X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlthrtut.pod;h=cdc409fd6e5d92f6a9e0a02f22bac5f90f48a2d1;hb=50de6d7ea6627af63fe7cc0e7a62e105a73a0565;hp=0b7092b39dca19a3c5b20cfc5f816f71ba8db9be;hpb=22d4bb9ccb8701e68f9243547d7e3a3c55f70908;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlthrtut.pod b/pod/perlthrtut.pod index 0b7092b..cdc409f 100644 --- a/pod/perlthrtut.pod +++ b/pod/perlthrtut.pod @@ -4,1064 +4,54 @@ perlthrtut - tutorial on threads in Perl =head1 DESCRIPTION - WARNING: Threading is an experimental feature. Both the interface - and implementation are subject to change drastically. In fact, this - documentation describes the flavor of threads that was in version - 5.005. Perl 5.6.0 and later have the beginnings of support for - interpreter threads, which (when finished) is expected to be - significantly different from what is described here. The information - contained here may therefore soon be obsolete. Use at your own risk! +B: this tutorial describes the new Perl threading flavour +introduced in Perl 5.6.0 called interpreter threads, or ithreads +for short. There is another older perl threading flavour called +the 5.005 model, unsurprisingly for 5.005 versions of Perl. -One of the most prominent new features of Perl 5.005 is the inclusion -of threads. Threads make a number of things a lot easier, and are a -very useful addition to your bag of programming tricks. +You can see which (or neither) threading flavour you have by +running C and look at the C section. +If you have C you have ithreads, if you +have C you have 5.005 threads. +If you have neither, you don't have any thread support built in. +If you have both, you are in trouble. -=head1 What Is A Thread Anyway? +This document is unfortunately rather sparse as of 2001-Sep-17. -A thread is a flow of control through a program with a single -execution point. - -Sounds an awful lot like a process, doesn't it? Well, it should. -Threads are one of the pieces of a process. Every process has at least -one thread and, up until now, every process running Perl had only one -thread. With 5.005, though, you can create extra threads. We're going -to show you how, when, and why. - -=head1 Threaded Program Models - -There are three basic ways that you can structure a threaded -program. Which model you choose depends on what you need your program -to do. For many non-trivial threaded programs you'll need to choose -different models for different pieces of your program. - -=head2 Boss/Worker - -The boss/worker model usually has one `boss' thread and one or more -`worker' threads. The boss thread gathers or generates tasks that need -to be done, then parcels those tasks out to the appropriate worker -thread. - -This model is common in GUI and server programs, where a main thread -waits for some event and then passes that event to the appropriate -worker threads for processing. Once the event has been passed on, the -boss thread goes back to waiting for another event. - -The boss thread does relatively little work. While tasks aren't -necessarily performed faster than with any other method, it tends to -have the best user-response times. - -=head2 Work Crew - -In the work crew model, several threads are created that do -essentially the same thing to different pieces of data. It closely -mirrors classical parallel processing and vector processors, where a -large array of processors do the exact same thing to many pieces of -data. - -This model is particularly useful if the system running the program -will distribute multiple threads across different processors. It can -also be useful in ray tracing or rendering engines, where the -individual threads can pass on interim results to give the user visual -feedback. - -=head2 Pipeline - -The pipeline model divides up a task into a series of steps, and -passes the results of one step on to the thread processing the -next. Each thread does one thing to each piece of data and passes the -results to the next thread in line. - -This model makes the most sense if you have multiple processors so two -or more threads will be executing in parallel, though it can often -make sense in other contexts as well. It tends to keep the individual -tasks small and simple, as well as allowing some parts of the pipeline -to block (on I/O or system calls, for example) while other parts keep -going. If you're running different parts of the pipeline on different -processors you may also take advantage of the caches on each -processor. - -This model is also handy for a form of recursive programming where, -rather than having a subroutine call itself, it instead creates -another thread. Prime and Fibonacci generators both map well to this -form of the pipeline model. (A version of a prime number generator is -presented later on.) - -=head1 Native threads - -There are several different ways to implement threads on a system. How -threads are implemented depends both on the vendor and, in some cases, -the version of the operating system. Often the first implementation -will be relatively simple, but later versions of the OS will be more -sophisticated. - -While the information in this section is useful, it's not necessary, -so you can skip it if you don't feel up to it. - -There are three basic categories of threads-user-mode threads, kernel -threads, and multiprocessor kernel threads. - -User-mode threads are threads that live entirely within a program and -its libraries. In this model, the OS knows nothing about threads. As -far as it's concerned, your process is just a process. - -This is the easiest way to implement threads, and the way most OSes -start. The big disadvantage is that, since the OS knows nothing about -threads, if one thread blocks they all do. Typical blocking activities -include most system calls, most I/O, and things like sleep(). - -Kernel threads are the next step in thread evolution. The OS knows -about kernel threads, and makes allowances for them. The main -difference between a kernel thread and a user-mode thread is -blocking. With kernel threads, things that block a single thread don't -block other threads. This is not the case with user-mode threads, -where the kernel blocks at the process level and not the thread level. - -This is a big step forward, and can give a threaded program quite a -performance boost over non-threaded programs. Threads that block -performing I/O, for example, won't block threads that are doing other -things. Each process still has only one thread running at once, -though, regardless of how many CPUs a system might have. - -Since kernel threading can interrupt a thread at any time, they will -uncover some of the implicit locking assumptions you may make in your -program. For example, something as simple as C<$a = $a + 2> can behave -unpredictably with kernel threads if $a is visible to other -threads, as another thread may have changed $a between the time it -was fetched on the right hand side and the time the new value is -stored. - -Multiprocessor Kernel Threads are the final step in thread -support. With multiprocessor kernel threads on a machine with multiple -CPUs, the OS may schedule two or more threads to run simultaneously on -different CPUs. - -This can give a serious performance boost to your threaded program, -since more than one thread will be executing at the same time. As a -tradeoff, though, any of those nagging synchronization issues that -might not have shown with basic kernel threads will appear with a -vengeance. - -In addition to the different levels of OS involvement in threads, -different OSes (and different thread implementations for a particular -OS) allocate CPU cycles to threads in different ways. - -Cooperative multitasking systems have running threads give up control -if one of two things happen. If a thread calls a yield function, it -gives up control. It also gives up control if the thread does -something that would cause it to block, such as perform I/O. In a -cooperative multitasking implementation, one thread can starve all the -others for CPU time if it so chooses. - -Preemptive multitasking systems interrupt threads at regular intervals -while the system decides which thread should run next. In a preemptive -multitasking system, one thread usually won't monopolize the CPU. - -On some systems, there can be cooperative and preemptive threads -running simultaneously. (Threads running with realtime priorities -often behave cooperatively, for example, while threads running at -normal priorities behave preemptively.) - -=head1 What kind of threads are perl threads? - -If you have experience with other thread implementations, you might -find that things aren't quite what you expect. It's very important to -remember when dealing with Perl threads that Perl Threads Are Not X -Threads, for all values of X. They aren't POSIX threads, or -DecThreads, or Java's Green threads, or Win32 threads. There are -similarities, and the broad concepts are the same, but if you start -looking for implementation details you're going to be either -disappointed or confused. Possibly both. - -This is not to say that Perl threads are completely different from -everything that's ever come before--they're not. Perl's threading -model owes a lot to other thread models, especially POSIX. Just as -Perl is not C, though, Perl threads are not POSIX threads. So if you -find yourself looking for mutexes, or thread priorities, it's time to -step back a bit and think about what you want to do and how Perl can -do it. - -=head1 Threadsafe Modules - -The addition of threads has changed Perl's internals -substantially. There are implications for people who write -modules--especially modules with XS code or external libraries. While -most modules won't encounter any problems, modules that aren't -explicitly tagged as thread-safe should be tested before being used in -production code. - -Not all modules that you might use are thread-safe, and you should -always assume a module is unsafe unless the documentation says -otherwise. This includes modules that are distributed as part of the -core. Threads are a beta feature, and even some of the standard -modules aren't thread-safe. - -If you're using a module that's not thread-safe for some reason, you -can protect yourself by using semaphores and lots of programming -discipline to control access to the module. Semaphores are covered -later in the article. Perl Threads Are Different - -=head1 Thread Basics - -The core Thread module provides the basic functions you need to write -threaded programs. In the following sections we'll cover the basics, -showing you what you need to do to create a threaded program. After -that, we'll go over some of the features of the Thread module that -make threaded programming easier. - -=head2 Basic Thread Support - -Thread support is a Perl compile-time option-it's something that's -turned on or off when Perl is built at your site, rather than when -your programs are compiled. If your Perl wasn't compiled with thread -support enabled, then any attempt to use threads will fail. - -Remember that the threading support in 5.005 is in beta release, and -should be treated as such. You should expect that it may not function -entirely properly, and the thread interface may well change some -before it is a fully supported, production release. The beta version -shouldn't be used for mission-critical projects. Having said that, -threaded Perl is pretty nifty, and worth a look. - -Your programs can use the Config module to check whether threads are -enabled. If your program can't run without them, you can say something -like: - - $Config{usethreads} or die "Recompile Perl with threads to run this program."; - -A possibly-threaded program using a possibly-threaded module might -have code like this: - - use Config; - use MyMod; - - if ($Config{usethreads}) { - # We have threads - require MyMod_threaded; - import MyMod_threaded; - } else { - require MyMod_unthreaded; - import MyMod_unthreaded; - } - -Since code that runs both with and without threads is usually pretty -messy, it's best to isolate the thread-specific code in its own -module. In our example above, that's what MyMod_threaded is, and it's -only imported if we're running on a threaded Perl. - -=head2 Creating Threads - -The Thread package provides the tools you need to create new -threads. Like any other module, you need to tell Perl you want to use -it; use Thread imports all the pieces you need to create basic -threads. - -The simplest, straightforward way to create a thread is with new(): - - use Thread; - - $thr = new Thread \&sub1; - - sub sub1 { - print "In the thread\n"; - } - -The new() method takes a reference to a subroutine and creates a new -thread, which starts executing in the referenced subroutine. Control -then passes both to the subroutine and the caller. - -If you need to, your program can pass parameters to the subroutine as -part of the thread startup. Just include the list of parameters as -part of the C call, like this: - - use Thread; - $Param3 = "foo"; - $thr = new Thread \&sub1, "Param 1", "Param 2", $Param3; - $thr = new Thread \&sub1, @ParamList; - $thr = new Thread \&sub1, qw(Param1 Param2 $Param3); - - sub sub1 { - my @InboundParameters = @_; - print "In the thread\n"; - print "got parameters >", join("<>", @InboundParameters), "<\n"; - } - - -The subroutine runs like a normal Perl subroutine, and the call to new -Thread returns whatever the subroutine returns. - -The last example illustrates another feature of threads. You can spawn -off several threads using the same subroutine. Each thread executes -the same subroutine, but in a separate thread with a separate -environment and potentially separate arguments. - -The other way to spawn a new thread is with async(), which is a way to -spin off a chunk of code like eval(), but into its own thread: - - use Thread qw(async); - - $LineCount = 0; - - $thr = async { - while(<>) {$LineCount++} - print "Got $LineCount lines\n"; - }; - - print "Waiting for the linecount to end\n"; - $thr->join; - print "All done\n"; - -You'll notice we did a use Thread qw(async) in that example. async is -not exported by default, so if you want it, you'll either need to -import it before you use it or fully qualify it as -Thread::async. You'll also note that there's a semicolon after the -closing brace. That's because async() treats the following block as an -anonymous subroutine, so the semicolon is necessary. - -Like eval(), the code executes in the same context as it would if it -weren't spun off. Since both the code inside and after the async start -executing, you need to be careful with any shared resources. Locking -and other synchronization techniques are covered later. - -=head2 Giving up control - -There are times when you may find it useful to have a thread -explicitly give up the CPU to another thread. Your threading package -might not support preemptive multitasking for threads, for example, or -you may be doing something compute-intensive and want to make sure -that the user-interface thread gets called frequently. Regardless, -there are times that you might want a thread to give up the processor. - -Perl's threading package provides the yield() function that does -this. yield() is pretty straightforward, and works like this: - - use Thread qw(yield async); - async { - my $foo = 50; - while ($foo--) { print "first async\n" } - yield; - $foo = 50; - while ($foo--) { print "first async\n" } - }; - async { - my $foo = 50; - while ($foo--) { print "second async\n" } - yield; - $foo = 50; - while ($foo--) { print "second async\n" } - }; - -=head2 Waiting For A Thread To Exit - -Since threads are also subroutines, they can return values. To wait -for a thread to exit and extract any scalars it might return, you can -use the join() method. - - use Thread; - $thr = new Thread \&sub1; - - @ReturnData = $thr->join; - print "Thread returned @ReturnData"; - - sub sub1 { return "Fifty-six", "foo", 2; } - -In the example above, the join() method returns as soon as the thread -ends. In addition to waiting for a thread to finish and gathering up -any values that the thread might have returned, join() also performs -any OS cleanup necessary for the thread. That cleanup might be -important, especially for long-running programs that spawn lots of -threads. If you don't want the return values and don't want to wait -for the thread to finish, you should call the detach() method -instead. detach() is covered later in the article. - -=head2 Errors In Threads - -So what happens when an error occurs in a thread? Any errors that -could be caught with eval() are postponed until the thread is -joined. If your program never joins, the errors appear when your -program exits. - -Errors deferred until a join() can be caught with eval(): - - use Thread qw(async); - $thr = async {$b = 3/0}; # Divide by zero error - $foo = eval {$thr->join}; - if ($@) { - print "died with error $@\n"; - } else { - print "Hey, why aren't you dead?\n"; - } - -eval() passes any results from the joined thread back unmodified, so -if you want the return value of the thread, this is your only chance -to get them. - -=head2 Ignoring A Thread - -join() does three things: it waits for a thread to exit, cleans up -after it, and returns any data the thread may have produced. But what -if you're not interested in the thread's return values, and you don't -really care when the thread finishes? All you want is for the thread -to get cleaned up after when it's done. - -In this case, you use the detach() method. Once a thread is detached, -it'll run until it's finished, then Perl will clean up after it -automatically. - - use Thread; - $thr = new Thread \&sub1; # Spawn the thread - - $thr->detach; # Now we officially don't care any more - - sub sub1 { - $a = 0; - while (1) { - $a++; - print "\$a is $a\n"; - sleep 1; - } - } - - -Once a thread is detached, it may not be joined, and any output that -it might have produced (if it was done and waiting for a join) is -lost. - -=head1 Threads And Data - -Now that we've covered the basics of threads, it's time for our next -topic: data. Threading introduces a couple of complications to data -access that non-threaded programs never need to worry about. - -=head2 Shared And Unshared Data - -The single most important thing to remember when using threads is that -all threads potentially have access to all the data anywhere in your -program. While this is true with a nonthreaded Perl program as well, -it's especially important to remember with a threaded program, since -more than one thread can be accessing this data at once. - -Perl's scoping rules don't change because you're using threads. If a -subroutine (or block, in the case of async()) could see a variable if -you weren't running with threads, it can see it if you are. This is -especially important for the subroutines that create, and makes C -variables even more important. Remember--if your variables aren't -lexically scoped (declared with C) you're probably sharing them -between threads. - -=head2 Thread Pitfall: Races - -While threads bring a new set of useful tools, they also bring a -number of pitfalls. One pitfall is the race condition: - - use Thread; - $a = 1; - $thr1 = Thread->new(\&sub1); - $thr2 = Thread->new(\&sub2); - - sleep 10; - print "$a\n"; - - sub sub1 { $foo = $a; $a = $foo + 1; } - sub sub2 { $bar = $a; $a = $bar + 1; } - -What do you think $a will be? The answer, unfortunately, is "it -depends." Both sub1() and sub2() access the global variable $a, once -to read and once to write. Depending on factors ranging from your -thread implementation's scheduling algorithm to the phase of the moon, -$a can be 2 or 3. - -Race conditions are caused by unsynchronized access to shared -data. Without explicit synchronization, there's no way to be sure that -nothing has happened to the shared data between the time you access it -and the time you update it. Even this simple code fragment has the -possibility of error: - - use Thread qw(async); - $a = 2; - async{ $b = $a; $a = $b + 1; }; - async{ $c = $a; $a = $c + 1; }; - -Two threads both access $a. Each thread can potentially be interrupted -at any point, or be executed in any order. At the end, $a could be 3 -or 4, and both $b and $c could be 2 or 3. - -Whenever your program accesses data or resources that can be accessed -by other threads, you must take steps to coordinate access or risk -data corruption and race conditions. - -=head2 Controlling access: lock() - -The lock() function takes a variable (or subroutine, but we'll get to -that later) and puts a lock on it. No other thread may lock the -variable until the locking thread exits the innermost block containing -the lock. Using lock() is straightforward: - - use Thread qw(async); - $a = 4; - $thr1 = async { - $foo = 12; - { - lock ($a); # Block until we get access to $a - $b = $a; - $a = $b * $foo; - } - print "\$foo was $foo\n"; - }; - $thr2 = async { - $bar = 7; - { - lock ($a); # Block until we can get access to $a - $c = $a; - $a = $c * $bar; - } - print "\$bar was $bar\n"; - }; - $thr1->join; - $thr2->join; - print "\$a is $a\n"; - -lock() blocks the thread until the variable being locked is -available. When lock() returns, your thread can be sure that no other -thread can lock that variable until the innermost block containing the -lock exits. - -It's important to note that locks don't prevent access to the variable -in question, only lock attempts. This is in keeping with Perl's -longstanding tradition of courteous programming, and the advisory file -locking that flock() gives you. Locked subroutines behave differently, -however. We'll cover that later in the article. - -You may lock arrays and hashes as well as scalars. Locking an array, -though, will not block subsequent locks on array elements, just lock -attempts on the array itself. - -Finally, locks are recursive, which means it's okay for a thread to -lock a variable more than once. The lock will last until the outermost -lock() on the variable goes out of scope. - -=head2 Thread Pitfall: Deadlocks - -Locks are a handy tool to synchronize access to data. Using them -properly is the key to safe shared data. Unfortunately, locks aren't -without their dangers. Consider the following code: - - use Thread qw(async yield); - $a = 4; - $b = "foo"; - async { - lock($a); - yield; - sleep 20; - lock ($b); - }; - async { - lock($b); - yield; - sleep 20; - lock ($a); - }; - -This program will probably hang until you kill it. The only way it -won't hang is if one of the two async() routines acquires both locks -first. A guaranteed-to-hang version is more complicated, but the -principle is the same. - -The first thread spawned by async() will grab a lock on $a then, a -second or two later, try to grab a lock on $b. Meanwhile, the second -thread grabs a lock on $b, then later tries to grab a lock on $a. The -second lock attempt for both threads will block, each waiting for the -other to release its lock. - -This condition is called a deadlock, and it occurs whenever two or -more threads are trying to get locks on resources that the others -own. Each thread will block, waiting for the other to release a lock -on a resource. That never happens, though, since the thread with the -resource is itself waiting for a lock to be released. - -There are a number of ways to handle this sort of problem. The best -way is to always have all threads acquire locks in the exact same -order. If, for example, you lock variables $a, $b, and $c, always lock -$a before $b, and $b before $c. It's also best to hold on to locks for -as short a period of time to minimize the risks of deadlock. - -=head2 Queues: Passing Data Around - -A queue is a special thread-safe object that lets you put data in one -end and take it out the other without having to worry about -synchronization issues. They're pretty straightforward, and look like -this: - - use Thread qw(async); - use Thread::Queue; - - my $DataQueue = new Thread::Queue; - $thr = async { - while ($DataElement = $DataQueue->dequeue) { - print "Popped $DataElement off the queue\n"; - } - }; - - $DataQueue->enqueue(12); - $DataQueue->enqueue("A", "B", "C"); - $DataQueue->enqueue(\$thr); - sleep 10; - $DataQueue->enqueue(undef); - -You create the queue with new Thread::Queue. Then you can add lists of -scalars onto the end with enqueue(), and pop scalars off the front of -it with dequeue(). A queue has no fixed size, and can grow as needed -to hold everything pushed on to it. - -If a queue is empty, dequeue() blocks until another thread enqueues -something. This makes queues ideal for event loops and other -communications between threads. - -=head1 Threads And Code - -In addition to providing thread-safe access to data via locks and -queues, threaded Perl also provides general-purpose semaphores for -coarser synchronization than locks provide and thread-safe access to -entire subroutines. - -=head2 Semaphores: Synchronizing Data Access - -Semaphores are a kind of generic locking mechanism. Unlike lock, which -gets a lock on a particular scalar, Perl doesn't associate any -particular thing with a semaphore so you can use them to control -access to anything you like. In addition, semaphores can allow more -than one thread to access a resource at once, though by default -semaphores only allow one thread access at a time. +In the meanwhile, you can read up on threading basics (while keeping +in mind the above caveat about the changing threading flavours) in +L =over 4 -=item Basic semaphores +=item * -Semaphores have two methods, down and up. down decrements the resource -count, while up increments it. down calls will block if the -semaphore's current count would decrement below zero. This program -gives a quick demonstration: +L - use Thread qw(yield); - use Thread::Semaphore; - my $semaphore = new Thread::Semaphore; - $GlobalVariable = 0; +=item * - $thr1 = new Thread \&sample_sub, 1; - $thr2 = new Thread \&sample_sub, 2; - $thr3 = new Thread \&sample_sub, 3; +L - sub sample_sub { - my $SubNumber = shift @_; - my $TryCount = 10; - my $LocalCopy; - sleep 1; - while ($TryCount--) { - $semaphore->down; - $LocalCopy = $GlobalVariable; - print "$TryCount tries left for sub $SubNumber (\$GlobalVariable is $GlobalVariable)\n"; - yield; - sleep 2; - $LocalCopy++; - $GlobalVariable = $LocalCopy; - $semaphore->up; - } - } +=item * -The three invocations of the subroutine all operate in sync. The -semaphore, though, makes sure that only one thread is accessing the -global variable at once. +L -=item Advanced Semaphores +=item * -By default, semaphores behave like locks, letting only one thread -down() them at a time. However, there are other uses for semaphores. +L -Each semaphore has a counter attached to it. down() decrements the -counter and up() increments the counter. By default, semaphores are -created with the counter set to one, down() decrements by one, and -up() increments by one. If down() attempts to decrement the counter -below zero, it blocks until the counter is large enough. Note that -while a semaphore can be created with a starting count of zero, any -up() or down() always changes the counter by at least -one. $semaphore->down(0) is the same as $semaphore->down(1). -The question, of course, is why would you do something like this? Why -create a semaphore with a starting count that's not one, or why -decrement/increment it by more than one? The answer is resource -availability. Many resources that you want to manage access for can be -safely used by more than one thread at once. +=item * -For example, let's take a GUI driven program. It has a semaphore that -it uses to synchronize access to the display, so only one thread is -ever drawing at once. Handy, but of course you don't want any thread -to start drawing until things are properly set up. In this case, you -can create a semaphore with a counter set to zero, and up it when -things are ready for drawing. - -Semaphores with counters greater than one are also useful for -establishing quotas. Say, for example, that you have a number of -threads that can do I/O at once. You don't want all the threads -reading or writing at once though, since that can potentially swamp -your I/O channels, or deplete your process' quota of filehandles. You -can use a semaphore initialized to the number of concurrent I/O -requests (or open files) that you want at any one time, and have your -threads quietly block and unblock themselves. - -Larger increments or decrements are handy in those cases where a -thread needs to check out or return a number of resources at once. +L =back -=head2 Attributes: Restricting Access To Subroutines - -In addition to synchronizing access to data or resources, you might -find it useful to synchronize access to subroutines. You may be -accessing a singular machine resource (perhaps a vector processor), or -find it easier to serialize calls to a particular subroutine than to -have a set of locks and semaphores. - -One of the additions to Perl 5.005 is subroutine attributes. The -Thread package uses these to provide several flavors of -serialization. It's important to remember that these attributes are -used in the compilation phase of your program so you can't change a -subroutine's behavior while your program is actually running. - -=head2 Subroutine Locks - -The basic subroutine lock looks like this: - - sub test_sub :locked { - } - -This ensures that only one thread will be executing this subroutine at -any one time. Once a thread calls this subroutine, any other thread -that calls it will block until the thread in the subroutine exits -it. A more elaborate example looks like this: - - use Thread qw(yield); - - new Thread \&thread_sub, 1; - new Thread \&thread_sub, 2; - new Thread \&thread_sub, 3; - new Thread \&thread_sub, 4; - - sub sync_sub :locked { - my $CallingThread = shift @_; - print "In sync_sub for thread $CallingThread\n"; - yield; - sleep 3; - print "Leaving sync_sub for thread $CallingThread\n"; - } - - sub thread_sub { - my $ThreadID = shift @_; - print "Thread $ThreadID calling sync_sub\n"; - sync_sub($ThreadID); - print "$ThreadID is done with sync_sub\n"; - } - -The C attribute tells perl to lock sync_sub(), and if you run -this, you can see that only one thread is in it at any one time. - -=head2 Methods - -Locking an entire subroutine can sometimes be overkill, especially -when dealing with Perl objects. When calling a method for an object, -for example, you want to serialize calls to a method, so that only one -thread will be in the subroutine for a particular object, but threads -calling that subroutine for a different object aren't blocked. The -method attribute indicates whether the subroutine is really a method. - - use Thread; - - sub tester { - my $thrnum = shift @_; - my $bar = new Foo; - foreach (1..10) { - print "$thrnum calling per_object\n"; - $bar->per_object($thrnum); - print "$thrnum out of per_object\n"; - yield; - print "$thrnum calling one_at_a_time\n"; - $bar->one_at_a_time($thrnum); - print "$thrnum out of one_at_a_time\n"; - yield; - } - } - - foreach my $thrnum (1..10) { - new Thread \&tester, $thrnum; - } - - package Foo; - sub new { - my $class = shift @_; - return bless [@_], $class; - } - - sub per_object :locked :method { - my ($class, $thrnum) = @_; - print "In per_object for thread $thrnum\n"; - yield; - sleep 2; - print "Exiting per_object for thread $thrnum\n"; - } - - sub one_at_a_time :locked { - my ($class, $thrnum) = @_; - print "In one_at_a_time for thread $thrnum\n"; - yield; - sleep 2; - print "Exiting one_at_a_time for thread $thrnum\n"; - } - -As you can see from the output (omitted for brevity; it's 800 lines) -all the threads can be in per_object() simultaneously, but only one -thread is ever in one_at_a_time() at once. - -=head2 Locking A Subroutine - -You can lock a subroutine as you would lock a variable. Subroutine locks -work the same as specifying a C attribute for the subroutine, -and block all access to the subroutine for other threads until the -lock goes out of scope. When the subroutine isn't locked, any number -of threads can be in it at once, and getting a lock on a subroutine -doesn't affect threads already in the subroutine. Getting a lock on a -subroutine looks like this: - - lock(\&sub_to_lock); - -Simple enough. Unlike the C attribute, which is a compile time -option, locking and unlocking a subroutine can be done at runtime at your -discretion. There is some runtime penalty to using lock(\&sub) instead -of the C attribute, so make sure you're choosing the proper -method to do the locking. - -You'd choose lock(\&sub) when writing modules and code to run on both -threaded and unthreaded Perl, especially for code that will run on -5.004 or earlier Perls. In that case, it's useful to have subroutines -that should be serialized lock themselves if they're running threaded, -like so: - - package Foo; - use Config; - $Running_Threaded = 0; - - BEGIN { $Running_Threaded = $Config{'usethreads'} } - - sub sub1 { lock(\&sub1) if $Running_Threaded } - - -This way you can ensure single-threadedness regardless of which -version of Perl you're running. - -=head1 General Thread Utility Routines - -We've covered the workhorse parts of Perl's threading package, and -with these tools you should be well on your way to writing threaded -code and packages. There are a few useful little pieces that didn't -really fit in anyplace else. - -=head2 What Thread Am I In? - -The Thread->self method provides your program with a way to get an -object representing the thread it's currently in. You can use this -object in the same way as the ones returned from the thread creation. - -=head2 Thread IDs - -tid() is a thread object method that returns the thread ID of the -thread the object represents. Thread IDs are integers, with the main -thread in a program being 0. Currently Perl assigns a unique tid to -every thread ever created in your program, assigning the first thread -to be created a tid of 1, and increasing the tid by 1 for each new -thread that's created. - -=head2 Are These Threads The Same? - -The equal() method takes two thread objects and returns true -if the objects represent the same thread, and false if they don't. - -=head2 What Threads Are Running? - -Thread->list returns a list of thread objects, one for each thread -that's currently running. Handy for a number of things, including -cleaning up at the end of your program: - - # Loop through all the threads - foreach $thr (Thread->list) { - # Don't join the main thread or ourselves - if ($thr->tid && !Thread::equal($thr, Thread->self)) { - $thr->join; - } - } - -The example above is just for illustration. It isn't strictly -necessary to join all the threads you create, since Perl detaches all -the threads before it exits. - -=head1 A Complete Example - -Confused yet? It's time for an example program to show some of the -things we've covered. This program finds prime numbers using threads. - - 1 #!/usr/bin/perl -w - 2 # prime-pthread, courtesy of Tom Christiansen - 3 - 4 use strict; - 5 - 6 use Thread; - 7 use Thread::Queue; - 8 - 9 my $stream = new Thread::Queue; - 10 my $kid = new Thread(\&check_num, $stream, 2); - 11 - 12 for my $i ( 3 .. 1000 ) { - 13 $stream->enqueue($i); - 14 } - 15 - 16 $stream->enqueue(undef); - 17 $kid->join(); - 18 - 19 sub check_num { - 20 my ($upstream, $cur_prime) = @_; - 21 my $kid; - 22 my $downstream = new Thread::Queue; - 23 while (my $num = $upstream->dequeue) { - 24 next unless $num % $cur_prime; - 25 if ($kid) { - 26 $downstream->enqueue($num); - 27 } else { - 28 print "Found prime $num\n"; - 29 $kid = new Thread(\&check_num, $downstream, $num); - 30 } - 31 } - 32 $downstream->enqueue(undef) if $kid; - 33 $kid->join() if $kid; - 34 } - -This program uses the pipeline model to generate prime numbers. Each -thread in the pipeline has an input queue that feeds numbers to be -checked, a prime number that it's responsible for, and an output queue -that it funnels numbers that have failed the check into. If the thread -has a number that's failed its check and there's no child thread, then -the thread must have found a new prime number. In that case, a new -child thread is created for that prime and stuck on the end of the -pipeline. - -This probably sounds a bit more confusing than it really is, so lets -go through this program piece by piece and see what it does. (For -those of you who might be trying to remember exactly what a prime -number is, it's a number that's only evenly divisible by itself and 1) - -The bulk of the work is done by the check_num() subroutine, which -takes a reference to its input queue and a prime number that it's -responsible for. After pulling in the input queue and the prime that -the subroutine's checking (line 20), we create a new queue (line 22) -and reserve a scalar for the thread that we're likely to create later -(line 21). - -The while loop from lines 23 to line 31 grabs a scalar off the input -queue and checks against the prime this thread is responsible -for. Line 24 checks to see if there's a remainder when we modulo the -number to be checked against our prime. If there is one, the number -must not be evenly divisible by our prime, so we need to either pass -it on to the next thread if we've created one (line 26) or create a -new thread if we haven't. - -The new thread creation is line 29. We pass on to it a reference to -the queue we've created, and the prime number we've found. - -Finally, once the loop terminates (because we got a 0 or undef in the -queue, which serves as a note to die), we pass on the notice to our -child and wait for it to exit if we've created a child (Lines 32 and -37). - -Meanwhile, back in the main thread, we create a queue (line 9) and the -initial child thread (line 10), and pre-seed it with the first prime: -2. Then we queue all the numbers from 3 to 1000 for checking (lines -12-14), then queue a die notice (line 16) and wait for the first child -thread to terminate (line 17). Because a child won't die until its -child has died, we know that we're done once we return from the join. - -That's how it works. It's pretty simple; as with many Perl programs, -the explanation is much longer than the program. - -=head1 Conclusion - -A complete thread tutorial could fill a book (and has, many times), -but this should get you well on your way. The final authority on how -Perl's threads behave is the documentation bundled with the Perl -distribution, but with what we've covered in this article, you should -be well on your way to becoming a threaded Perl expert. - -=head1 Bibliography - -Here's a short bibliography courtesy of Jürgen Christoffel: - -=head2 Introductory Texts - -Birrell, Andrew D. An Introduction to Programming with -Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report -#35 online as -http://www.research.digital.com/SRC/staff/birrell/bib.html (highly -recommended) - -Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A -Guide to Concurrency, Communication, and -Multithreading. Prentice-Hall, 1996. - -Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with -Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written -introduction to threads). - -Nelson, Greg (editor). Systems Programming with Modula-3. Prentice -Hall, 1991, ISBN 0-13-590464-1. - -Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell. -Pthreads Programming. O'Reilly & Associates, 1996, ISBN 156592-115-1 -(covers POSIX threads). - -=head2 OS-Related References - -Boykin, Joseph, David Kirschen, Alan Langerman, and Susan -LoVerso. Programming under Mach. Addison-Wesley, 1994, ISBN -0-201-52739-1. - -Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall, -1995, ISBN 0-13-219908-4 (great textbook). - -Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts, -4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4 - -=head2 Other References - -Arnold, Ken and James Gosling. The Java Programming Language, 2nd -ed. Addison-Wesley, 1998, ISBN 0-201-31006-6. - -Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage -Collection on Virtually Shared Memory Architectures" in Memory -Management: Proc. of the International Workshop IWMM 92, St. Malo, -France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer, -1992, ISBN 3540-55940-X (real-life thread applications). - -=head1 Acknowledgements - -Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy -Sarathy, Ilya Zakharevich, Benjamin Sugars, Jürgen Christoffel, Joshua -Pritikin, and Alan Burlison, for their help in reality-checking and -polishing this article. Big thanks to Tom Christiansen for his rewrite -of the prime number generator. - -=head1 AUTHOR - -Dan Sugalski Esugalskd@ous.eduE - -=head1 Copyrights - -This article originally appeared in The Perl Journal #10, and is -copyright 1998 The Perl Journal. It appears courtesy of Jon Orwant and -The Perl Journal. This document may be distributed under the same terms -as Perl itself. +When C reaches L is when +you should slow down and remember to mentally read C +when C says C. The C was the old +5.005-style threading module, the C is the new ithreads +style threading module. +For more information please see L and L.