=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!
-
-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.
+B<NOTE>: this tutorial describes the new Perl threading flavour
+introduced in Perl 5.6.0 called interpreter threads, or B<ithreads>
+for short. In this model each thread runs in its own Perl interpreter,
+and any data sharing between threads must be explicit.
+
+There is another older Perl threading flavour called the 5.005 model,
+unsurprisingly for 5.005 versions of Perl. The old model is known to
+have problems, deprecated, and will probably be removed around release
+5.10. You are strongly encouraged to migrate any existing 5.005
+threads code to the new model as soon as possible.
+
+You can see which (or neither) threading flavour you have by
+running C<perl -V> and looking 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.
+
+The user-level interface to the 5.005 threads was via the L<Threads>
+class, while ithreads uses the L<threads> class. Note the change in case.
+
+=head1 Status
+
+The ithreads code has been available since Perl 5.6.0, and is considered
+stable. The user-level interface to ithreads (the L<threads> classes)
+appeared in the 5.8.0 release, and as of this time is considered stable
+although it should be treated with caution as with all new features.
=head1 What Is A Thread Anyway?
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
+thread. With 5.8, though, you can create extra threads. We're going
to show you how, when, and why.
=head1 Threaded Program Models
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
+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
was fetched on the right hand side and the time the new value is
stored.
-Multiprocessor Kernel Threads are the final step in thread
+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.
often behave cooperatively, for example, while threads running at
normal priorities behave preemptively.)
-=head1 What kind of threads are perl threads?
+=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
step back a bit and think about what you want to do and how Perl can
do it.
-=head1 Threadsafe Modules
+However it is important to remember that Perl threads cannot magically
+do things unless your operating systems threads allows it. So if your
+system blocks the entire process on sleep(), Perl usually will as well.
+
+Perl Threads Are Different.
+
+=head1 Thread-Safe 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.
+substantially. There are implications for people who write
+modules with XS code or external libraries. However, since perl data is
+not shared among threads by default, Perl modules stand a high chance of
+being thread-safe or can be made thread-safe easily. Modules that are not
+tagged as thread-safe should be tested or code reviewed 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
+core. Threads are a new feature, and even some of the standard
modules aren't thread-safe.
+Even if a module is thread-safe, it doesn't mean that the module is optimized
+to work well with threads. A module could possibly be rewritten to utilize
+the new features in threaded Perl to increase performance in a threaded
+environment.
+
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
+can protect yourself by using it from one, and only one thread at all.
+If you need multiple threads to access such a module, you can use semaphores and
+lots of programming discipline to control access to it. Semaphores
+are covered in L</"Basic semaphores">.
+
+See also L</"Thread-Safety of System Libraries">.
=head1 Thread Basics
-The core Thread module provides the basic functions you need to write
+The core L<threads> 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
+that, we'll go over some of the features of the L<threads> module that
make threaded programming easier.
=head2 Basic Thread Support
-Thread support is a Perl compile-time option-it's something that's
+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.";
+ $Config{useithreads} or die "Recompile Perl with threads to run this program.";
A possibly-threaded program using a possibly-threaded module might
have code like this:
- use Config;
- use MyMod;
-
- if ($Config{usethreads}) {
- # We have threads
- require MyMod_threaded;
- import MyMod_threaded;
- } else {
- require MyMod_unthreaded;
- import MyMod_unthreaded;
- }
+ use Config;
+ use MyMod;
+
+ BEGIN {
+ if ($Config{useithreads}) {
+ # 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 A Note about the Examples
+
+Although thread support is considered to be stable, there are still a number
+of quirks that may startle you when you try out any of the examples below.
+In a real situation, care should be taken that all threads are finished
+executing before the program exits. That care has B<not> been taken in these
+examples in the interest of simplicity. Running these examples "as is" will
+produce error messages, usually caused by the fact that there are still
+threads running when the program exits. You should not be alarmed by this.
+Future versions of Perl may fix this problem.
+
=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
+The L<threads> package provides the tools you need to create new
+threads. Like any other module, you need to tell Perl that you want to use
+it; C<use threads> imports all the pieces you need to create basic
threads.
-The simplest, straightforward way to create a thread is with new():
+The simplest, most straightforward way to create a thread is with new():
- use Thread;
+ use threads;
- $thr = new Thread \&sub1;
+ $thr = threads->new(\&sub1);
sub sub1 {
print "In the thread\n";
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:
+part of the C<threads::new> call, like this:
+
+ use threads;
- 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);
+ $thr = threads->new(\&sub1, "Param 1", "Param 2", $Param3);
+ $thr = threads->new(\&sub1, @ParamList);
+ $thr = threads->new(\&sub1, qw(Param1 Param2 Param3));
sub sub1 {
my @InboundParameters = @_;
}
-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.
+C<create()> is a synonym for C<new()>.
=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
+you may be doing something processor-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" }
- };
+ use threads;
+
+ sub loop {
+ my $thread = shift;
+ my $foo = 50;
+ while($foo--) { print "in thread $thread\n" }
+ threads->yield;
+ $foo = 50;
+ while($foo--) { print "in thread $thread\n" }
+ }
+
+ my $thread1 = threads->new(\&loop, 'first');
+ my $thread2 = threads->new(\&loop, 'second');
+ my $thread3 = threads->new(\&loop, 'third');
+
+It is important to remember that yield() is only a hint to give up the CPU,
+it depends on your hardware, OS and threading libraries what actually happens.
+Therefore it is important to note that one should not build the scheduling of
+the threads around yield() calls. It might work on your platform but it won't
+work on another platform.
=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.
+for a thread to exit and extract any values it might return, you can
+use the join() method:
+
+ use threads;
- use Thread;
- $thr = new Thread \&sub1;
+ $thr = threads->new(\&sub1);
@ReturnData = $thr->join;
print "Thread returned @ReturnData";
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.
+instead, as described next.
=head2 Ignoring A Thread
it'll run until it's finished, then Perl will clean up after it
automatically.
- use Thread;
- $thr = new Thread \&sub1; # Spawn the thread
+ use threads;
+
+ $thr = threads->new(\&sub1); # Spawn the thread
$thr->detach; # Now we officially don't care any more
- sub sub1 {
+ sub sub1 {
$a = 0;
while (1) {
$a++;
}
}
-
-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
+Once a thread is detached, it may not be joined, and any return data
+that it might have produced (if it was done and waiting for a join) is
lost.
=head1 Threads And Data
=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.
+The biggest difference between Perl ithreads and the old 5.005 style
+threading, or for that matter, to most other threading systems out there,
+is that by default, no data is shared. When a new perl thread is created,
+all the data associated with the current thread is copied to the new
+thread, and is subsequently private to that new thread!
+This is similar in feel to what happens when a UNIX process forks,
+except that in this case, the data is just copied to a different part of
+memory within the same process rather than a real fork taking place.
+
+To make use of threading however, one usually wants the threads to share
+at least some data between themselves. This is done with the
+L<threads::shared> module and the C< : shared> attribute:
+
+ use threads;
+ use threads::shared;
+
+ my $foo : shared = 1;
+ my $bar = 1;
+ threads->new(sub { $foo++; $bar++ })->join;
+
+ print "$foo\n"; #prints 2 since $foo is shared
+ print "$bar\n"; #prints 1 since $bar is not shared
+
+In the case of a shared array, all the array's elements are shared, and for
+a shared hash, all the keys and values are shared. This places
+restrictions on what may be assigned to shared array and hash elements: only
+simple values or references to shared variables are allowed - this is
+so that a private variable can't accidentally become shared. A bad
+assignment will cause the thread to die. For example:
+
+ use threads;
+ use threads::shared;
-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.
+ my $var = 1;
+ my $svar : shared = 2;
+ my %hash : shared;
-=head2 Thread Pitfall: Races
+ ... create some threads ...
+
+ $hash{a} = 1; # all threads see exists($hash{a}) and $hash{a} == 1
+ $hash{a} = $var # okay - copy-by-value: same effect as previous
+ $hash{a} = $svar # okay - copy-by-value: same effect as previous
+ $hash{a} = \$svar # okay - a reference to a shared variable
+ $hash{a} = \$var # This will die
+ delete $hash{a} # okay - all threads will see !exists($hash{a})
+
+Note that a shared variable guarantees that if two or more threads try to
+modify it at the same time, the internal state of the variable will not
+become corrupted. However, there are no guarantees beyond this, as
+explained in the next section.
+
+=head2 Thread Pitfalls: 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);
+ use threads;
+ use threads::shared;
- sleep 10;
+ my $a : shared = 1;
+ $thr1 = threads->new(\&sub1);
+ $thr2 = threads->new(\&sub2);
+
+ $thr1->join;
+ $thr2->join;
print "$a\n";
- sub sub1 { $foo = $a; $a = $foo + 1; }
- sub sub2 { $bar = $a; $a = $bar + 1; }
+ sub sub1 { my $foo = $a; $a = $foo + 1; }
+ sub sub2 { my $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
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; };
+ use threads;
+ my $a : shared = 2;
+ my $b : shared;
+ my $c : shared;
+ my $thr1 = threads->create(sub { $b = $a; $a = $b + 1; });
+ my $thr2 = threads->create(sub { $c = $a; $a = $c + 1; });
+ $thr1->join;
+ $thr2->join;
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 $c could be 2 or 3.
+Even C<$a += 5> or C<$a++> are not guaranteed to be atomic.
+
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.
+data inconsistency and race conditions. Note that Perl will protect its
+internals from your race conditions, but it won't protect you from you.
+
+=head1 Synchronization and control
+
+Perl provides a number of mechanisms to coordinate the interactions
+between themselves and their data, to avoid race conditions and the like.
+Some of these are designed to resemble the common techniques used in thread
+libraries such as C<pthreads>; others are Perl-specific. Often, the
+standard techniques are clumsy and difficult to get right (such as
+condition waits). Where possible, it is usually easier to use Perlish
+techniques such as queues, which remove some of the hard work involved.
=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";
+The lock() function takes a shared variable and puts a lock on it.
+No other thread may lock the variable until the variable is unlocked
+by the thread holding the lock. Unlocking happens automatically
+when the locking thread exits the outermost block that contains
+C<lock()> function. Using lock() is straightforward: this example has
+several threads doing some calculations in parallel, and occasionally
+updating a running total:
+
+ use threads;
+ use threads::shared;
+
+ my $total : shared = 0;
+
+ sub calc {
+ for (;;) {
+ my $result;
+ # (... do some calculations and set $result ...)
+ {
+ lock($total); # block until we obtain the lock
+ $total += $result;
+ } # lock implicitly released at end of scope
+ last if $result == 0;
+ }
+ }
+
+ my $thr1 = threads->new(\&calc);
+ my $thr2 = threads->new(\&calc);
+ my $thr3 = threads->new(\&calc);
+ $thr1->join;
+ $thr2->join;
+ $thr3->join;
+ print "total=$total\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
+thread can lock that variable until the outermost 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.
+locking that flock() gives you.
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
+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.
+lock() on the variable goes out of scope. For example:
+
+ my $x : shared;
+ doit();
+
+ sub doit {
+ {
+ {
+ lock($x); # wait for lock
+ lock($x); # NOOP - we already have the lock
+ {
+ lock($x); # NOOP
+ {
+ lock($x); # NOOP
+ lockit_some_more();
+ }
+ }
+ } # *** implicit unlock here ***
+ }
+ }
-=head2 Thread Pitfall: Deadlocks
+ sub lockit_some_more {
+ lock($x); # NOOP
+ } # nothing happens here
-Locks are a handy tool to synchronize access to data. Using them
+Note that there is no unlock() function - the only way to unlock a
+variable is to allow it to go out of scope.
+
+A lock can either be used to guard the data contained within the variable
+being locked, or it can be used to guard something else, like a section
+of code. In this latter case, the variable in question does not hold any
+useful data, and exists only for the purpose of being locked. In this
+respect, the variable behaves like the mutexes and basic semaphores of
+traditional thread libraries.
+
+=head2 A Thread Pitfall: Deadlocks
+
+Locks are a handy tool to synchronize access to data, and using them
properly is the key to safe shared data. Unfortunately, locks aren't
-without their dangers. Consider the following code:
+without their dangers, especially when multiple locks are involved.
+Consider the following code:
+
+ use threads;
- use Thread qw(async yield);
- $a = 4;
- $b = "foo";
- async {
+ my $a : shared = 4;
+ my $b : shared = "foo";
+ my $thr1 = threads->new(sub {
lock($a);
- yield;
+ threads->yield;
sleep 20;
- lock ($b);
- };
- async {
lock($b);
- yield;
+ });
+ my $thr2 = threads->new(sub {
+ lock($b);
+ threads->yield;
sleep 20;
- lock ($a);
- };
+ lock($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
+won't hang is if one of the two threads 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.
+The first thread will grab a lock on $a, then, after a pause during which
+the second thread has probably had time to do some work, 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
$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.
+The other synchronization primitives described below can suffer from
+similar problems.
+
=head2 Queues: Passing Data Around
A queue is a special thread-safe object that lets you put data in one
synchronization issues. They're pretty straightforward, and look like
this:
- use Thread qw(async);
+ use threads;
use Thread::Queue;
- my $DataQueue = new Thread::Queue;
- $thr = async {
+ my $DataQueue = Thread::Queue->new;
+ $thr = threads->new(sub {
while ($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);
+ $thr->join;
-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.
+You create the queue with C<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.
+Semaphores are a kind of generic locking mechanism. In their most basic
+form, they behave very much like lockable scalars, except that thay
+can't hold data, and that they must be explicitly unlocked. In their
+advanced form, they act like a kind of counter, and can allow multiple
+threads to have the 'lock' at any one time.
-=over 4
+=head2 Basic semaphores
-=item Basic semaphores
-
-Semaphores have two methods, down and up. down decrements the resource
-count, while up increments it. down calls will block if the
+Semaphores have two methods, down() and up(): down() decrements the resource
+count, while up increments it. Calls to down() will block if the
semaphore's current count would decrement below zero. This program
gives a quick demonstration:
- use Thread qw(yield);
+ use threads qw(yield);
use Thread::Semaphore;
+
my $semaphore = new Thread::Semaphore;
- $GlobalVariable = 0;
+ my $GlobalVariable : shared = 0;
- $thr1 = new Thread \&sample_sub, 1;
- $thr2 = new Thread \&sample_sub, 2;
- $thr3 = new Thread \&sample_sub, 3;
+ $thr1 = new threads \&sample_sub, 1;
+ $thr2 = new threads \&sample_sub, 2;
+ $thr3 = new threads \&sample_sub, 3;
sub sample_sub {
my $SubNumber = shift @_;
}
}
+ $thr1->join;
+ $thr2->join;
+ $thr3->join;
+
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.
-=item Advanced Semaphores
+=head2 Advanced Semaphores
By default, semaphores behave like locks, letting only one thread
down() them at a time. However, there are other uses for semaphores.
-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).
+Each semaphore has a counter attached to it. By default, semaphores are
+created with the counter set to one, down() decrements the counter by
+one, and up() increments by one. However, we can override any or all
+of these defaults simply by passing in different values:
+
+ use threads;
+ use Thread::Semaphore;
+ my $semaphore = Thread::Semaphore->new(5);
+ # Creates a semaphore with the counter set to five
+
+ $thr1 = threads->new(\&sub1);
+ $thr2 = threads->new(\&sub1);
+
+ sub sub1 {
+ $semaphore->down(5); # Decrements the counter by five
+ # Do stuff here
+ $semaphore->up(5); # Increment the counter by five
+ }
+
+ $thr1->detach;
+ $thr2->detach;
+
+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, and so $semaphore->down(0) is the same as
+$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
Larger increments or decrements are handy in those cases where a
thread needs to check out or return a number of resources at once.
-=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<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;
+=head2 cond_wait() and cond_signal()
- 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.
+These two functions can be used in conjunction with locks to notify
+co-operating threads that a resource has become available. They are
+very similar in use to the functions found in C<pthreads>. However
+for most purposes, queues are simpler to use and more intuitive. See
+L<threads::shared> for more details.
=head1 General Thread Utility Routines
=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.
+The C<< threads->self >> class 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 thread creation.
=head2 Thread IDs
The equal() method takes two thread objects and returns true
if the objects represent the same thread, and false if they don't.
+Thread objects also have an overloaded == comparison so that you can do
+comparison on them as you would with normal objects.
+
=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:
+C<< threads->list >> returns a list of thread objects, one for each thread
+that's currently running and not detached. Handy for a number of things,
+including cleaning up at the end of your program:
# Loop through all the threads
- foreach $thr (Thread->list) {
+ foreach $thr (threads->list) {
# Don't join the main thread or ourselves
- if ($thr->tid && !Thread::equal($thr, Thread->self)) {
+ if ($thr->tid && !threads::equal($thr, threads->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.
+If some threads have not finished running when the main Perl thread
+ends, Perl will warn you about it and die, since it is impossible for Perl
+to clean up itself while other threads are running
=head1 A Complete Example
3
4 use strict;
5
- 6 use Thread;
+ 6 use threads;
7 use Thread::Queue;
8
9 my $stream = new Thread::Queue;
- 10 my $kid = new Thread(\&check_num, $stream, 2);
+ 10 my $kid = new threads(\&check_num, $stream, 2);
11
12 for my $i ( 3 .. 1000 ) {
13 $stream->enqueue($i);
14 }
15
16 $stream->enqueue(undef);
- 17 $kid->join();
+ 17 $kid->join;
18
19 sub check_num {
20 my ($upstream, $cur_prime) = @_;
26 $downstream->enqueue($num);
27 } else {
28 print "Found prime $num\n";
- 29 $kid = new Thread(\&check_num, $downstream, $num);
+ 29 $kid = new threads(\&check_num, $downstream, $num);
30 }
31 }
32 $downstream->enqueue(undef) if $kid;
- 33 $kid->join() 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
+into which it funnels numbers that have failed the check. 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
+This probably sounds a bit more confusing than it really is, so let's
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)
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
+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
That's how it works. It's pretty simple; as with many Perl programs,
the explanation is much longer than the program.
+=head1 Performance considerations
+
+The main thing to bear in mind when comparing ithreads to other threading
+models is the fact that for each new thread created, a complete copy of
+all the variables and data of the parent thread has to be taken. Thus
+thread creation can be quite expensive, both in terms of memory usage and
+time spent in creation. The ideal way to reduce these costs is to have a
+relatively short number of long-lived threads, all created fairly early
+on - before the base thread has accumulated too much data. Of course, this
+may not always be possible, so compromises have to be made. However, after
+a thread has been created, its performance and extra memory usage should
+be little different than ordinary code.
+
+Also note that under the current implementation, shared variables
+use a little more memory and are a little slower than ordinary variables.
+
+=head1 Process-scope Changes
+
+Note that while threads themselves are separate execution threads and
+Perl data is thread-private unless explicitly shared, the threads can
+affect process-scope state, affecting all the threads.
+
+The most common example of this is changing the current working
+directory using chdir(). One thread calls chdir(), and the working
+directory of all the threads changes.
+
+Even more drastic example of a process-scope change is chroot():
+the root directory of all the threads changes, and no thread can
+undo it (as opposed to chdir()).
+
+Further examples of process-scope changes include umask() and
+changing uids/gids.
+
+Thinking of mixing fork() and threads? Please lie down and wait
+until the feeling passes-- but in case you really want to know,
+the semantics is that fork() duplicates all the threads.
+(In UNIX, at least, other platforms will do something different.)
+
+Similarly, mixing signals and threads should not be attempted.
+Implementations are platform-dependent, and even the POSIX
+semantics may not be what you expect (and Perl doesn't even
+give you the full POSIX API).
+
+=head1 Thread-Safety of System Libraries
+
+Whether various library calls are thread-safe is outside the control
+of Perl. Calls often suffering from not being thread-safe include:
+localtime(), gmtime(), get{gr,host,net,proto,serv,pw}*(), readdir(),
+rand(), and srand() -- in general, calls that depend on some global
+external state.
+
+If the system Perl is compiled in has thread-safe variants of such
+calls, they will be used. Beyond that, Perl is at the mercy of
+the thread-safety or -unsafety of the calls. Please consult your
+C library call documentation.
+
+On some platforms the thread-safe library interfaces may fail if the
+result buffer is too small (for example the user group databases may
+be rather large, and the reentrant interfaces may have to carry around
+a full snapshot of those databases). Perl will start with a small
+buffer, but keep retrying and growing the result buffer
+until the result fits. If this limitless growing sounds bad for
+security or memory consumption reasons you can recompile Perl with
+PERL_REENTRANT_MAXSIZE defined to the maximum number of bytes you will
+allow.
+
=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.
+but with what we've covered in this introduction, you should be well
+on your way to becoming a threaded Perl expert.
=head1 Bibliography
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)
+http://gatekeeper.dec.com/pub/DEC/SRC/research-reports/abstracts/src-rr-035.html
+(highly recommended)
Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A
Guide to Concurrency, Communication, and
Arnold, Ken and James Gosling. The Java Programming Language, 2nd
ed. Addison-Wesley, 1998, ISBN 0-201-31006-6.
+comp.programming.threads FAQ,
+L<http://www.serpentine.com/~bos/threads-faq/>
+
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).
+Artur Bergman, "Where Wizards Fear To Tread", June 11, 2002,
+L<http://www.perl.com/pub/a/2002/06/11/threads.html>
+
=head1 Acknowledgements
Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy
=head1 AUTHOR
-Dan Sugalski E<lt>sugalskd@ous.eduE<gt>
+Dan Sugalski E<lt>dan@sidhe.org<gt>
+
+Slightly modified by Arthur Bergman to fit the new thread model/module.
+
+Reworked slightly by Jörg Walter E<lt>jwalt@cpan.org<gt> to be more concise
+about thread-safety of perl code.
=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.
+The original version of 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.
+For more information please see L<threads> and L<threads::shared>.
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