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| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "PERLOTHRTUT 1" |
| 132 | .TH PERLOTHRTUT 1 "2006-01-07" "perl v5.8.8" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | perlothrtut \- old tutorial on threads in Perl |
| 135 | .SH "DESCRIPTION" |
| 136 | .IX Header "DESCRIPTION" |
| 137 | \&\fB\s-1WARNING\s0\fR: |
| 138 | This tutorial describes the old-style thread model that was introduced in |
| 139 | release 5.005. This model is now deprecated, and will be removed, probably |
| 140 | in version 5.10. The interfaces described here were considered |
| 141 | experimental, and are likely to be buggy. |
| 142 | .PP |
| 143 | For information about the new interpreter threads (\*(L"ithreads\*(R") model, see |
| 144 | the \fIperlthrtut\fR tutorial, and the threads and threads::shared |
| 145 | modules. |
| 146 | .PP |
| 147 | You are strongly encouraged to migrate any existing threads code to the |
| 148 | new model as soon as possible. |
| 149 | .SH "What Is A Thread Anyway?" |
| 150 | .IX Header "What Is A Thread Anyway?" |
| 151 | A thread is a flow of control through a program with a single |
| 152 | execution point. |
| 153 | .PP |
| 154 | Sounds an awful lot like a process, doesn't it? Well, it should. |
| 155 | Threads are one of the pieces of a process. Every process has at least |
| 156 | one thread and, up until now, every process running Perl had only one |
| 157 | thread. With 5.005, though, you can create extra threads. We're going |
| 158 | to show you how, when, and why. |
| 159 | .SH "Threaded Program Models" |
| 160 | .IX Header "Threaded Program Models" |
| 161 | There are three basic ways that you can structure a threaded |
| 162 | program. Which model you choose depends on what you need your program |
| 163 | to do. For many non-trivial threaded programs you'll need to choose |
| 164 | different models for different pieces of your program. |
| 165 | .Sh "Boss/Worker" |
| 166 | .IX Subsection "Boss/Worker" |
| 167 | The boss/worker model usually has one `boss' thread and one or more |
| 168 | `worker' threads. The boss thread gathers or generates tasks that need |
| 169 | to be done, then parcels those tasks out to the appropriate worker |
| 170 | thread. |
| 171 | .PP |
| 172 | This model is common in \s-1GUI\s0 and server programs, where a main thread |
| 173 | waits for some event and then passes that event to the appropriate |
| 174 | worker threads for processing. Once the event has been passed on, the |
| 175 | boss thread goes back to waiting for another event. |
| 176 | .PP |
| 177 | The boss thread does relatively little work. While tasks aren't |
| 178 | necessarily performed faster than with any other method, it tends to |
| 179 | have the best user-response times. |
| 180 | .Sh "Work Crew" |
| 181 | .IX Subsection "Work Crew" |
| 182 | In the work crew model, several threads are created that do |
| 183 | essentially the same thing to different pieces of data. It closely |
| 184 | mirrors classical parallel processing and vector processors, where a |
| 185 | large array of processors do the exact same thing to many pieces of |
| 186 | data. |
| 187 | .PP |
| 188 | This model is particularly useful if the system running the program |
| 189 | will distribute multiple threads across different processors. It can |
| 190 | also be useful in ray tracing or rendering engines, where the |
| 191 | individual threads can pass on interim results to give the user visual |
| 192 | feedback. |
| 193 | .Sh "Pipeline" |
| 194 | .IX Subsection "Pipeline" |
| 195 | The pipeline model divides up a task into a series of steps, and |
| 196 | passes the results of one step on to the thread processing the |
| 197 | next. Each thread does one thing to each piece of data and passes the |
| 198 | results to the next thread in line. |
| 199 | .PP |
| 200 | This model makes the most sense if you have multiple processors so two |
| 201 | or more threads will be executing in parallel, though it can often |
| 202 | make sense in other contexts as well. It tends to keep the individual |
| 203 | tasks small and simple, as well as allowing some parts of the pipeline |
| 204 | to block (on I/O or system calls, for example) while other parts keep |
| 205 | going. If you're running different parts of the pipeline on different |
| 206 | processors you may also take advantage of the caches on each |
| 207 | processor. |
| 208 | .PP |
| 209 | This model is also handy for a form of recursive programming where, |
| 210 | rather than having a subroutine call itself, it instead creates |
| 211 | another thread. Prime and Fibonacci generators both map well to this |
| 212 | form of the pipeline model. (A version of a prime number generator is |
| 213 | presented later on.) |
| 214 | .SH "Native threads" |
| 215 | .IX Header "Native threads" |
| 216 | There are several different ways to implement threads on a system. How |
| 217 | threads are implemented depends both on the vendor and, in some cases, |
| 218 | the version of the operating system. Often the first implementation |
| 219 | will be relatively simple, but later versions of the \s-1OS\s0 will be more |
| 220 | sophisticated. |
| 221 | .PP |
| 222 | While the information in this section is useful, it's not necessary, |
| 223 | so you can skip it if you don't feel up to it. |
| 224 | .PP |
| 225 | There are three basic categories of threads-user-mode threads, kernel |
| 226 | threads, and multiprocessor kernel threads. |
| 227 | .PP |
| 228 | User-mode threads are threads that live entirely within a program and |
| 229 | its libraries. In this model, the \s-1OS\s0 knows nothing about threads. As |
| 230 | far as it's concerned, your process is just a process. |
| 231 | .PP |
| 232 | This is the easiest way to implement threads, and the way most OSes |
| 233 | start. The big disadvantage is that, since the \s-1OS\s0 knows nothing about |
| 234 | threads, if one thread blocks they all do. Typical blocking activities |
| 235 | include most system calls, most I/O, and things like \fIsleep()\fR. |
| 236 | .PP |
| 237 | Kernel threads are the next step in thread evolution. The \s-1OS\s0 knows |
| 238 | about kernel threads, and makes allowances for them. The main |
| 239 | difference between a kernel thread and a user-mode thread is |
| 240 | blocking. With kernel threads, things that block a single thread don't |
| 241 | block other threads. This is not the case with user-mode threads, |
| 242 | where the kernel blocks at the process level and not the thread level. |
| 243 | .PP |
| 244 | This is a big step forward, and can give a threaded program quite a |
| 245 | performance boost over non-threaded programs. Threads that block |
| 246 | performing I/O, for example, won't block threads that are doing other |
| 247 | things. Each process still has only one thread running at once, |
| 248 | though, regardless of how many CPUs a system might have. |
| 249 | .PP |
| 250 | Since kernel threading can interrupt a thread at any time, they will |
| 251 | uncover some of the implicit locking assumptions you may make in your |
| 252 | program. For example, something as simple as \f(CW\*(C`$a = $a + 2\*(C'\fR can behave |
| 253 | unpredictably with kernel threads if \f(CW$a\fR is visible to other |
| 254 | threads, as another thread may have changed \f(CW$a\fR between the time it |
| 255 | was fetched on the right hand side and the time the new value is |
| 256 | stored. |
| 257 | .PP |
| 258 | Multiprocessor Kernel Threads are the final step in thread |
| 259 | support. With multiprocessor kernel threads on a machine with multiple |
| 260 | CPUs, the \s-1OS\s0 may schedule two or more threads to run simultaneously on |
| 261 | different CPUs. |
| 262 | .PP |
| 263 | This can give a serious performance boost to your threaded program, |
| 264 | since more than one thread will be executing at the same time. As a |
| 265 | tradeoff, though, any of those nagging synchronization issues that |
| 266 | might not have shown with basic kernel threads will appear with a |
| 267 | vengeance. |
| 268 | .PP |
| 269 | In addition to the different levels of \s-1OS\s0 involvement in threads, |
| 270 | different OSes (and different thread implementations for a particular |
| 271 | \&\s-1OS\s0) allocate \s-1CPU\s0 cycles to threads in different ways. |
| 272 | .PP |
| 273 | Cooperative multitasking systems have running threads give up control |
| 274 | if one of two things happen. If a thread calls a yield function, it |
| 275 | gives up control. It also gives up control if the thread does |
| 276 | something that would cause it to block, such as perform I/O. In a |
| 277 | cooperative multitasking implementation, one thread can starve all the |
| 278 | others for \s-1CPU\s0 time if it so chooses. |
| 279 | .PP |
| 280 | Preemptive multitasking systems interrupt threads at regular intervals |
| 281 | while the system decides which thread should run next. In a preemptive |
| 282 | multitasking system, one thread usually won't monopolize the \s-1CPU\s0. |
| 283 | .PP |
| 284 | On some systems, there can be cooperative and preemptive threads |
| 285 | running simultaneously. (Threads running with realtime priorities |
| 286 | often behave cooperatively, for example, while threads running at |
| 287 | normal priorities behave preemptively.) |
| 288 | .SH "What kind of threads are perl threads?" |
| 289 | .IX Header "What kind of threads are perl threads?" |
| 290 | If you have experience with other thread implementations, you might |
| 291 | find that things aren't quite what you expect. It's very important to |
| 292 | remember when dealing with Perl threads that Perl Threads Are Not X |
| 293 | Threads, for all values of X. They aren't \s-1POSIX\s0 threads, or |
| 294 | DecThreads, or Java's Green threads, or Win32 threads. There are |
| 295 | similarities, and the broad concepts are the same, but if you start |
| 296 | looking for implementation details you're going to be either |
| 297 | disappointed or confused. Possibly both. |
| 298 | .PP |
| 299 | This is not to say that Perl threads are completely different from |
| 300 | everything that's ever come before\*(--they're not. Perl's threading |
| 301 | model owes a lot to other thread models, especially \s-1POSIX\s0. Just as |
| 302 | Perl is not C, though, Perl threads are not \s-1POSIX\s0 threads. So if you |
| 303 | find yourself looking for mutexes, or thread priorities, it's time to |
| 304 | step back a bit and think about what you want to do and how Perl can |
| 305 | do it. |
| 306 | .SH "Threadsafe Modules" |
| 307 | .IX Header "Threadsafe Modules" |
| 308 | The addition of threads has changed Perl's internals |
| 309 | substantially. There are implications for people who write |
| 310 | modules\*(--especially modules with \s-1XS\s0 code or external libraries. While |
| 311 | most modules won't encounter any problems, modules that aren't |
| 312 | explicitly tagged as thread-safe should be tested before being used in |
| 313 | production code. |
| 314 | .PP |
| 315 | Not all modules that you might use are thread\-safe, and you should |
| 316 | always assume a module is unsafe unless the documentation says |
| 317 | otherwise. This includes modules that are distributed as part of the |
| 318 | core. Threads are a beta feature, and even some of the standard |
| 319 | modules aren't thread\-safe. |
| 320 | .PP |
| 321 | If you're using a module that's not thread-safe for some reason, you |
| 322 | can protect yourself by using semaphores and lots of programming |
| 323 | discipline to control access to the module. Semaphores are covered |
| 324 | later in the article. Perl Threads Are Different |
| 325 | .SH "Thread Basics" |
| 326 | .IX Header "Thread Basics" |
| 327 | The core Thread module provides the basic functions you need to write |
| 328 | threaded programs. In the following sections we'll cover the basics, |
| 329 | showing you what you need to do to create a threaded program. After |
| 330 | that, we'll go over some of the features of the Thread module that |
| 331 | make threaded programming easier. |
| 332 | .Sh "Basic Thread Support" |
| 333 | .IX Subsection "Basic Thread Support" |
| 334 | Thread support is a Perl compile-time option\-it's something that's |
| 335 | turned on or off when Perl is built at your site, rather than when |
| 336 | your programs are compiled. If your Perl wasn't compiled with thread |
| 337 | support enabled, then any attempt to use threads will fail. |
| 338 | .PP |
| 339 | Remember that the threading support in 5.005 is in beta release, and |
| 340 | should be treated as such. You should expect that it may not function |
| 341 | entirely properly, and the thread interface may well change some |
| 342 | before it is a fully supported, production release. The beta version |
| 343 | shouldn't be used for mission-critical projects. Having said that, |
| 344 | threaded Perl is pretty nifty, and worth a look. |
| 345 | .PP |
| 346 | Your programs can use the Config module to check whether threads are |
| 347 | enabled. If your program can't run without them, you can say something |
| 348 | like: |
| 349 | .PP |
| 350 | .Vb 1 |
| 351 | \& $Config{usethreads} or die "Recompile Perl with threads to run this program."; |
| 352 | .Ve |
| 353 | .PP |
| 354 | A possibly-threaded program using a possibly-threaded module might |
| 355 | have code like this: |
| 356 | .PP |
| 357 | .Vb 2 |
| 358 | \& use Config; |
| 359 | \& use MyMod; |
| 360 | .Ve |
| 361 | .PP |
| 362 | .Vb 8 |
| 363 | \& if ($Config{usethreads}) { |
| 364 | \& # We have threads |
| 365 | \& require MyMod_threaded; |
| 366 | \& import MyMod_threaded; |
| 367 | \& } else { |
| 368 | \& require MyMod_unthreaded; |
| 369 | \& import MyMod_unthreaded; |
| 370 | \& } |
| 371 | .Ve |
| 372 | .PP |
| 373 | Since code that runs both with and without threads is usually pretty |
| 374 | messy, it's best to isolate the thread-specific code in its own |
| 375 | module. In our example above, that's what MyMod_threaded is, and it's |
| 376 | only imported if we're running on a threaded Perl. |
| 377 | .Sh "Creating Threads" |
| 378 | .IX Subsection "Creating Threads" |
| 379 | The Thread package provides the tools you need to create new |
| 380 | threads. Like any other module, you need to tell Perl you want to use |
| 381 | it; use Thread imports all the pieces you need to create basic |
| 382 | threads. |
| 383 | .PP |
| 384 | The simplest, straightforward way to create a thread is with \fInew()\fR: |
| 385 | .PP |
| 386 | .Vb 1 |
| 387 | \& use Thread; |
| 388 | .Ve |
| 389 | .PP |
| 390 | .Vb 1 |
| 391 | \& $thr = new Thread \e&sub1; |
| 392 | .Ve |
| 393 | .PP |
| 394 | .Vb 3 |
| 395 | \& sub sub1 { |
| 396 | \& print "In the thread\en"; |
| 397 | \& } |
| 398 | .Ve |
| 399 | .PP |
| 400 | The \fInew()\fR method takes a reference to a subroutine and creates a new |
| 401 | thread, which starts executing in the referenced subroutine. Control |
| 402 | then passes both to the subroutine and the caller. |
| 403 | .PP |
| 404 | If you need to, your program can pass parameters to the subroutine as |
| 405 | part of the thread startup. Just include the list of parameters as |
| 406 | part of the \f(CW\*(C`Thread::new\*(C'\fR call, like this: |
| 407 | .PP |
| 408 | .Vb 5 |
| 409 | \& use Thread; |
| 410 | \& $Param3 = "foo"; |
| 411 | \& $thr = new Thread \e&sub1, "Param 1", "Param 2", $Param3; |
| 412 | \& $thr = new Thread \e&sub1, @ParamList; |
| 413 | \& $thr = new Thread \e&sub1, qw(Param1 Param2 $Param3); |
| 414 | .Ve |
| 415 | .PP |
| 416 | .Vb 5 |
| 417 | \& sub sub1 { |
| 418 | \& my @InboundParameters = @_; |
| 419 | \& print "In the thread\en"; |
| 420 | \& print "got parameters >", join("<>", @InboundParameters), "<\en"; |
| 421 | \& } |
| 422 | .Ve |
| 423 | .PP |
| 424 | The subroutine runs like a normal Perl subroutine, and the call to new |
| 425 | Thread returns whatever the subroutine returns. |
| 426 | .PP |
| 427 | The last example illustrates another feature of threads. You can spawn |
| 428 | off several threads using the same subroutine. Each thread executes |
| 429 | the same subroutine, but in a separate thread with a separate |
| 430 | environment and potentially separate arguments. |
| 431 | .PP |
| 432 | The other way to spawn a new thread is with \fIasync()\fR, which is a way to |
| 433 | spin off a chunk of code like \fIeval()\fR, but into its own thread: |
| 434 | .PP |
| 435 | .Vb 1 |
| 436 | \& use Thread qw(async); |
| 437 | .Ve |
| 438 | .PP |
| 439 | .Vb 1 |
| 440 | \& $LineCount = 0; |
| 441 | .Ve |
| 442 | .PP |
| 443 | .Vb 4 |
| 444 | \& $thr = async { |
| 445 | \& while(<>) {$LineCount++} |
| 446 | \& print "Got $LineCount lines\en"; |
| 447 | \& }; |
| 448 | .Ve |
| 449 | .PP |
| 450 | .Vb 3 |
| 451 | \& print "Waiting for the linecount to end\en"; |
| 452 | \& $thr->join; |
| 453 | \& print "All done\en"; |
| 454 | .Ve |
| 455 | .PP |
| 456 | You'll notice we did a use Thread qw(async) in that example. async is |
| 457 | not exported by default, so if you want it, you'll either need to |
| 458 | import it before you use it or fully qualify it as |
| 459 | Thread::async. You'll also note that there's a semicolon after the |
| 460 | closing brace. That's because \fIasync()\fR treats the following block as an |
| 461 | anonymous subroutine, so the semicolon is necessary. |
| 462 | .PP |
| 463 | Like \fIeval()\fR, the code executes in the same context as it would if it |
| 464 | weren't spun off. Since both the code inside and after the async start |
| 465 | executing, you need to be careful with any shared resources. Locking |
| 466 | and other synchronization techniques are covered later. |
| 467 | .Sh "Giving up control" |
| 468 | .IX Subsection "Giving up control" |
| 469 | There are times when you may find it useful to have a thread |
| 470 | explicitly give up the \s-1CPU\s0 to another thread. Your threading package |
| 471 | might not support preemptive multitasking for threads, for example, or |
| 472 | you may be doing something compute-intensive and want to make sure |
| 473 | that the user-interface thread gets called frequently. Regardless, |
| 474 | there are times that you might want a thread to give up the processor. |
| 475 | .PP |
| 476 | Perl's threading package provides the \fIyield()\fR function that does |
| 477 | this. \fIyield()\fR is pretty straightforward, and works like this: |
| 478 | .PP |
| 479 | .Vb 15 |
| 480 | \& use Thread qw(yield async); |
| 481 | \& async { |
| 482 | \& my $foo = 50; |
| 483 | \& while ($foo--) { print "first async\en" } |
| 484 | \& yield; |
| 485 | \& $foo = 50; |
| 486 | \& while ($foo--) { print "first async\en" } |
| 487 | \& }; |
| 488 | \& async { |
| 489 | \& my $foo = 50; |
| 490 | \& while ($foo--) { print "second async\en" } |
| 491 | \& yield; |
| 492 | \& $foo = 50; |
| 493 | \& while ($foo--) { print "second async\en" } |
| 494 | \& }; |
| 495 | .Ve |
| 496 | .Sh "Waiting For A Thread To Exit" |
| 497 | .IX Subsection "Waiting For A Thread To Exit" |
| 498 | Since threads are also subroutines, they can return values. To wait |
| 499 | for a thread to exit and extract any scalars it might return, you can |
| 500 | use the \fIjoin()\fR method. |
| 501 | .PP |
| 502 | .Vb 2 |
| 503 | \& use Thread; |
| 504 | \& $thr = new Thread \e&sub1; |
| 505 | .Ve |
| 506 | .PP |
| 507 | .Vb 2 |
| 508 | \& @ReturnData = $thr->join; |
| 509 | \& print "Thread returned @ReturnData"; |
| 510 | .Ve |
| 511 | .PP |
| 512 | .Vb 1 |
| 513 | \& sub sub1 { return "Fifty-six", "foo", 2; } |
| 514 | .Ve |
| 515 | .PP |
| 516 | In the example above, the \fIjoin()\fR method returns as soon as the thread |
| 517 | ends. In addition to waiting for a thread to finish and gathering up |
| 518 | any values that the thread might have returned, \fIjoin()\fR also performs |
| 519 | any \s-1OS\s0 cleanup necessary for the thread. That cleanup might be |
| 520 | important, especially for long-running programs that spawn lots of |
| 521 | threads. If you don't want the return values and don't want to wait |
| 522 | for the thread to finish, you should call the \fIdetach()\fR method |
| 523 | instead. \fIdetach()\fR is covered later in the article. |
| 524 | .Sh "Errors In Threads" |
| 525 | .IX Subsection "Errors In Threads" |
| 526 | So what happens when an error occurs in a thread? Any errors that |
| 527 | could be caught with \fIeval()\fR are postponed until the thread is |
| 528 | joined. If your program never joins, the errors appear when your |
| 529 | program exits. |
| 530 | .PP |
| 531 | Errors deferred until a \fIjoin()\fR can be caught with \fIeval()\fR: |
| 532 | .PP |
| 533 | .Vb 8 |
| 534 | \& use Thread qw(async); |
| 535 | \& $thr = async {$b = 3/0}; # Divide by zero error |
| 536 | \& $foo = eval {$thr->join}; |
| 537 | \& if ($@) { |
| 538 | \& print "died with error $@\en"; |
| 539 | \& } else { |
| 540 | \& print "Hey, why aren't you dead?\en"; |
| 541 | \& } |
| 542 | .Ve |
| 543 | .PP |
| 544 | \&\fIeval()\fR passes any results from the joined thread back unmodified, so |
| 545 | if you want the return value of the thread, this is your only chance |
| 546 | to get them. |
| 547 | .Sh "Ignoring A Thread" |
| 548 | .IX Subsection "Ignoring A Thread" |
| 549 | \&\fIjoin()\fR does three things: it waits for a thread to exit, cleans up |
| 550 | after it, and returns any data the thread may have produced. But what |
| 551 | if you're not interested in the thread's return values, and you don't |
| 552 | really care when the thread finishes? All you want is for the thread |
| 553 | to get cleaned up after when it's done. |
| 554 | .PP |
| 555 | In this case, you use the \fIdetach()\fR method. Once a thread is detached, |
| 556 | it'll run until it's finished, then Perl will clean up after it |
| 557 | automatically. |
| 558 | .PP |
| 559 | .Vb 2 |
| 560 | \& use Thread; |
| 561 | \& $thr = new Thread \e&sub1; # Spawn the thread |
| 562 | .Ve |
| 563 | .PP |
| 564 | .Vb 1 |
| 565 | \& $thr->detach; # Now we officially don't care any more |
| 566 | .Ve |
| 567 | .PP |
| 568 | .Vb 8 |
| 569 | \& sub sub1 { |
| 570 | \& $a = 0; |
| 571 | \& while (1) { |
| 572 | \& $a++; |
| 573 | \& print "\e$a is $a\en"; |
| 574 | \& sleep 1; |
| 575 | \& } |
| 576 | \& } |
| 577 | .Ve |
| 578 | .PP |
| 579 | Once a thread is detached, it may not be joined, and any output that |
| 580 | it might have produced (if it was done and waiting for a join) is |
| 581 | lost. |
| 582 | .SH "Threads And Data" |
| 583 | .IX Header "Threads And Data" |
| 584 | Now that we've covered the basics of threads, it's time for our next |
| 585 | topic: data. Threading introduces a couple of complications to data |
| 586 | access that non-threaded programs never need to worry about. |
| 587 | .Sh "Shared And Unshared Data" |
| 588 | .IX Subsection "Shared And Unshared Data" |
| 589 | The single most important thing to remember when using threads is that |
| 590 | all threads potentially have access to all the data anywhere in your |
| 591 | program. While this is true with a nonthreaded Perl program as well, |
| 592 | it's especially important to remember with a threaded program, since |
| 593 | more than one thread can be accessing this data at once. |
| 594 | .PP |
| 595 | Perl's scoping rules don't change because you're using threads. If a |
| 596 | subroutine (or block, in the case of \fIasync()\fR) could see a variable if |
| 597 | you weren't running with threads, it can see it if you are. This is |
| 598 | especially important for the subroutines that create, and makes \f(CW\*(C`my\*(C'\fR |
| 599 | variables even more important. Remember\*(--if your variables aren't |
| 600 | lexically scoped (declared with \f(CW\*(C`my\*(C'\fR) you're probably sharing them |
| 601 | between threads. |
| 602 | .Sh "Thread Pitfall: Races" |
| 603 | .IX Subsection "Thread Pitfall: Races" |
| 604 | While threads bring a new set of useful tools, they also bring a |
| 605 | number of pitfalls. One pitfall is the race condition: |
| 606 | .PP |
| 607 | .Vb 4 |
| 608 | \& use Thread; |
| 609 | \& $a = 1; |
| 610 | \& $thr1 = Thread->new(\e&sub1); |
| 611 | \& $thr2 = Thread->new(\e&sub2); |
| 612 | .Ve |
| 613 | .PP |
| 614 | .Vb 2 |
| 615 | \& sleep 10; |
| 616 | \& print "$a\en"; |
| 617 | .Ve |
| 618 | .PP |
| 619 | .Vb 2 |
| 620 | \& sub sub1 { $foo = $a; $a = $foo + 1; } |
| 621 | \& sub sub2 { $bar = $a; $a = $bar + 1; } |
| 622 | .Ve |
| 623 | .PP |
| 624 | What do you think \f(CW$a\fR will be? The answer, unfortunately, is \*(L"it |
| 625 | depends.\*(R" Both \fIsub1()\fR and \fIsub2()\fR access the global variable \f(CW$a\fR, once |
| 626 | to read and once to write. Depending on factors ranging from your |
| 627 | thread implementation's scheduling algorithm to the phase of the moon, |
| 628 | \&\f(CW$a\fR can be 2 or 3. |
| 629 | .PP |
| 630 | Race conditions are caused by unsynchronized access to shared |
| 631 | data. Without explicit synchronization, there's no way to be sure that |
| 632 | nothing has happened to the shared data between the time you access it |
| 633 | and the time you update it. Even this simple code fragment has the |
| 634 | possibility of error: |
| 635 | .PP |
| 636 | .Vb 4 |
| 637 | \& use Thread qw(async); |
| 638 | \& $a = 2; |
| 639 | \& async{ $b = $a; $a = $b + 1; }; |
| 640 | \& async{ $c = $a; $a = $c + 1; }; |
| 641 | .Ve |
| 642 | .PP |
| 643 | Two threads both access \f(CW$a\fR. Each thread can potentially be interrupted |
| 644 | at any point, or be executed in any order. At the end, \f(CW$a\fR could be 3 |
| 645 | or 4, and both \f(CW$b\fR and \f(CW$c\fR could be 2 or 3. |
| 646 | .PP |
| 647 | Whenever your program accesses data or resources that can be accessed |
| 648 | by other threads, you must take steps to coordinate access or risk |
| 649 | data corruption and race conditions. |
| 650 | .Sh "Controlling access: \fIlock()\fP" |
| 651 | .IX Subsection "Controlling access: lock()" |
| 652 | The \fIlock()\fR function takes a variable (or subroutine, but we'll get to |
| 653 | that later) and puts a lock on it. No other thread may lock the |
| 654 | variable until the locking thread exits the innermost block containing |
| 655 | the lock. Using \fIlock()\fR is straightforward: |
| 656 | .PP |
| 657 | .Vb 23 |
| 658 | \& use Thread qw(async); |
| 659 | \& $a = 4; |
| 660 | \& $thr1 = async { |
| 661 | \& $foo = 12; |
| 662 | \& { |
| 663 | \& lock ($a); # Block until we get access to $a |
| 664 | \& $b = $a; |
| 665 | \& $a = $b * $foo; |
| 666 | \& } |
| 667 | \& print "\e$foo was $foo\en"; |
| 668 | \& }; |
| 669 | \& $thr2 = async { |
| 670 | \& $bar = 7; |
| 671 | \& { |
| 672 | \& lock ($a); # Block until we can get access to $a |
| 673 | \& $c = $a; |
| 674 | \& $a = $c * $bar; |
| 675 | \& } |
| 676 | \& print "\e$bar was $bar\en"; |
| 677 | \& }; |
| 678 | \& $thr1->join; |
| 679 | \& $thr2->join; |
| 680 | \& print "\e$a is $a\en"; |
| 681 | .Ve |
| 682 | .PP |
| 683 | \&\fIlock()\fR blocks the thread until the variable being locked is |
| 684 | available. When \fIlock()\fR returns, your thread can be sure that no other |
| 685 | thread can lock that variable until the innermost block containing the |
| 686 | lock exits. |
| 687 | .PP |
| 688 | It's important to note that locks don't prevent access to the variable |
| 689 | in question, only lock attempts. This is in keeping with Perl's |
| 690 | longstanding tradition of courteous programming, and the advisory file |
| 691 | locking that \fIflock()\fR gives you. Locked subroutines behave differently, |
| 692 | however. We'll cover that later in the article. |
| 693 | .PP |
| 694 | You may lock arrays and hashes as well as scalars. Locking an array, |
| 695 | though, will not block subsequent locks on array elements, just lock |
| 696 | attempts on the array itself. |
| 697 | .PP |
| 698 | Finally, locks are recursive, which means it's okay for a thread to |
| 699 | lock a variable more than once. The lock will last until the outermost |
| 700 | \&\fIlock()\fR on the variable goes out of scope. |
| 701 | .Sh "Thread Pitfall: Deadlocks" |
| 702 | .IX Subsection "Thread Pitfall: Deadlocks" |
| 703 | Locks are a handy tool to synchronize access to data. Using them |
| 704 | properly is the key to safe shared data. Unfortunately, locks aren't |
| 705 | without their dangers. Consider the following code: |
| 706 | .PP |
| 707 | .Vb 15 |
| 708 | \& use Thread qw(async yield); |
| 709 | \& $a = 4; |
| 710 | \& $b = "foo"; |
| 711 | \& async { |
| 712 | \& lock($a); |
| 713 | \& yield; |
| 714 | \& sleep 20; |
| 715 | \& lock ($b); |
| 716 | \& }; |
| 717 | \& async { |
| 718 | \& lock($b); |
| 719 | \& yield; |
| 720 | \& sleep 20; |
| 721 | \& lock ($a); |
| 722 | \& }; |
| 723 | .Ve |
| 724 | .PP |
| 725 | This program will probably hang until you kill it. The only way it |
| 726 | won't hang is if one of the two \fIasync()\fR routines acquires both locks |
| 727 | first. A guaranteed-to-hang version is more complicated, but the |
| 728 | principle is the same. |
| 729 | .PP |
| 730 | The first thread spawned by \fIasync()\fR will grab a lock on \f(CW$a\fR then, a |
| 731 | second or two later, try to grab a lock on \f(CW$b\fR. Meanwhile, the second |
| 732 | thread grabs a lock on \f(CW$b\fR, then later tries to grab a lock on \f(CW$a\fR. The |
| 733 | second lock attempt for both threads will block, each waiting for the |
| 734 | other to release its lock. |
| 735 | .PP |
| 736 | This condition is called a deadlock, and it occurs whenever two or |
| 737 | more threads are trying to get locks on resources that the others |
| 738 | own. Each thread will block, waiting for the other to release a lock |
| 739 | on a resource. That never happens, though, since the thread with the |
| 740 | resource is itself waiting for a lock to be released. |
| 741 | .PP |
| 742 | There are a number of ways to handle this sort of problem. The best |
| 743 | way is to always have all threads acquire locks in the exact same |
| 744 | order. If, for example, you lock variables \f(CW$a\fR, \f(CW$b\fR, and \f(CW$c\fR, always lock |
| 745 | \&\f(CW$a\fR before \f(CW$b\fR, and \f(CW$b\fR before \f(CW$c\fR. It's also best to hold on to locks for |
| 746 | as short a period of time to minimize the risks of deadlock. |
| 747 | .Sh "Queues: Passing Data Around" |
| 748 | .IX Subsection "Queues: Passing Data Around" |
| 749 | A queue is a special thread-safe object that lets you put data in one |
| 750 | end and take it out the other without having to worry about |
| 751 | synchronization issues. They're pretty straightforward, and look like |
| 752 | this: |
| 753 | .PP |
| 754 | .Vb 2 |
| 755 | \& use Thread qw(async); |
| 756 | \& use Thread::Queue; |
| 757 | .Ve |
| 758 | .PP |
| 759 | .Vb 6 |
| 760 | \& my $DataQueue = new Thread::Queue; |
| 761 | \& $thr = async { |
| 762 | \& while ($DataElement = $DataQueue->dequeue) { |
| 763 | \& print "Popped $DataElement off the queue\en"; |
| 764 | \& } |
| 765 | \& }; |
| 766 | .Ve |
| 767 | .PP |
| 768 | .Vb 5 |
| 769 | \& $DataQueue->enqueue(12); |
| 770 | \& $DataQueue->enqueue("A", "B", "C"); |
| 771 | \& $DataQueue->enqueue(\e$thr); |
| 772 | \& sleep 10; |
| 773 | \& $DataQueue->enqueue(undef); |
| 774 | .Ve |
| 775 | .PP |
| 776 | You create the queue with new Thread::Queue. Then you can add lists of |
| 777 | scalars onto the end with \fIenqueue()\fR, and pop scalars off the front of |
| 778 | it with \fIdequeue()\fR. A queue has no fixed size, and can grow as needed |
| 779 | to hold everything pushed on to it. |
| 780 | .PP |
| 781 | If a queue is empty, \fIdequeue()\fR blocks until another thread enqueues |
| 782 | something. This makes queues ideal for event loops and other |
| 783 | communications between threads. |
| 784 | .SH "Threads And Code" |
| 785 | .IX Header "Threads And Code" |
| 786 | In addition to providing thread-safe access to data via locks and |
| 787 | queues, threaded Perl also provides general-purpose semaphores for |
| 788 | coarser synchronization than locks provide and thread-safe access to |
| 789 | entire subroutines. |
| 790 | .Sh "Semaphores: Synchronizing Data Access" |
| 791 | .IX Subsection "Semaphores: Synchronizing Data Access" |
| 792 | Semaphores are a kind of generic locking mechanism. Unlike lock, which |
| 793 | gets a lock on a particular scalar, Perl doesn't associate any |
| 794 | particular thing with a semaphore so you can use them to control |
| 795 | access to anything you like. In addition, semaphores can allow more |
| 796 | than one thread to access a resource at once, though by default |
| 797 | semaphores only allow one thread access at a time. |
| 798 | .IP "Basic semaphores" 4 |
| 799 | .IX Item "Basic semaphores" |
| 800 | Semaphores have two methods, down and up. down decrements the resource |
| 801 | count, while up increments it. down calls will block if the |
| 802 | semaphore's current count would decrement below zero. This program |
| 803 | gives a quick demonstration: |
| 804 | .Sp |
| 805 | .Vb 4 |
| 806 | \& use Thread qw(yield); |
| 807 | \& use Thread::Semaphore; |
| 808 | \& my $semaphore = new Thread::Semaphore; |
| 809 | \& $GlobalVariable = 0; |
| 810 | .Ve |
| 811 | .Sp |
| 812 | .Vb 3 |
| 813 | \& $thr1 = new Thread \e&sample_sub, 1; |
| 814 | \& $thr2 = new Thread \e&sample_sub, 2; |
| 815 | \& $thr3 = new Thread \e&sample_sub, 3; |
| 816 | .Ve |
| 817 | .Sp |
| 818 | .Vb 16 |
| 819 | \& sub sample_sub { |
| 820 | \& my $SubNumber = shift @_; |
| 821 | \& my $TryCount = 10; |
| 822 | \& my $LocalCopy; |
| 823 | \& sleep 1; |
| 824 | \& while ($TryCount--) { |
| 825 | \& $semaphore->down; |
| 826 | \& $LocalCopy = $GlobalVariable; |
| 827 | \& print "$TryCount tries left for sub $SubNumber (\e$GlobalVariable is $GlobalVariable)\en"; |
| 828 | \& yield; |
| 829 | \& sleep 2; |
| 830 | \& $LocalCopy++; |
| 831 | \& $GlobalVariable = $LocalCopy; |
| 832 | \& $semaphore->up; |
| 833 | \& } |
| 834 | \& } |
| 835 | .Ve |
| 836 | .Sp |
| 837 | The three invocations of the subroutine all operate in sync. The |
| 838 | semaphore, though, makes sure that only one thread is accessing the |
| 839 | global variable at once. |
| 840 | .IP "Advanced Semaphores" 4 |
| 841 | .IX Item "Advanced Semaphores" |
| 842 | By default, semaphores behave like locks, letting only one thread |
| 843 | \&\fIdown()\fR them at a time. However, there are other uses for semaphores. |
| 844 | .Sp |
| 845 | Each semaphore has a counter attached to it. \fIdown()\fR decrements the |
| 846 | counter and \fIup()\fR increments the counter. By default, semaphores are |
| 847 | created with the counter set to one, \fIdown()\fR decrements by one, and |
| 848 | \&\fIup()\fR increments by one. If \fIdown()\fR attempts to decrement the counter |
| 849 | below zero, it blocks until the counter is large enough. Note that |
| 850 | while a semaphore can be created with a starting count of zero, any |
| 851 | \&\fIup()\fR or \fIdown()\fR always changes the counter by at least |
| 852 | one. \f(CW$semaphore\fR\->\fIdown\fR\|(0) is the same as \f(CW$semaphore\fR\->\fIdown\fR\|(1). |
| 853 | .Sp |
| 854 | The question, of course, is why would you do something like this? Why |
| 855 | create a semaphore with a starting count that's not one, or why |
| 856 | decrement/increment it by more than one? The answer is resource |
| 857 | availability. Many resources that you want to manage access for can be |
| 858 | safely used by more than one thread at once. |
| 859 | .Sp |
| 860 | For example, let's take a \s-1GUI\s0 driven program. It has a semaphore that |
| 861 | it uses to synchronize access to the display, so only one thread is |
| 862 | ever drawing at once. Handy, but of course you don't want any thread |
| 863 | to start drawing until things are properly set up. In this case, you |
| 864 | can create a semaphore with a counter set to zero, and up it when |
| 865 | things are ready for drawing. |
| 866 | .Sp |
| 867 | Semaphores with counters greater than one are also useful for |
| 868 | establishing quotas. Say, for example, that you have a number of |
| 869 | threads that can do I/O at once. You don't want all the threads |
| 870 | reading or writing at once though, since that can potentially swamp |
| 871 | your I/O channels, or deplete your process' quota of filehandles. You |
| 872 | can use a semaphore initialized to the number of concurrent I/O |
| 873 | requests (or open files) that you want at any one time, and have your |
| 874 | threads quietly block and unblock themselves. |
| 875 | .Sp |
| 876 | Larger increments or decrements are handy in those cases where a |
| 877 | thread needs to check out or return a number of resources at once. |
| 878 | .Sh "Attributes: Restricting Access To Subroutines" |
| 879 | .IX Subsection "Attributes: Restricting Access To Subroutines" |
| 880 | In addition to synchronizing access to data or resources, you might |
| 881 | find it useful to synchronize access to subroutines. You may be |
| 882 | accessing a singular machine resource (perhaps a vector processor), or |
| 883 | find it easier to serialize calls to a particular subroutine than to |
| 884 | have a set of locks and semaphores. |
| 885 | .PP |
| 886 | One of the additions to Perl 5.005 is subroutine attributes. The |
| 887 | Thread package uses these to provide several flavors of |
| 888 | serialization. It's important to remember that these attributes are |
| 889 | used in the compilation phase of your program so you can't change a |
| 890 | subroutine's behavior while your program is actually running. |
| 891 | .Sh "Subroutine Locks" |
| 892 | .IX Subsection "Subroutine Locks" |
| 893 | The basic subroutine lock looks like this: |
| 894 | .PP |
| 895 | .Vb 2 |
| 896 | \& sub test_sub :locked { |
| 897 | \& } |
| 898 | .Ve |
| 899 | .PP |
| 900 | This ensures that only one thread will be executing this subroutine at |
| 901 | any one time. Once a thread calls this subroutine, any other thread |
| 902 | that calls it will block until the thread in the subroutine exits |
| 903 | it. A more elaborate example looks like this: |
| 904 | .PP |
| 905 | .Vb 1 |
| 906 | \& use Thread qw(yield); |
| 907 | .Ve |
| 908 | .PP |
| 909 | .Vb 4 |
| 910 | \& new Thread \e&thread_sub, 1; |
| 911 | \& new Thread \e&thread_sub, 2; |
| 912 | \& new Thread \e&thread_sub, 3; |
| 913 | \& new Thread \e&thread_sub, 4; |
| 914 | .Ve |
| 915 | .PP |
| 916 | .Vb 7 |
| 917 | \& sub sync_sub :locked { |
| 918 | \& my $CallingThread = shift @_; |
| 919 | \& print "In sync_sub for thread $CallingThread\en"; |
| 920 | \& yield; |
| 921 | \& sleep 3; |
| 922 | \& print "Leaving sync_sub for thread $CallingThread\en"; |
| 923 | \& } |
| 924 | .Ve |
| 925 | .PP |
| 926 | .Vb 6 |
| 927 | \& sub thread_sub { |
| 928 | \& my $ThreadID = shift @_; |
| 929 | \& print "Thread $ThreadID calling sync_sub\en"; |
| 930 | \& sync_sub($ThreadID); |
| 931 | \& print "$ThreadID is done with sync_sub\en"; |
| 932 | \& } |
| 933 | .Ve |
| 934 | .PP |
| 935 | The \f(CW\*(C`locked\*(C'\fR attribute tells perl to lock \fIsync_sub()\fR, and if you run |
| 936 | this, you can see that only one thread is in it at any one time. |
| 937 | .Sh "Methods" |
| 938 | .IX Subsection "Methods" |
| 939 | Locking an entire subroutine can sometimes be overkill, especially |
| 940 | when dealing with Perl objects. When calling a method for an object, |
| 941 | for example, you want to serialize calls to a method, so that only one |
| 942 | thread will be in the subroutine for a particular object, but threads |
| 943 | calling that subroutine for a different object aren't blocked. The |
| 944 | method attribute indicates whether the subroutine is really a method. |
| 945 | .PP |
| 946 | .Vb 1 |
| 947 | \& use Thread; |
| 948 | .Ve |
| 949 | .PP |
| 950 | .Vb 14 |
| 951 | \& sub tester { |
| 952 | \& my $thrnum = shift @_; |
| 953 | \& my $bar = new Foo; |
| 954 | \& foreach (1..10) { |
| 955 | \& print "$thrnum calling per_object\en"; |
| 956 | \& $bar->per_object($thrnum); |
| 957 | \& print "$thrnum out of per_object\en"; |
| 958 | \& yield; |
| 959 | \& print "$thrnum calling one_at_a_time\en"; |
| 960 | \& $bar->one_at_a_time($thrnum); |
| 961 | \& print "$thrnum out of one_at_a_time\en"; |
| 962 | \& yield; |
| 963 | \& } |
| 964 | \& } |
| 965 | .Ve |
| 966 | .PP |
| 967 | .Vb 3 |
| 968 | \& foreach my $thrnum (1..10) { |
| 969 | \& new Thread \e&tester, $thrnum; |
| 970 | \& } |
| 971 | .Ve |
| 972 | .PP |
| 973 | .Vb 5 |
| 974 | \& package Foo; |
| 975 | \& sub new { |
| 976 | \& my $class = shift @_; |
| 977 | \& return bless [@_], $class; |
| 978 | \& } |
| 979 | .Ve |
| 980 | .PP |
| 981 | .Vb 7 |
| 982 | \& sub per_object :locked :method { |
| 983 | \& my ($class, $thrnum) = @_; |
| 984 | \& print "In per_object for thread $thrnum\en"; |
| 985 | \& yield; |
| 986 | \& sleep 2; |
| 987 | \& print "Exiting per_object for thread $thrnum\en"; |
| 988 | \& } |
| 989 | .Ve |
| 990 | .PP |
| 991 | .Vb 7 |
| 992 | \& sub one_at_a_time :locked { |
| 993 | \& my ($class, $thrnum) = @_; |
| 994 | \& print "In one_at_a_time for thread $thrnum\en"; |
| 995 | \& yield; |
| 996 | \& sleep 2; |
| 997 | \& print "Exiting one_at_a_time for thread $thrnum\en"; |
| 998 | \& } |
| 999 | .Ve |
| 1000 | .PP |
| 1001 | As you can see from the output (omitted for brevity; it's 800 lines) |
| 1002 | all the threads can be in \fIper_object()\fR simultaneously, but only one |
| 1003 | thread is ever in \fIone_at_a_time()\fR at once. |
| 1004 | .Sh "Locking A Subroutine" |
| 1005 | .IX Subsection "Locking A Subroutine" |
| 1006 | You can lock a subroutine as you would lock a variable. Subroutine locks |
| 1007 | work the same as specifying a \f(CW\*(C`locked\*(C'\fR attribute for the subroutine, |
| 1008 | and block all access to the subroutine for other threads until the |
| 1009 | lock goes out of scope. When the subroutine isn't locked, any number |
| 1010 | of threads can be in it at once, and getting a lock on a subroutine |
| 1011 | doesn't affect threads already in the subroutine. Getting a lock on a |
| 1012 | subroutine looks like this: |
| 1013 | .PP |
| 1014 | .Vb 1 |
| 1015 | \& lock(\e&sub_to_lock); |
| 1016 | .Ve |
| 1017 | .PP |
| 1018 | Simple enough. Unlike the \f(CW\*(C`locked\*(C'\fR attribute, which is a compile time |
| 1019 | option, locking and unlocking a subroutine can be done at runtime at your |
| 1020 | discretion. There is some runtime penalty to using lock(\e&sub) instead |
| 1021 | of the \f(CW\*(C`locked\*(C'\fR attribute, so make sure you're choosing the proper |
| 1022 | method to do the locking. |
| 1023 | .PP |
| 1024 | You'd choose lock(\e&sub) when writing modules and code to run on both |
| 1025 | threaded and unthreaded Perl, especially for code that will run on |
| 1026 | 5.004 or earlier Perls. In that case, it's useful to have subroutines |
| 1027 | that should be serialized lock themselves if they're running threaded, |
| 1028 | like so: |
| 1029 | .PP |
| 1030 | .Vb 3 |
| 1031 | \& package Foo; |
| 1032 | \& use Config; |
| 1033 | \& $Running_Threaded = 0; |
| 1034 | .Ve |
| 1035 | .PP |
| 1036 | .Vb 1 |
| 1037 | \& BEGIN { $Running_Threaded = $Config{'usethreads'} } |
| 1038 | .Ve |
| 1039 | .PP |
| 1040 | .Vb 1 |
| 1041 | \& sub sub1 { lock(\e&sub1) if $Running_Threaded } |
| 1042 | .Ve |
| 1043 | .PP |
| 1044 | This way you can ensure single-threadedness regardless of which |
| 1045 | version of Perl you're running. |
| 1046 | .SH "General Thread Utility Routines" |
| 1047 | .IX Header "General Thread Utility Routines" |
| 1048 | We've covered the workhorse parts of Perl's threading package, and |
| 1049 | with these tools you should be well on your way to writing threaded |
| 1050 | code and packages. There are a few useful little pieces that didn't |
| 1051 | really fit in anyplace else. |
| 1052 | .Sh "What Thread Am I In?" |
| 1053 | .IX Subsection "What Thread Am I In?" |
| 1054 | The Thread\->self method provides your program with a way to get an |
| 1055 | object representing the thread it's currently in. You can use this |
| 1056 | object in the same way as the ones returned from the thread creation. |
| 1057 | .Sh "Thread IDs" |
| 1058 | .IX Subsection "Thread IDs" |
| 1059 | \&\fItid()\fR is a thread object method that returns the thread \s-1ID\s0 of the |
| 1060 | thread the object represents. Thread IDs are integers, with the main |
| 1061 | thread in a program being 0. Currently Perl assigns a unique tid to |
| 1062 | every thread ever created in your program, assigning the first thread |
| 1063 | to be created a tid of 1, and increasing the tid by 1 for each new |
| 1064 | thread that's created. |
| 1065 | .Sh "Are These Threads The Same?" |
| 1066 | .IX Subsection "Are These Threads The Same?" |
| 1067 | The \fIequal()\fR method takes two thread objects and returns true |
| 1068 | if the objects represent the same thread, and false if they don't. |
| 1069 | .Sh "What Threads Are Running?" |
| 1070 | .IX Subsection "What Threads Are Running?" |
| 1071 | Thread\->list returns a list of thread objects, one for each thread |
| 1072 | that's currently running. Handy for a number of things, including |
| 1073 | cleaning up at the end of your program: |
| 1074 | .PP |
| 1075 | .Vb 7 |
| 1076 | \& # Loop through all the threads |
| 1077 | \& foreach $thr (Thread->list) { |
| 1078 | \& # Don't join the main thread or ourselves |
| 1079 | \& if ($thr->tid && !Thread::equal($thr, Thread->self)) { |
| 1080 | \& $thr->join; |
| 1081 | \& } |
| 1082 | \& } |
| 1083 | .Ve |
| 1084 | .PP |
| 1085 | The example above is just for illustration. It isn't strictly |
| 1086 | necessary to join all the threads you create, since Perl detaches all |
| 1087 | the threads before it exits. |
| 1088 | .SH "A Complete Example" |
| 1089 | .IX Header "A Complete Example" |
| 1090 | Confused yet? It's time for an example program to show some of the |
| 1091 | things we've covered. This program finds prime numbers using threads. |
| 1092 | .PP |
| 1093 | .Vb 34 |
| 1094 | \& 1 #!/usr/bin/perl -w |
| 1095 | \& 2 # prime-pthread, courtesy of Tom Christiansen |
| 1096 | \& 3 |
| 1097 | \& 4 use strict; |
| 1098 | \& 5 |
| 1099 | \& 6 use Thread; |
| 1100 | \& 7 use Thread::Queue; |
| 1101 | \& 8 |
| 1102 | \& 9 my $stream = new Thread::Queue; |
| 1103 | \& 10 my $kid = new Thread(\e&check_num, $stream, 2); |
| 1104 | \& 11 |
| 1105 | \& 12 for my $i ( 3 .. 1000 ) { |
| 1106 | \& 13 $stream->enqueue($i); |
| 1107 | \& 14 } |
| 1108 | \& 15 |
| 1109 | \& 16 $stream->enqueue(undef); |
| 1110 | \& 17 $kid->join(); |
| 1111 | \& 18 |
| 1112 | \& 19 sub check_num { |
| 1113 | \& 20 my ($upstream, $cur_prime) = @_; |
| 1114 | \& 21 my $kid; |
| 1115 | \& 22 my $downstream = new Thread::Queue; |
| 1116 | \& 23 while (my $num = $upstream->dequeue) { |
| 1117 | \& 24 next unless $num % $cur_prime; |
| 1118 | \& 25 if ($kid) { |
| 1119 | \& 26 $downstream->enqueue($num); |
| 1120 | \& 27 } else { |
| 1121 | \& 28 print "Found prime $num\en"; |
| 1122 | \& 29 $kid = new Thread(\e&check_num, $downstream, $num); |
| 1123 | \& 30 } |
| 1124 | \& 31 } |
| 1125 | \& 32 $downstream->enqueue(undef) if $kid; |
| 1126 | \& 33 $kid->join() if $kid; |
| 1127 | \& 34 } |
| 1128 | .Ve |
| 1129 | .PP |
| 1130 | This program uses the pipeline model to generate prime numbers. Each |
| 1131 | thread in the pipeline has an input queue that feeds numbers to be |
| 1132 | checked, a prime number that it's responsible for, and an output queue |
| 1133 | that it funnels numbers that have failed the check into. If the thread |
| 1134 | has a number that's failed its check and there's no child thread, then |
| 1135 | the thread must have found a new prime number. In that case, a new |
| 1136 | child thread is created for that prime and stuck on the end of the |
| 1137 | pipeline. |
| 1138 | .PP |
| 1139 | This probably sounds a bit more confusing than it really is, so lets |
| 1140 | go through this program piece by piece and see what it does. (For |
| 1141 | those of you who might be trying to remember exactly what a prime |
| 1142 | number is, it's a number that's only evenly divisible by itself and 1) |
| 1143 | .PP |
| 1144 | The bulk of the work is done by the \fIcheck_num()\fR subroutine, which |
| 1145 | takes a reference to its input queue and a prime number that it's |
| 1146 | responsible for. After pulling in the input queue and the prime that |
| 1147 | the subroutine's checking (line 20), we create a new queue (line 22) |
| 1148 | and reserve a scalar for the thread that we're likely to create later |
| 1149 | (line 21). |
| 1150 | .PP |
| 1151 | The while loop from lines 23 to line 31 grabs a scalar off the input |
| 1152 | queue and checks against the prime this thread is responsible |
| 1153 | for. Line 24 checks to see if there's a remainder when we modulo the |
| 1154 | number to be checked against our prime. If there is one, the number |
| 1155 | must not be evenly divisible by our prime, so we need to either pass |
| 1156 | it on to the next thread if we've created one (line 26) or create a |
| 1157 | new thread if we haven't. |
| 1158 | .PP |
| 1159 | The new thread creation is line 29. We pass on to it a reference to |
| 1160 | the queue we've created, and the prime number we've found. |
| 1161 | .PP |
| 1162 | Finally, once the loop terminates (because we got a 0 or undef in the |
| 1163 | queue, which serves as a note to die), we pass on the notice to our |
| 1164 | child and wait for it to exit if we've created a child (Lines 32 and |
| 1165 | 37). |
| 1166 | .PP |
| 1167 | Meanwhile, back in the main thread, we create a queue (line 9) and the |
| 1168 | initial child thread (line 10), and pre-seed it with the first prime: |
| 1169 | 2. Then we queue all the numbers from 3 to 1000 for checking (lines |
| 1170 | 12\-14), then queue a die notice (line 16) and wait for the first child |
| 1171 | thread to terminate (line 17). Because a child won't die until its |
| 1172 | child has died, we know that we're done once we return from the join. |
| 1173 | .PP |
| 1174 | That's how it works. It's pretty simple; as with many Perl programs, |
| 1175 | the explanation is much longer than the program. |
| 1176 | .SH "Conclusion" |
| 1177 | .IX Header "Conclusion" |
| 1178 | A complete thread tutorial could fill a book (and has, many times), |
| 1179 | but this should get you well on your way. The final authority on how |
| 1180 | Perl's threads behave is the documentation bundled with the Perl |
| 1181 | distribution, but with what we've covered in this article, you should |
| 1182 | be well on your way to becoming a threaded Perl expert. |
| 1183 | .SH "Bibliography" |
| 1184 | .IX Header "Bibliography" |
| 1185 |