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1 =head1 NAME 2 3 perlothrtut - old tutorial on threads in Perl 4 5 =head1 DESCRIPTION 6 7 B<WARNING>: 8 This tutorial describes the old-style thread model that was introduced in 9 release 5.005. This model is deprecated, and has been removed 10 for version 5.10. The interfaces described here were considered 11 experimental, and are likely to be buggy. 12 13 For information about the new interpreter threads ("ithreads") model, see 14 the F<perlthrtut> tutorial, and the L<threads> and L<threads::shared> 15 modules. 16 17 You are strongly encouraged to migrate any existing threads code to the 18 new model as soon as possible. 19 20 =head1 What Is A Thread Anyway? 21 22 A thread is a flow of control through a program with a single 23 execution point. 24 25 Sounds an awful lot like a process, doesn't it? Well, it should. 26 Threads are one of the pieces of a process. Every process has at least 27 one thread and, up until now, every process running Perl had only one 28 thread. With 5.005, though, you can create extra threads. We're going 29 to show you how, when, and why. 30 31 =head1 Threaded Program Models 32 33 There are three basic ways that you can structure a threaded 34 program. Which model you choose depends on what you need your program 35 to do. For many non-trivial threaded programs you'll need to choose 36 different models for different pieces of your program. 37 38 =head2 Boss/Worker 39 40 The boss/worker model usually has one `boss' thread and one or more 41 `worker' threads. The boss thread gathers or generates tasks that need 42 to be done, then parcels those tasks out to the appropriate worker 43 thread. 44 45 This model is common in GUI and server programs, where a main thread 46 waits for some event and then passes that event to the appropriate 47 worker threads for processing. Once the event has been passed on, the 48 boss thread goes back to waiting for another event. 49 50 The boss thread does relatively little work. While tasks aren't 51 necessarily performed faster than with any other method, it tends to 52 have the best user-response times. 53 54 =head2 Work Crew 55 56 In the work crew model, several threads are created that do 57 essentially the same thing to different pieces of data. It closely 58 mirrors classical parallel processing and vector processors, where a 59 large array of processors do the exact same thing to many pieces of 60 data. 61 62 This model is particularly useful if the system running the program 63 will distribute multiple threads across different processors. It can 64 also be useful in ray tracing or rendering engines, where the 65 individual threads can pass on interim results to give the user visual 66 feedback. 67 68 =head2 Pipeline 69 70 The pipeline model divides up a task into a series of steps, and 71 passes the results of one step on to the thread processing the 72 next. Each thread does one thing to each piece of data and passes the 73 results to the next thread in line. 74 75 This model makes the most sense if you have multiple processors so two 76 or more threads will be executing in parallel, though it can often 77 make sense in other contexts as well. It tends to keep the individual 78 tasks small and simple, as well as allowing some parts of the pipeline 79 to block (on I/O or system calls, for example) while other parts keep 80 going. If you're running different parts of the pipeline on different 81 processors you may also take advantage of the caches on each 82 processor. 83 84 This model is also handy for a form of recursive programming where, 85 rather than having a subroutine call itself, it instead creates 86 another thread. Prime and Fibonacci generators both map well to this 87 form of the pipeline model. (A version of a prime number generator is 88 presented later on.) 89 90 =head1 Native threads 91 92 There are several different ways to implement threads on a system. How 93 threads are implemented depends both on the vendor and, in some cases, 94 the version of the operating system. Often the first implementation 95 will be relatively simple, but later versions of the OS will be more 96 sophisticated. 97 98 While the information in this section is useful, it's not necessary, 99 so you can skip it if you don't feel up to it. 100 101 There are three basic categories of threads-user-mode threads, kernel 102 threads, and multiprocessor kernel threads. 103 104 User-mode threads are threads that live entirely within a program and 105 its libraries. In this model, the OS knows nothing about threads. As 106 far as it's concerned, your process is just a process. 107 108 This is the easiest way to implement threads, and the way most OSes 109 start. The big disadvantage is that, since the OS knows nothing about 110 threads, if one thread blocks they all do. Typical blocking activities 111 include most system calls, most I/O, and things like sleep(). 112 113 Kernel threads are the next step in thread evolution. The OS knows 114 about kernel threads, and makes allowances for them. The main 115 difference between a kernel thread and a user-mode thread is 116 blocking. With kernel threads, things that block a single thread don't 117 block other threads. This is not the case with user-mode threads, 118 where the kernel blocks at the process level and not the thread level. 119 120 This is a big step forward, and can give a threaded program quite a 121 performance boost over non-threaded programs. Threads that block 122 performing I/O, for example, won't block threads that are doing other 123 things. Each process still has only one thread running at once, 124 though, regardless of how many CPUs a system might have. 125 126 Since kernel threading can interrupt a thread at any time, they will 127 uncover some of the implicit locking assumptions you may make in your 128 program. For example, something as simple as C<$a = $a + 2> can behave 129 unpredictably with kernel threads if $a is visible to other 130 threads, as another thread may have changed $a between the time it 131 was fetched on the right hand side and the time the new value is 132 stored. 133 134 Multiprocessor Kernel Threads are the final step in thread 135 support. With multiprocessor kernel threads on a machine with multiple 136 CPUs, the OS may schedule two or more threads to run simultaneously on 137 different CPUs. 138 139 This can give a serious performance boost to your threaded program, 140 since more than one thread will be executing at the same time. As a 141 tradeoff, though, any of those nagging synchronization issues that 142 might not have shown with basic kernel threads will appear with a 143 vengeance. 144 145 In addition to the different levels of OS involvement in threads, 146 different OSes (and different thread implementations for a particular 147 OS) allocate CPU cycles to threads in different ways. 148 149 Cooperative multitasking systems have running threads give up control 150 if one of two things happen. If a thread calls a yield function, it 151 gives up control. It also gives up control if the thread does 152 something that would cause it to block, such as perform I/O. In a 153 cooperative multitasking implementation, one thread can starve all the 154 others for CPU time if it so chooses. 155 156 Preemptive multitasking systems interrupt threads at regular intervals 157 while the system decides which thread should run next. In a preemptive 158 multitasking system, one thread usually won't monopolize the CPU. 159 160 On some systems, there can be cooperative and preemptive threads 161 running simultaneously. (Threads running with realtime priorities 162 often behave cooperatively, for example, while threads running at 163 normal priorities behave preemptively.) 164 165 =head1 What kind of threads are perl threads? 166 167 If you have experience with other thread implementations, you might 168 find that things aren't quite what you expect. It's very important to 169 remember when dealing with Perl threads that Perl Threads Are Not X 170 Threads, for all values of X. They aren't POSIX threads, or 171 DecThreads, or Java's Green threads, or Win32 threads. There are 172 similarities, and the broad concepts are the same, but if you start 173 looking for implementation details you're going to be either 174 disappointed or confused. Possibly both. 175 176 This is not to say that Perl threads are completely different from 177 everything that's ever come before--they're not. Perl's threading 178 model owes a lot to other thread models, especially POSIX. Just as 179 Perl is not C, though, Perl threads are not POSIX threads. So if you 180 find yourself looking for mutexes, or thread priorities, it's time to 181 step back a bit and think about what you want to do and how Perl can 182 do it. 183 184 =head1 Threadsafe Modules 185 186 The addition of threads has changed Perl's internals 187 substantially. There are implications for people who write 188 modules--especially modules with XS code or external libraries. While 189 most modules won't encounter any problems, modules that aren't 190 explicitly tagged as thread-safe should be tested before being used in 191 production code. 192 193 Not all modules that you might use are thread-safe, and you should 194 always assume a module is unsafe unless the documentation says 195 otherwise. This includes modules that are distributed as part of the 196 core. Threads are a beta feature, and even some of the standard 197 modules aren't thread-safe. 198 199 If you're using a module that's not thread-safe for some reason, you 200 can protect yourself by using semaphores and lots of programming 201 discipline to control access to the module. Semaphores are covered 202 later in the article. Perl Threads Are Different 203 204 =head1 Thread Basics 205 206 The core Thread module provides the basic functions you need to write 207 threaded programs. In the following sections we'll cover the basics, 208 showing you what you need to do to create a threaded program. After 209 that, we'll go over some of the features of the Thread module that 210 make threaded programming easier. 211 212 =head2 Basic Thread Support 213 214 Thread support is a Perl compile-time option-it's something that's 215 turned on or off when Perl is built at your site, rather than when 216 your programs are compiled. If your Perl wasn't compiled with thread 217 support enabled, then any attempt to use threads will fail. 218 219 Remember that the threading support in 5.005 is in beta release, and 220 should be treated as such. You should expect that it may not function 221 entirely properly, and the thread interface may well change some 222 before it is a fully supported, production release. The beta version 223 shouldn't be used for mission-critical projects. Having said that, 224 threaded Perl is pretty nifty, and worth a look. 225 226 Your programs can use the Config module to check whether threads are 227 enabled. If your program can't run without them, you can say something 228 like: 229 230 $Config{usethreads} or die "Recompile Perl with threads to run this program."; 231 232 A possibly-threaded program using a possibly-threaded module might 233 have code like this: 234 235 use Config; 236 use MyMod; 237 238 if ($Config{usethreads}) { 239 # We have threads 240 require MyMod_threaded; 241 import MyMod_threaded; 242 } else { 243 require MyMod_unthreaded; 244 import MyMod_unthreaded; 245 } 246 247 Since code that runs both with and without threads is usually pretty 248 messy, it's best to isolate the thread-specific code in its own 249 module. In our example above, that's what MyMod_threaded is, and it's 250 only imported if we're running on a threaded Perl. 251 252 =head2 Creating Threads 253 254 The Thread package provides the tools you need to create new 255 threads. Like any other module, you need to tell Perl you want to use 256 it; use Thread imports all the pieces you need to create basic 257 threads. 258 259 The simplest, straightforward way to create a thread is with new(): 260 261 use Thread; 262 263 $thr = Thread->new( \&sub1 ); 264 265 sub sub1 { 266 print "In the thread\n"; 267 } 268 269 The new() method takes a reference to a subroutine and creates a new 270 thread, which starts executing in the referenced subroutine. Control 271 then passes both to the subroutine and the caller. 272 273 If you need to, your program can pass parameters to the subroutine as 274 part of the thread startup. Just include the list of parameters as 275 part of the C<Thread::new> call, like this: 276 277 use Thread; 278 $Param3 = "foo"; 279 $thr = Thread->new( \&sub1, "Param 1", "Param 2", $Param3 ); 280 $thr = Thread->new( \&sub1, @ParamList ); 281 $thr = Thread->new( \&sub1, qw(Param1 Param2 $Param3) ); 282 283 sub sub1 { 284 my @InboundParameters = @_; 285 print "In the thread\n"; 286 print "got parameters >", join("<>", @InboundParameters), "<\n"; 287 } 288 289 290 The subroutine runs like a normal Perl subroutine, and the call to new 291 Thread returns whatever the subroutine returns. 292 293 The last example illustrates another feature of threads. You can spawn 294 off several threads using the same subroutine. Each thread executes 295 the same subroutine, but in a separate thread with a separate 296 environment and potentially separate arguments. 297 298 The other way to spawn a new thread is with async(), which is a way to 299 spin off a chunk of code like eval(), but into its own thread: 300 301 use Thread qw(async); 302 303 $LineCount = 0; 304 305 $thr = async { 306 while(<>) {$LineCount++} 307 print "Got $LineCount lines\n"; 308 }; 309 310 print "Waiting for the linecount to end\n"; 311 $thr->join; 312 print "All done\n"; 313 314 You'll notice we did a use Thread qw(async) in that example. async is 315 not exported by default, so if you want it, you'll either need to 316 import it before you use it or fully qualify it as 317 Thread::async. You'll also note that there's a semicolon after the 318 closing brace. That's because async() treats the following block as an 319 anonymous subroutine, so the semicolon is necessary. 320 321 Like eval(), the code executes in the same context as it would if it 322 weren't spun off. Since both the code inside and after the async start 323 executing, you need to be careful with any shared resources. Locking 324 and other synchronization techniques are covered later. 325 326 =head2 Giving up control 327 328 There are times when you may find it useful to have a thread 329 explicitly give up the CPU to another thread. Your threading package 330 might not support preemptive multitasking for threads, for example, or 331 you may be doing something compute-intensive and want to make sure 332 that the user-interface thread gets called frequently. Regardless, 333 there are times that you might want a thread to give up the processor. 334 335 Perl's threading package provides the yield() function that does 336 this. yield() is pretty straightforward, and works like this: 337 338 use Thread qw(yield async); 339 async { 340 my $foo = 50; 341 while ($foo--) { print "first async\n" } 342 yield; 343 $foo = 50; 344 while ($foo--) { print "first async\n" } 345 }; 346 async { 347 my $foo = 50; 348 while ($foo--) { print "second async\n" } 349 yield; 350 $foo = 50; 351 while ($foo--) { print "second async\n" } 352 }; 353 354 =head2 Waiting For A Thread To Exit 355 356 Since threads are also subroutines, they can return values. To wait 357 for a thread to exit and extract any scalars it might return, you can 358 use the join() method. 359 360 use Thread; 361 $thr = Thread->new( \&sub1 ); 362 363 @ReturnData = $thr->join; 364 print "Thread returned @ReturnData"; 365 366 sub sub1 { return "Fifty-six", "foo", 2; } 367 368 In the example above, the join() method returns as soon as the thread 369 ends. In addition to waiting for a thread to finish and gathering up 370 any values that the thread might have returned, join() also performs 371 any OS cleanup necessary for the thread. That cleanup might be 372 important, especially for long-running programs that spawn lots of 373 threads. If you don't want the return values and don't want to wait 374 for the thread to finish, you should call the detach() method 375 instead. detach() is covered later in the article. 376 377 =head2 Errors In Threads 378 379 So what happens when an error occurs in a thread? Any errors that 380 could be caught with eval() are postponed until the thread is 381 joined. If your program never joins, the errors appear when your 382 program exits. 383 384 Errors deferred until a join() can be caught with eval(): 385 386 use Thread qw(async); 387 $thr = async {$b = 3/0}; # Divide by zero error 388 $foo = eval {$thr->join}; 389 if ($@) { 390 print "died with error $@\n"; 391 } else { 392 print "Hey, why aren't you dead?\n"; 393 } 394 395 eval() passes any results from the joined thread back unmodified, so 396 if you want the return value of the thread, this is your only chance 397 to get them. 398 399 =head2 Ignoring A Thread 400 401 join() does three things: it waits for a thread to exit, cleans up 402 after it, and returns any data the thread may have produced. But what 403 if you're not interested in the thread's return values, and you don't 404 really care when the thread finishes? All you want is for the thread 405 to get cleaned up after when it's done. 406 407 In this case, you use the detach() method. Once a thread is detached, 408 it'll run until it's finished, then Perl will clean up after it 409 automatically. 410 411 use Thread; 412 $thr = Thread->new( \&sub1 ); # Spawn the thread 413 414 $thr->detach; # Now we officially don't care any more 415 416 sub sub1 { 417 $a = 0; 418 while (1) { 419 $a++; 420 print "\$a is $a\n"; 421 sleep 1; 422 } 423 } 424 425 426 Once a thread is detached, it may not be joined, and any output that 427 it might have produced (if it was done and waiting for a join) is 428 lost. 429 430 =head1 Threads And Data 431 432 Now that we've covered the basics of threads, it's time for our next 433 topic: data. Threading introduces a couple of complications to data 434 access that non-threaded programs never need to worry about. 435 436 =head2 Shared And Unshared Data 437 438 The single most important thing to remember when using threads is that 439 all threads potentially have access to all the data anywhere in your 440 program. While this is true with a nonthreaded Perl program as well, 441 it's especially important to remember with a threaded program, since 442 more than one thread can be accessing this data at once. 443 444 Perl's scoping rules don't change because you're using threads. If a 445 subroutine (or block, in the case of async()) could see a variable if 446 you weren't running with threads, it can see it if you are. This is 447 especially important for the subroutines that create, and makes C<my> 448 variables even more important. Remember--if your variables aren't 449 lexically scoped (declared with C<my>) you're probably sharing them 450 between threads. 451 452 =head2 Thread Pitfall: Races 453 454 While threads bring a new set of useful tools, they also bring a 455 number of pitfalls. One pitfall is the race condition: 456 457 use Thread; 458 $a = 1; 459 $thr1 = Thread->new(\&sub1); 460 $thr2 = Thread->new(\&sub2); 461 462 sleep 10; 463 print "$a\n"; 464 465 sub sub1 { $foo = $a; $a = $foo + 1; } 466 sub sub2 { $bar = $a; $a = $bar + 1; } 467 468 What do you think $a will be? The answer, unfortunately, is "it 469 depends." Both sub1() and sub2() access the global variable $a, once 470 to read and once to write. Depending on factors ranging from your 471 thread implementation's scheduling algorithm to the phase of the moon, 472 $a can be 2 or 3. 473 474 Race conditions are caused by unsynchronized access to shared 475 data. Without explicit synchronization, there's no way to be sure that 476 nothing has happened to the shared data between the time you access it 477 and the time you update it. Even this simple code fragment has the 478 possibility of error: 479 480 use Thread qw(async); 481 $a = 2; 482 async{ $b = $a; $a = $b + 1; }; 483 async{ $c = $a; $a = $c + 1; }; 484 485 Two threads both access $a. Each thread can potentially be interrupted 486 at any point, or be executed in any order. At the end, $a could be 3 487 or 4, and both $b and $c could be 2 or 3. 488 489 Whenever your program accesses data or resources that can be accessed 490 by other threads, you must take steps to coordinate access or risk 491 data corruption and race conditions. 492 493 =head2 Controlling access: lock() 494 495 The lock() function takes a variable (or subroutine, but we'll get to 496 that later) and puts a lock on it. No other thread may lock the 497 variable until the locking thread exits the innermost block containing 498 the lock. Using lock() is straightforward: 499 500 use Thread qw(async); 501 $a = 4; 502 $thr1 = async { 503 $foo = 12; 504 { 505 lock ($a); # Block until we get access to $a 506 $b = $a; 507 $a = $b * $foo; 508 } 509 print "\$foo was $foo\n"; 510 }; 511 $thr2 = async { 512 $bar = 7; 513 { 514 lock ($a); # Block until we can get access to $a 515 $c = $a; 516 $a = $c * $bar; 517 } 518 print "\$bar was $bar\n"; 519 }; 520 $thr1->join; 521 $thr2->join; 522 print "\$a is $a\n"; 523 524 lock() blocks the thread until the variable being locked is 525 available. When lock() returns, your thread can be sure that no other 526 thread can lock that variable until the innermost block containing the 527 lock exits. 528 529 It's important to note that locks don't prevent access to the variable 530 in question, only lock attempts. This is in keeping with Perl's 531 longstanding tradition of courteous programming, and the advisory file 532 locking that flock() gives you. Locked subroutines behave differently, 533 however. We'll cover that later in the article. 534 535 You may lock arrays and hashes as well as scalars. Locking an array, 536 though, will not block subsequent locks on array elements, just lock 537 attempts on the array itself. 538 539 Finally, locks are recursive, which means it's okay for a thread to 540 lock a variable more than once. The lock will last until the outermost 541 lock() on the variable goes out of scope. 542 543 =head2 Thread Pitfall: Deadlocks 544 545 Locks are a handy tool to synchronize access to data. Using them 546 properly is the key to safe shared data. Unfortunately, locks aren't 547 without their dangers. Consider the following code: 548 549 use Thread qw(async yield); 550 $a = 4; 551 $b = "foo"; 552 async { 553 lock($a); 554 yield; 555 sleep 20; 556 lock ($b); 557 }; 558 async { 559 lock($b); 560 yield; 561 sleep 20; 562 lock ($a); 563 }; 564 565 This program will probably hang until you kill it. The only way it 566 won't hang is if one of the two async() routines acquires both locks 567 first. A guaranteed-to-hang version is more complicated, but the 568 principle is the same. 569 570 The first thread spawned by async() will grab a lock on $a then, a 571 second or two later, try to grab a lock on $b. Meanwhile, the second 572 thread grabs a lock on $b, then later tries to grab a lock on $a. The 573 second lock attempt for both threads will block, each waiting for the 574 other to release its lock. 575 576 This condition is called a deadlock, and it occurs whenever two or 577 more threads are trying to get locks on resources that the others 578 own. Each thread will block, waiting for the other to release a lock 579 on a resource. That never happens, though, since the thread with the 580 resource is itself waiting for a lock to be released. 581 582 There are a number of ways to handle this sort of problem. The best 583 way is to always have all threads acquire locks in the exact same 584 order. If, for example, you lock variables $a, $b, and $c, always lock 585 $a before $b, and $b before $c. It's also best to hold on to locks for 586 as short a period of time to minimize the risks of deadlock. 587 588 =head2 Queues: Passing Data Around 589 590 A queue is a special thread-safe object that lets you put data in one 591 end and take it out the other without having to worry about 592 synchronization issues. They're pretty straightforward, and look like 593 this: 594 595 use Thread qw(async); 596 use Thread::Queue; 597 598 my $DataQueue = Thread::Queue->new(); 599 $thr = async { 600 while ($DataElement = $DataQueue->dequeue) { 601 print "Popped $DataElement off the queue\n"; 602 } 603 }; 604 605 $DataQueue->enqueue(12); 606 $DataQueue->enqueue("A", "B", "C"); 607 sleep 10; 608 $DataQueue->enqueue(undef); 609 610 You create the queue with new Thread::Queue. Then you can add lists of 611 scalars onto the end with enqueue(), and pop scalars off the front of 612 it with dequeue(). A queue has no fixed size, and can grow as needed 613 to hold everything pushed on to it. 614 615 If a queue is empty, dequeue() blocks until another thread enqueues 616 something. This makes queues ideal for event loops and other 617 communications between threads. 618 619 =head1 Threads And Code 620 621 In addition to providing thread-safe access to data via locks and 622 queues, threaded Perl also provides general-purpose semaphores for 623 coarser synchronization than locks provide and thread-safe access to 624 entire subroutines. 625 626 =head2 Semaphores: Synchronizing Data Access 627 628 Semaphores are a kind of generic locking mechanism. Unlike lock, which 629 gets a lock on a particular scalar, Perl doesn't associate any 630 particular thing with a semaphore so you can use them to control 631 access to anything you like. In addition, semaphores can allow more 632 than one thread to access a resource at once, though by default 633 semaphores only allow one thread access at a time. 634 635 =over 4 636 637 =item Basic semaphores 638 639 Semaphores have two methods, down and up. down decrements the resource 640 count, while up increments it. down calls will block if the 641 semaphore's current count would decrement below zero. This program 642 gives a quick demonstration: 643 644 use Thread qw(yield); 645 use Thread::Semaphore; 646 my $semaphore = Thread::Semaphore->new(); 647 $GlobalVariable = 0; 648 649 $thr1 = Thread->new( \&sample_sub, 1 ); 650 $thr2 = Thread->new( \&sample_sub, 2 ); 651 $thr3 = Thread->new( \&sample_sub, 3 ); 652 653 sub sample_sub { 654 my $SubNumber = shift @_; 655 my $TryCount = 10; 656 my $LocalCopy; 657 sleep 1; 658 while ($TryCount--) { 659 $semaphore->down; 660 $LocalCopy = $GlobalVariable; 661 print "$TryCount tries left for sub $SubNumber (\$GlobalVariable is $GlobalVariable)\n"; 662 yield; 663 sleep 2; 664 $LocalCopy++; 665 $GlobalVariable = $LocalCopy; 666 $semaphore->up; 667 } 668 } 669 670 The three invocations of the subroutine all operate in sync. The 671 semaphore, though, makes sure that only one thread is accessing the 672 global variable at once. 673 674 =item Advanced Semaphores 675 676 By default, semaphores behave like locks, letting only one thread 677 down() them at a time. However, there are other uses for semaphores. 678 679 Each semaphore has a counter attached to it. down() decrements the 680 counter and up() increments the counter. By default, semaphores are 681 created with the counter set to one, down() decrements by one, and 682 up() increments by one. If down() attempts to decrement the counter 683 below zero, it blocks until the counter is large enough. Note that 684 while a semaphore can be created with a starting count of zero, any 685 up() or down() always changes the counter by at least 686 one. $semaphore->down(0) is the same as $semaphore->down(1). 687 688 The question, of course, is why would you do something like this? Why 689 create a semaphore with a starting count that's not one, or why 690 decrement/increment it by more than one? The answer is resource 691 availability. Many resources that you want to manage access for can be 692 safely used by more than one thread at once. 693 694 For example, let's take a GUI driven program. It has a semaphore that 695 it uses to synchronize access to the display, so only one thread is 696 ever drawing at once. Handy, but of course you don't want any thread 697 to start drawing until things are properly set up. In this case, you 698 can create a semaphore with a counter set to zero, and up it when 699 things are ready for drawing. 700 701 Semaphores with counters greater than one are also useful for 702 establishing quotas. Say, for example, that you have a number of 703 threads that can do I/O at once. You don't want all the threads 704 reading or writing at once though, since that can potentially swamp 705 your I/O channels, or deplete your process' quota of filehandles. You 706 can use a semaphore initialized to the number of concurrent I/O 707 requests (or open files) that you want at any one time, and have your 708 threads quietly block and unblock themselves. 709 710 Larger increments or decrements are handy in those cases where a 711 thread needs to check out or return a number of resources at once. 712 713 =back 714 715 =head2 Attributes: Restricting Access To Subroutines 716 717 In addition to synchronizing access to data or resources, you might 718 find it useful to synchronize access to subroutines. You may be 719 accessing a singular machine resource (perhaps a vector processor), or 720 find it easier to serialize calls to a particular subroutine than to 721 have a set of locks and semaphores. 722 723 One of the additions to Perl 5.005 is subroutine attributes. The 724 Thread package uses these to provide several flavors of 725 serialization. It's important to remember that these attributes are 726 used in the compilation phase of your program so you can't change a 727 subroutine's behavior while your program is actually running. 728 729 =head2 Subroutine Locks 730 731 The basic subroutine lock looks like this: 732 733 sub test_sub :locked { 734 } 735 736 This ensures that only one thread will be executing this subroutine at 737 any one time. Once a thread calls this subroutine, any other thread 738 that calls it will block until the thread in the subroutine exits 739 it. A more elaborate example looks like this: 740 741 use Thread qw(yield); 742 743 new Thread \&thread_sub, 1; 744 new Thread \&thread_sub, 2; 745 new Thread \&thread_sub, 3; 746 new Thread \&thread_sub, 4; 747 748 sub sync_sub :locked { 749 my $CallingThread = shift @_; 750 print "In sync_sub for thread $CallingThread\n"; 751 yield; 752 sleep 3; 753 print "Leaving sync_sub for thread $CallingThread\n"; 754 } 755 756 sub thread_sub { 757 my $ThreadID = shift @_; 758 print "Thread $ThreadID calling sync_sub\n"; 759 sync_sub($ThreadID); 760 print "$ThreadID is done with sync_sub\n"; 761 } 762 763 The C<locked> attribute tells perl to lock sync_sub(), and if you run 764 this, you can see that only one thread is in it at any one time. 765 766 =head2 Methods 767 768 Locking an entire subroutine can sometimes be overkill, especially 769 when dealing with Perl objects. When calling a method for an object, 770 for example, you want to serialize calls to a method, so that only one 771 thread will be in the subroutine for a particular object, but threads 772 calling that subroutine for a different object aren't blocked. The 773 method attribute indicates whether the subroutine is really a method. 774 775 use Thread; 776 777 sub tester { 778 my $thrnum = shift @_; 779 my $bar = Foo->new(); 780 foreach (1..10) { 781 print "$thrnum calling per_object\n"; 782 $bar->per_object($thrnum); 783 print "$thrnum out of per_object\n"; 784 yield; 785 print "$thrnum calling one_at_a_time\n"; 786 $bar->one_at_a_time($thrnum); 787 print "$thrnum out of one_at_a_time\n"; 788 yield; 789 } 790 } 791 792 foreach my $thrnum (1..10) { 793 new Thread \&tester, $thrnum; 794 } 795 796 package Foo; 797 sub new { 798 my $class = shift @_; 799 return bless [@_], $class; 800 } 801 802 sub per_object :locked :method { 803 my ($class, $thrnum) = @_; 804 print "In per_object for thread $thrnum\n"; 805 yield; 806 sleep 2; 807 print "Exiting per_object for thread $thrnum\n"; 808 } 809 810 sub one_at_a_time :locked { 811 my ($class, $thrnum) = @_; 812 print "In one_at_a_time for thread $thrnum\n"; 813 yield; 814 sleep 2; 815 print "Exiting one_at_a_time for thread $thrnum\n"; 816 } 817 818 As you can see from the output (omitted for brevity; it's 800 lines) 819 all the threads can be in per_object() simultaneously, but only one 820 thread is ever in one_at_a_time() at once. 821 822 =head2 Locking A Subroutine 823 824 You can lock a subroutine as you would lock a variable. Subroutine locks 825 work the same as specifying a C<locked> attribute for the subroutine, 826 and block all access to the subroutine for other threads until the 827 lock goes out of scope. When the subroutine isn't locked, any number 828 of threads can be in it at once, and getting a lock on a subroutine 829 doesn't affect threads already in the subroutine. Getting a lock on a 830 subroutine looks like this: 831 832 lock(\&sub_to_lock); 833 834 Simple enough. Unlike the C<locked> attribute, which is a compile time 835 option, locking and unlocking a subroutine can be done at runtime at your 836 discretion. There is some runtime penalty to using lock(\&sub) instead 837 of the C<locked> attribute, so make sure you're choosing the proper 838 method to do the locking. 839 840 You'd choose lock(\&sub) when writing modules and code to run on both 841 threaded and unthreaded Perl, especially for code that will run on 842 5.004 or earlier Perls. In that case, it's useful to have subroutines 843 that should be serialized lock themselves if they're running threaded, 844 like so: 845 846 package Foo; 847 use Config; 848 $Running_Threaded = 0; 849 850 BEGIN { $Running_Threaded = $Config{'usethreads'} } 851 852 sub sub1 { lock(\&sub1) if $Running_Threaded } 853 854 855 This way you can ensure single-threadedness regardless of which 856 version of Perl you're running. 857 858 =head1 General Thread Utility Routines 859 860 We've covered the workhorse parts of Perl's threading package, and 861 with these tools you should be well on your way to writing threaded 862 code and packages. There are a few useful little pieces that didn't 863 really fit in anyplace else. 864 865 =head2 What Thread Am I In? 866 867 The Thread->self method provides your program with a way to get an 868 object representing the thread it's currently in. You can use this 869 object in the same way as the ones returned from the thread creation. 870 871 =head2 Thread IDs 872 873 tid() is a thread object method that returns the thread ID of the 874 thread the object represents. Thread IDs are integers, with the main 875 thread in a program being 0. Currently Perl assigns a unique tid to 876 every thread ever created in your program, assigning the first thread 877 to be created a tid of 1, and increasing the tid by 1 for each new 878 thread that's created. 879 880 =head2 Are These Threads The Same? 881 882 The equal() method takes two thread objects and returns true 883 if the objects represent the same thread, and false if they don't. 884 885 =head2 What Threads Are Running? 886 887 Thread->list returns a list of thread objects, one for each thread 888 that's currently running. Handy for a number of things, including 889 cleaning up at the end of your program: 890 891 # Loop through all the threads 892 foreach $thr (Thread->list) { 893 # Don't join the main thread or ourselves 894 if ($thr->tid && !Thread::equal($thr, Thread->self)) { 895 $thr->join; 896 } 897 } 898 899 The example above is just for illustration. It isn't strictly 900 necessary to join all the threads you create, since Perl detaches all 901 the threads before it exits. 902 903 =head1 A Complete Example 904 905 Confused yet? It's time for an example program to show some of the 906 things we've covered. This program finds prime numbers using threads. 907 908 1 #!/usr/bin/perl -w 909 2 # prime-pthread, courtesy of Tom Christiansen 910 3 911 4 use strict; 912 5 913 6 use Thread; 914 7 use Thread::Queue; 915 8 916 9 my $stream = Thread::Queue->new(); 917 10 my $kid = Thread->new(\&check_num, $stream, 2); 918 11 919 12 for my $i ( 3 .. 1000 ) { 920 13 $stream->enqueue($i); 921 14 } 922 15 923 16 $stream->enqueue(undef); 924 17 $kid->join(); 925 18 926 19 sub check_num { 927 20 my ($upstream, $cur_prime) = @_; 928 21 my $kid; 929 22 my $downstream = Thread::Queue->new(); 930 23 while (my $num = $upstream->dequeue) { 931 24 next unless $num % $cur_prime; 932 25 if ($kid) { 933 26 $downstream->enqueue($num); 934 27 } else { 935 28 print "Found prime $num\n"; 936 29 $kid = Thread->new(\&check_num, $downstream, $num); 937 30 } 938 31 } 939 32 $downstream->enqueue(undef) if $kid; 940 33 $kid->join() if $kid; 941 34 } 942 943 This program uses the pipeline model to generate prime numbers. Each 944 thread in the pipeline has an input queue that feeds numbers to be 945 checked, a prime number that it's responsible for, and an output queue 946 that it funnels numbers that have failed the check into. If the thread 947 has a number that's failed its check and there's no child thread, then 948 the thread must have found a new prime number. In that case, a new 949 child thread is created for that prime and stuck on the end of the 950 pipeline. 951 952 This probably sounds a bit more confusing than it really is, so lets 953 go through this program piece by piece and see what it does. (For 954 those of you who might be trying to remember exactly what a prime 955 number is, it's a number that's only evenly divisible by itself and 1) 956 957 The bulk of the work is done by the check_num() subroutine, which 958 takes a reference to its input queue and a prime number that it's 959 responsible for. After pulling in the input queue and the prime that 960 the subroutine's checking (line 20), we create a new queue (line 22) 961 and reserve a scalar for the thread that we're likely to create later 962 (line 21). 963 964 The while loop from lines 23 to line 31 grabs a scalar off the input 965 queue and checks against the prime this thread is responsible 966 for. Line 24 checks to see if there's a remainder when we modulo the 967 number to be checked against our prime. If there is one, the number 968 must not be evenly divisible by our prime, so we need to either pass 969 it on to the next thread if we've created one (line 26) or create a 970 new thread if we haven't. 971 972 The new thread creation is line 29. We pass on to it a reference to 973 the queue we've created, and the prime number we've found. 974 975 Finally, once the loop terminates (because we got a 0 or undef in the 976 queue, which serves as a note to die), we pass on the notice to our 977 child and wait for it to exit if we've created a child (Lines 32 and 978 37). 979 980 Meanwhile, back in the main thread, we create a queue (line 9) and the 981 initial child thread (line 10), and pre-seed it with the first prime: 982 2. Then we queue all the numbers from 3 to 1000 for checking (lines 983 12-14), then queue a die notice (line 16) and wait for the first child 984 thread to terminate (line 17). Because a child won't die until its 985 child has died, we know that we're done once we return from the join. 986 987 That's how it works. It's pretty simple; as with many Perl programs, 988 the explanation is much longer than the program. 989 990 =head1 Conclusion 991 992 A complete thread tutorial could fill a book (and has, many times), 993 but this should get you well on your way. The final authority on how 994 Perl's threads behave is the documentation bundled with the Perl 995 distribution, but with what we've covered in this article, you should 996 be well on your way to becoming a threaded Perl expert. 997 998 =head1 Bibliography 999 1000 Here's a short bibliography courtesy of Jürgen Christoffel: 1001 1002 =head2 Introductory Texts 1003 1004 Birrell, Andrew D. An Introduction to Programming with 1005 Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report 1006 #35 online as 1007 http://www.research.digital.com/SRC/staff/birrell/bib.html (highly 1008 recommended) 1009 1010 Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A 1011 Guide to Concurrency, Communication, and 1012 Multithreading. Prentice-Hall, 1996. 1013 1014 Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with 1015 Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written 1016 introduction to threads). 1017 1018 Nelson, Greg (editor). Systems Programming with Modula-3. Prentice 1019 Hall, 1991, ISBN 0-13-590464-1. 1020 1021 Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell. 1022 Pthreads Programming. O'Reilly & Associates, 1996, ISBN 156592-115-1 1023 (covers POSIX threads). 1024 1025 =head2 OS-Related References 1026 1027 Boykin, Joseph, David Kirschen, Alan Langerman, and Susan 1028 LoVerso. Programming under Mach. Addison-Wesley, 1994, ISBN 1029 0-201-52739-1. 1030 1031 Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall, 1032 1995, ISBN 0-13-219908-4 (great textbook). 1033 1034 Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts, 1035 4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4 1036 1037 =head2 Other References 1038 1039 Arnold, Ken and James Gosling. The Java Programming Language, 2nd 1040 ed. Addison-Wesley, 1998, ISBN 0-201-31006-6. 1041 1042 Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage 1043 Collection on Virtually Shared Memory Architectures" in Memory 1044 Management: Proc. of the International Workshop IWMM 92, St. Malo, 1045 France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer, 1046 1992, ISBN 3540-55940-X (real-life thread applications). 1047 1048 =head1 Acknowledgements 1049 1050 Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy 1051 Sarathy, Ilya Zakharevich, Benjamin Sugars, Jürgen Christoffel, Joshua 1052 Pritikin, and Alan Burlison, for their help in reality-checking and 1053 polishing this article. Big thanks to Tom Christiansen for his rewrite 1054 of the prime number generator. 1055 1056 =head1 AUTHOR 1057 1058 Dan Sugalski E<lt>sugalskd@ous.eduE<gt> 1059 1060 =head1 Copyrights 1061 1062 This article originally appeared in The Perl Journal #10, and is 1063 copyright 1998 The Perl Journal. It appears courtesy of Jon Orwant and 1064 The Perl Journal. This document may be distributed under the same terms 1065 as Perl itself. 1066 1067
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