without having to worry about
681synchronization issues. They're pretty straightforward, and look like
682this:
683
684 use threads;
83272a45 685 use Thread::Queue;
c975c451 686
83272a45 687 my $DataQueue = Thread::Queue->new;
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688 $thr = threads->new(sub {
689 while ($DataElement = $DataQueue->dequeue) {
690 print "Popped $DataElement off the queue\n";
691 }
692 });
693
694 $DataQueue->enqueue(12);
695 $DataQueue->enqueue("A", "B", "C");
696 $DataQueue->enqueue(\$thr);
697 sleep 10;
698 $DataQueue->enqueue(undef);
8f95bfb9 699 $thr->join;
c975c451 700
83272a45 701You create the queue with C<new Thread::Queue>. Then you can
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702add lists of scalars onto the end with enqueue(), and pop scalars off
703the front of it with dequeue(). A queue has no fixed size, and can grow
704as needed to hold everything pushed on to it.
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705
706If a queue is empty, dequeue() blocks until another thread enqueues
707something. This makes queues ideal for event loops and other
708communications between threads.
709
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710=head2 Semaphores: Synchronizing Data Access
711
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712Semaphores are a kind of generic locking mechanism. In their most basic
713form, they behave very much like lockable scalars, except that thay
714can't hold data, and that they must be explicitly unlocked. In their
715advanced form, they act like a kind of counter, and can allow multiple
716threads to have the 'lock' at any one time.
2605996a 717
bfce6503 718=head2 Basic semaphores
2605996a 719
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720Semaphores have two methods, down() and up(): down() decrements the resource
721count, while up increments it. Calls to down() will block if the
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722semaphore's current count would decrement below zero. This program
723gives a quick demonstration:
724
725 use threads qw(yield);
83272a45 726 use Thread::Semaphore;
bfce6503 727
83272a45 728 my $semaphore = new Thread::Semaphore;
bfce6503 729 my $GlobalVariable : shared = 0;
2605996a 730
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731 $thr1 = new threads \&sample_sub, 1;
732 $thr2 = new threads \&sample_sub, 2;
733 $thr3 = new threads \&sample_sub, 3;
2605996a 734
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735 sub sample_sub {
736 my $SubNumber = shift @_;
737 my $TryCount = 10;
738 my $LocalCopy;
739 sleep 1;
740 while ($TryCount--) {
741 $semaphore->down;
742 $LocalCopy = $GlobalVariable;
743 print "$TryCount tries left for sub $SubNumber (\$GlobalVariable is $GlobalVariable)\n";
744 yield;
745 sleep 2;
746 $LocalCopy++;
747 $GlobalVariable = $LocalCopy;
748 $semaphore->up;
749 }
750 }
6eded8f3 751
8f95bfb9
EM
752 $thr1->join;
753 $thr2->join;
754 $thr3->join;
2605996a 755
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756The three invocations of the subroutine all operate in sync. The
757semaphore, though, makes sure that only one thread is accessing the
758global variable at once.
2605996a 759
bfce6503 760=head2 Advanced Semaphores
2605996a 761
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762By default, semaphores behave like locks, letting only one thread
763down() them at a time. However, there are other uses for semaphores.
2605996a 764
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765Each semaphore has a counter attached to it. By default, semaphores are
766created with the counter set to one, down() decrements the counter by
767one, and up() increments by one. However, we can override any or all
768of these defaults simply by passing in different values:
769
770 use threads;
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771 use Thread::Semaphore;
772 my $semaphore = Thread::Semaphore->new(5);
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773 # Creates a semaphore with the counter set to five
774
775 $thr1 = threads->new(\&sub1);
776 $thr2 = threads->new(\&sub1);
777
778 sub sub1 {
779 $semaphore->down(5); # Decrements the counter by five
780 # Do stuff here
781 $semaphore->up(5); # Increment the counter by five
782 }
783
8f95bfb9
EM
784 $thr1->detach;
785 $thr2->detach;
6eded8f3
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786
787If down() attempts to decrement the counter below zero, it blocks until
788the counter is large enough. Note that while a semaphore can be created
789with a starting count of zero, any up() or down() always changes the
790counter by at least one, and so $semaphore->down(0) is the same as
791$semaphore->down(1).
2605996a 792
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793The question, of course, is why would you do something like this? Why
794create a semaphore with a starting count that's not one, or why
795decrement/increment it by more than one? The answer is resource
796availability. Many resources that you want to manage access for can be
797safely used by more than one thread at once.
2605996a 798
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799For example, let's take a GUI driven program. It has a semaphore that
800it uses to synchronize access to the display, so only one thread is
801ever drawing at once. Handy, but of course you don't want any thread
802to start drawing until things are properly set up. In this case, you
803can create a semaphore with a counter set to zero, and up it when
804things are ready for drawing.
2605996a 805
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806Semaphores with counters greater than one are also useful for
807establishing quotas. Say, for example, that you have a number of
808threads that can do I/O at once. You don't want all the threads
809reading or writing at once though, since that can potentially swamp
810your I/O channels, or deplete your process' quota of filehandles. You
811can use a semaphore initialized to the number of concurrent I/O
812requests (or open files) that you want at any one time, and have your
813threads quietly block and unblock themselves.
2605996a 814
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815Larger increments or decrements are handy in those cases where a
816thread needs to check out or return a number of resources at once.
2605996a 817
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818=head2 cond_wait() and cond_signal()
819
820These two functions can be used in conjunction with locks to notify
821co-operating threads that a resource has become available. They are
822very similar in use to the functions found in C<pthreads>. However
823for most purposes, queues are simpler to use and more intuitive. See
824L<threads::shared> for more details.
2605996a 825
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826=head1 General Thread Utility Routines
827
828We've covered the workhorse parts of Perl's threading package, and
829with these tools you should be well on your way to writing threaded
830code and packages. There are a few useful little pieces that didn't
831really fit in anyplace else.
832
833=head2 What Thread Am I In?
834
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835The C<< threads->self >> class method provides your program with a way to
836get an object representing the thread it's currently in. You can use this
6eded8f3 837object in the same way as the ones returned from thread creation.
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838
839=head2 Thread IDs
840
841tid() is a thread object method that returns the thread ID of the
842thread the object represents. Thread IDs are integers, with the main
843thread in a program being 0. Currently Perl assigns a unique tid to
844every thread ever created in your program, assigning the first thread
845to be created a tid of 1, and increasing the tid by 1 for each new
846thread that's created.
847
848=head2 Are These Threads The Same?
849
850The equal() method takes two thread objects and returns true
851if the objects represent the same thread, and false if they don't.
852
853Thread objects also have an overloaded == comparison so that you can do
854comparison on them as you would with normal objects.
855
856=head2 What Threads Are Running?
857
bfce6503 858C<< threads->list >> returns a list of thread objects, one for each thread
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859that's currently running and not detached. Handy for a number of things,
860including cleaning up at the end of your program:
861
862 # Loop through all the threads
863 foreach $thr (threads->list) {
864 # Don't join the main thread or ourselves
865 if ($thr->tid && !threads::equal($thr, threads->self)) {
866 $thr->join;
867 }
868 }
869
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870If some threads have not finished running when the main Perl thread
871ends, Perl will warn you about it and die, since it is impossible for Perl
6eded8f3 872to clean up itself while other threads are running
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873
874=head1 A Complete Example
875
876Confused yet? It's time for an example program to show some of the
877things we've covered. This program finds prime numbers using threads.
878
879 1 #!/usr/bin/perl -w
880 2 # prime-pthread, courtesy of Tom Christiansen
881 3
882 4 use strict;
883 5
884 6 use threads;
83272a45 885 7 use Thread::Queue;
c975c451 886 8
83272a45 887 9 my $stream = new Thread::Queue;
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888 10 my $kid = new threads(\&check_num, $stream, 2);
889 11
890 12 for my $i ( 3 .. 1000 ) {
891 13 $stream->enqueue($i);
892 14 }
893 15
894 16 $stream->enqueue(undef);
8f95bfb9 895 17 $kid->join;
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896 18
897 19 sub check_num {
898 20 my ($upstream, $cur_prime) = @_;
899 21 my $kid;
83272a45 900 22 my $downstream = new Thread::Queue;
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901 23 while (my $num = $upstream->dequeue) {
902 24 next unless $num % $cur_prime;
903 25 if ($kid) {
904 26 $downstream->enqueue($num);
905 27 } else {
906 28 print "Found prime $num\n";
907 29 $kid = new threads(\&check_num, $downstream, $num);
908 30 }
909 31 }
910 32 $downstream->enqueue(undef) if $kid;
8f95bfb9 911 33 $kid->join if $kid;
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912 34 }
913
914This program uses the pipeline model to generate prime numbers. Each
915thread in the pipeline has an input queue that feeds numbers to be
916checked, a prime number that it's responsible for, and an output queue
9e75ef81 917into which it funnels numbers that have failed the check. If the thread
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918has a number that's failed its check and there's no child thread, then
919the thread must have found a new prime number. In that case, a new
920child thread is created for that prime and stuck on the end of the
921pipeline.
922
6eded8f3 923This probably sounds a bit more confusing than it really is, so let's
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924go through this program piece by piece and see what it does. (For
925those of you who might be trying to remember exactly what a prime
926number is, it's a number that's only evenly divisible by itself and 1)
927
928The bulk of the work is done by the check_num() subroutine, which
929takes a reference to its input queue and a prime number that it's
930responsible for. After pulling in the input queue and the prime that
931the subroutine's checking (line 20), we create a new queue (line 22)
932and reserve a scalar for the thread that we're likely to create later
933(line 21).
934
935The while loop from lines 23 to line 31 grabs a scalar off the input
936queue and checks against the prime this thread is responsible
937for. Line 24 checks to see if there's a remainder when we modulo the
938number to be checked against our prime. If there is one, the number
939must not be evenly divisible by our prime, so we need to either pass
940it on to the next thread if we've created one (line 26) or create a
941new thread if we haven't.
942
943The new thread creation is line 29. We pass on to it a reference to
944the queue we've created, and the prime number we've found.
945
946Finally, once the loop terminates (because we got a 0 or undef in the
947queue, which serves as a note to die), we pass on the notice to our
6eded8f3 948child and wait for it to exit if we've created a child (lines 32 and
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94937).
950
951Meanwhile, back in the main thread, we create a queue (line 9) and the
952initial child thread (line 10), and pre-seed it with the first prime:
9532. Then we queue all the numbers from 3 to 1000 for checking (lines
95412-14), then queue a die notice (line 16) and wait for the first child
955thread to terminate (line 17). Because a child won't die until its
956child has died, we know that we're done once we return from the join.
957
958That's how it works. It's pretty simple; as with many Perl programs,
959the explanation is much longer than the program.
960
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961=head1 Performance considerations
962
963The main thing to bear in mind when comparing ithreads to other threading
964models is the fact that for each new thread created, a complete copy of
965all the variables and data of the parent thread has to be taken. Thus
966thread creation can be quite expensive, both in terms of memory usage and
967time spent in creation. The ideal way to reduce these costs is to have a
968relatively short number of long-lived threads, all created fairly early
969on - before the base thread has accumulated too much data. Of course, this
970may not always be possible, so compromises have to be made. However, after
971a thread has been created, its performance and extra memory usage should
972be little different than ordinary code.
973
974Also note that under the current implementation, shared variables
975use a little more memory and are a little slower than ordinary variables.
976
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977=head1 Threadsafety of System Libraries
978
979Whether various library calls are threadsafe is outside the control
80bbcbc4 980of Perl. Calls often suffering from not being threadsafe include:
bdcfa4c7 981localtime(), gmtime(), get{gr,host,net,proto,serv,pw}*(), readdir(),
80bbcbc4
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982rand(), and srand() -- in general, calls that depend on some external
983state.
984
985If the system Perl is compiled in has threadsafe variants of such
986calls, they will be used. Beyond that, Perl is at the mercy of
987the threadsafety or unsafety of the calls. Please consult your
988C library call documentation.
989
990In some platforms the threadsafe interfaces may fail if the result
991buffer is too small (for example getgrent() may return quite large
992group member lists). Perl will retry growing the result buffer
993a few times, but only up to 64k (for safety reasons).
bdcfa4c7 994
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995=head1 Conclusion
996
997A complete thread tutorial could fill a book (and has, many times),
6eded8f3
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998but with what we've covered in this introduction, you should be well
999on your way to becoming a threaded Perl expert.
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1000
1001=head1 Bibliography
1002
1003Here's a short bibliography courtesy of Jürgen Christoffel:
1004
1005=head2 Introductory Texts
1006
1007Birrell, Andrew D. An Introduction to Programming with
1008Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report
1009#35 online as
6eded8f3
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1010http://gatekeeper.dec.com/pub/DEC/SRC/research-reports/abstracts/src-rr-035.html
1011(highly recommended)
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1012
1013Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A
1014Guide to Concurrency, Communication, and
1015Multithreading. Prentice-Hall, 1996.
1016
1017Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with
1018Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written
1019introduction to threads).
1020
1021Nelson, Greg (editor). Systems Programming with Modula-3. Prentice
1022Hall, 1991, ISBN 0-13-590464-1.
1023
1024Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell.
1025Pthreads Programming. O'Reilly & Associates, 1996, ISBN 156592-115-1
1026(covers POSIX threads).
1027
1028=head2 OS-Related References
1029
1030Boykin, Joseph, David Kirschen, Alan Langerman, and Susan
1031LoVerso. Programming under Mach. Addison-Wesley, 1994, ISBN
10320-201-52739-1.
1033
1034Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall,
10351995, ISBN 0-13-219908-4 (great textbook).
1036
1037Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts,
10384th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4
1039
1040=head2 Other References
1041
1042Arnold, Ken and James Gosling. The Java Programming Language, 2nd
1043ed. Addison-Wesley, 1998, ISBN 0-201-31006-6.
1044
1045Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage
1046Collection on Virtually Shared Memory Architectures" in Memory
1047Management: Proc. of the International Workshop IWMM 92, St. Malo,
1048France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer,
10491992, ISBN 3540-55940-X (real-life thread applications).
1050
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1051Artur Bergman, "Where Wizards Fear To Tread", June 11, 2002,
1052L<http://www.perl.com/pub/a/2002/06/11/threads.html>
1053
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1054=head1 Acknowledgements
1055
1056Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy
1057Sarathy, Ilya Zakharevich, Benjamin Sugars, Jürgen Christoffel, Joshua
1058Pritikin, and Alan Burlison, for their help in reality-checking and
1059polishing this article. Big thanks to Tom Christiansen for his rewrite
1060of the prime number generator.
1061
1062=head1 AUTHOR
1063
9316ed2f 1064Dan Sugalski E<lt>dan@sidhe.org<gt>
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1065
1066Slightly modified by Arthur Bergman to fit the new thread model/module.
1067
1068=head1 Copyrights
1069
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1070The original version of this article originally appeared in The Perl
1071Journal #10, and is copyright 1998 The Perl Journal. It appears courtesy
1072of Jon Orwant and The Perl Journal. This document may be distributed
1073under the same terms as Perl itself.
2605996a 1074
53d7eaa8 1075For more information please see L<threads> and L<threads::shared>.
2605996a 1076