=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<NOTE>: 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<perl -V> and look at the C<Platform> section.
+If you have C<useithreads=define> you have ithreads, if you
+have C<use5005threads=define> 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<Thread::new> 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<my>
-variables even more important. Remember--if your variables aren't
-lexically scoped (declared with C<my>) 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<perlothrtut>
=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<perlothrtut/What Is A Thread Anyway?>
- 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<perlothrtut/Threaded Program Models>
- 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<perlothrtut/Native threads>
-=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<perlothrtut/What kind of threads are perl threads?>
-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<perlothrtut/Threadsafe Modules>
=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 sempahores.
-
-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<locked> 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<locked> 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<locked> 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<locked> 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 documention 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 0-13-143934-0 (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 E<lt>sugalskd@ous.eduE<gt>
-
-=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<perlothrut> reaches L<perlothrtut/Thread Basics> is when
+you should slow down and remember to mentally read C<threads>
+when C<perlothrtut> says C<Thread>. The C<Thread> was the old
+5.005-style threading module, the C<threads> is the new ithreads
+style threading module.
+For more information please see L<threads> and L<threads::shared>.
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