=head1 NAME
-perlthrtut - tutorial on threads in Perl
+perlthrtut - Tutorial on threads in Perl
=head1 DESCRIPTION
-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.
+This tutorial describes the use of Perl interpreter threads (sometimes
+referred to as I<ithreads>) that was first introduced in Perl 5.6.0. In this
+model, each thread runs in its own Perl interpreter, and any data sharing
+between threads must be explicit. The user-level interface for I<ithreads>
+uses the L<threads> class.
-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.
+B<NOTE>: There is another older Perl threading flavor called the 5.005 model
+that used the L<Threads> class. This old model is known to have problems, is
+deprecated, and support for it will be removed in 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 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.
+The L<threads> and L<threads::shared> modules are included in the core Perl
+distribution. Additionally, they are maintained as a separate modules on
+CPAN, so you can check there for any updates.
=head1 What Is A Thread Anyway?
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
+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
+The boss/worker model usually has one I<boss> thread and one or more
+I<worker> threads. The boss thread gathers or generates tasks that need
to be done, then parcels those tasks out to the appropriate worker
thread.
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
+remember when dealing with Perl threads that I<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
+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.
-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.
+However, it is important to remember that Perl threads cannot magically
+do things unless your operating system's threads allows it. So if your
+system blocks the entire process on C<sleep()>, Perl usually will, as well.
-Perl Threads Are Different.
+B<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 with XS code or external libraries. However, since perl data is
+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
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 new feature, and even some of the standard
+core. Threads are a relatively 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
=head1 Thread Basics
-The core L<threads> module provides the basic functions you need to write
-threaded programs. In the following sections we'll cover the basics,
+The 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 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.
enabled. If your program can't run without them, you can say something
like:
- $Config{useithreads} or die "Recompile Perl with threads to run this program.";
+ use Config;
+ $Config{useithreads} or die('Recompile Perl with threads to run this program.');
A possibly-threaded program using a possibly-threaded module might
have code like this:
if ($Config{useithreads}) {
# We have threads
require MyMod_threaded;
- import MyMod_threaded;
+ import MyMod_threaded;
} else {
- require MyMod_unthreaded;
- import MyMod_unthreaded;
+ 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
+module. In our example above, that's what C<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
+examples in the interest of simplicity. Running these examples I<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 L<threads> package provides the tools you need to create new
+The L<threads> module 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
+it; C<use threads;> imports all the pieces you need to create basic
threads.
-The simplest, most straightforward way to create a thread is with new():
+The simplest, most straightforward way to create a thread is with C<create()>:
- use threads;
+ use threads;
- $thr = threads->new(\&sub1);
+ my $thr = threads->create(\&sub1);
- sub sub1 {
- print "In the thread\n";
+ 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
+The C<create()> method takes a reference to a subroutine and creates a new
+thread that 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<threads::new> call, like this:
+part of the C<threads-E<gt>create()> call, like this:
- use threads;
+ use threads;
- $Param3 = "foo";
- $thr = threads->new(\&sub1, "Param 1", "Param 2", $Param3);
- $thr = threads->new(\&sub1, @ParamList);
- $thr = threads->new(\&sub1, qw(Param1 Param2 Param3));
+ my $Param3 = 'foo';
+ my $thr1 = threads->create(\&sub1, 'Param 1', 'Param 2', $Param3);
+ my @ParamList = (42, 'Hello', 3.14);
+ my $thr2 = threads->create(\&sub1, @ParamList);
+ my $thr3 = threads->create(\&sub1, qw(Param1 Param2 Param3));
- sub sub1 {
- my @InboundParameters = @_;
- print "In the thread\n";
- print "got parameters >", join("<>", @InboundParameters), "<\n";
+ sub sub1 {
+ my @InboundParameters = @_;
+ print("In the thread\n");
+ print('Got parameters >', join('<>', @InboundParameters), "<\n");
}
-
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.
-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 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 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.
+C<new()> is a synonym for C<create()>.
=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 values it might return, you can
-use the join() method:
+use the C<join()> method:
- use threads;
+ use threads;
- $thr = threads->new(\&sub1);
+ my ($thr) = threads->create(\&sub1);
- @ReturnData = $thr->join;
- print "Thread returned @ReturnData";
+ my @ReturnData = $thr->join();
+ print('Thread returned ', join(', ', @ReturnData), "\n");
- sub sub1 { return "Fifty-six", "foo", 2; }
+ sub sub1 { return ('Fifty-six', 'foo', 2); }
-In the example above, the join() method returns as soon as the thread
+In the example above, the C<join()> method returns as soon as the thread
ends. In addition to waiting for a thread to finish and gathering up
-any values that the thread might have returned, join() also performs
+any values that the thread might have returned, C<join()> also performs
any OS cleanup necessary for the thread. That cleanup might be
important, especially for long-running programs that spawn lots of
threads. If you don't want the return values and don't want to wait
-for the thread to finish, you should call the detach() method
+for the thread to finish, you should call the C<detach()> method
instead, as described next.
+NOTE: In the example above, the thread returns a list, thus necessitating
+that the thread creation call be made in list context (i.e., C<my ($thr)>).
+See L<threads/"$thr->join()"> and L<threads/"THREAD CONTEXT"> for more
+details on thread context and return values.
+
=head2 Ignoring A Thread
-join() does three things: it waits for a thread to exit, cleans up
+C<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
+In this case, you use the C<detach()> method. Once a thread is detached,
+it'll run until it's finished; then Perl will clean up after it
automatically.
- use threads;
+ use threads;
+
+ my $thr = threads->create(\&sub1); # Spawn the thread
- $thr = threads->new(\&sub1); # Spawn the thread
+ $thr->detach(); # Now we officially don't care any more
- $thr->detach; # Now we officially don't care any more
+ sleep(15); # Let thread run for awhile
sub sub1 {
- $a = 0;
- while (1) {
- $a++;
- print "\$a is $a\n";
- sleep 1;
- }
+ $a = 0;
+ while (1) {
+ $a++;
+ print("\$a is $a\n");
+ sleep(1);
+ }
}
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.
+C<detach()> can also be called as a class method to allow a thread to
+detach itself:
+
+ use threads;
+
+ my $thr = threads->create(\&sub1);
+
+ sub sub1 {
+ threads->detach();
+ # Do more work
+ }
+
=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
+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 biggest difference between Perl ithreads and the old 5.005 style
+The biggest difference between Perl I<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,
+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
+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:
+L<threads::shared> module and the C<:shared> attribute:
use threads;
use threads::shared;
- my $foo : shared = 1;
+ my $foo :shared = 1;
my $bar = 1;
- threads->new(sub { $foo++; $bar++ })->join;
+ threads->create(sub { $foo++; $bar++; })->join();
- print "$foo\n"; #prints 2 since $foo is shared
- print "$bar\n"; #prints 1 since $bar is not shared
+ print("$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
use threads;
use threads::shared;
- my $var = 1;
- my $svar : shared = 2;
- my %hash : shared;
+ my $var = 1;
+ my $svar :shared = 2;
+ my %hash :shared;
... 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})
+ $hash{a} = 1; # All threads see exists($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
While threads bring a new set of useful tools, they also bring a
number of pitfalls. One pitfall is the race condition:
- use threads;
+ use threads;
use threads::shared;
- my $a : shared = 1;
- $thr1 = threads->new(\&sub1);
- $thr2 = threads->new(\&sub2);
+ my $a :shared = 1;
+ my $thr1 = threads->create(\&sub1);
+ my $thr2 = threads->create(\&sub2);
$thr1->join;
$thr2->join;
- print "$a\n";
+ print("$a\n");
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 $a, once
+What do you think C<$a> will be? The answer, unfortunately, is I<it
+depends>. Both C<sub1()> and C<sub2()> access the global variable C<$a>, once
to read and once to write. Depending on factors ranging from your
thread implementation's scheduling algorithm to the phase of the moon,
-$a can be 2 or 3.
+C<$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
and the time you update it. Even this simple code fragment has the
possibility of error:
- use threads;
- my $a : shared = 2;
- my $b : shared;
- my $c : shared;
- my $thr1 = threads->create(sub { $b = $a; $a = $b + 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 $c could be 2 or 3.
+Two threads both access C<$a>. Each thread can potentially be interrupted
+at any point, or be executed in any order. At the end, C<$a> could be 3
+or 4, and both C<$b> and C<$c> could be 2 or 3.
Even C<$a += 5> or C<$a++> are not guaranteed to be atomic.
=head2 Controlling access: lock()
-The lock() function takes a shared variable and puts a lock on it.
+The C<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
+when the locking thread exits the block that contains the call to the
+C<lock()> function. Using C<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;
+ 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;
- }
+ while (1) {
+ 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 outermost block containing the
+ my $thr1 = threads->create(\&calc);
+ my $thr2 = threads->create(\&calc);
+ my $thr3 = threads->create(\&calc);
+ $thr1->join();
+ $thr2->join();
+ $thr3->join();
+ print("total=$total\n");
+
+C<lock()> blocks the thread until the variable being locked is
+available. When C<lock()> returns, your thread can be sure that no other
+thread can lock that variable until the 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.
+locking that C<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
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. For example:
+C<lock()> on the variable goes out of scope. For example:
- my $x : shared;
+ my $x :shared;
doit();
sub doit {
- {
- {
- lock($x); # wait for lock
- lock($x); # NOOP - we already have the lock
- {
- lock($x); # NOOP
- {
- lock($x); # NOOP
- lockit_some_more();
- }
- }
- } # *** implicit unlock here ***
- }
+ {
+ {
+ lock($x); # Wait for lock
+ lock($x); # NOOP - we already have the lock
+ {
+ lock($x); # NOOP
+ {
+ lock($x); # NOOP
+ lockit_some_more();
+ }
+ }
+ } # *** Implicit unlock here ***
+ }
}
sub lockit_some_more {
- lock($x); # NOOP
- } # nothing happens here
+ lock($x); # NOOP
+ } # Nothing happens here
-Note that there is no unlock() function - the only way to unlock a
-variable is to allow it to go out of scope.
+Note that there is no C<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
without their dangers, especially when multiple locks are involved.
Consider the following code:
- use threads;
-
- my $a : shared = 4;
- my $b : shared = "foo";
- my $thr1 = threads->new(sub {
- lock($a);
- threads->yield;
- sleep 20;
- lock($b);
- });
- my $thr2 = threads->new(sub {
- lock($b);
- threads->yield;
- sleep 20;
- lock($a);
+ use threads;
+
+ my $a :shared = 4;
+ my $b :shared = 'foo';
+ my $thr1 = threads->create(sub {
+ lock($a);
+ sleep(20);
+ lock($b);
+ });
+ my $thr2 = threads->create(sub {
+ lock($b);
+ sleep(20);
+ lock($a);
});
This program will probably hang until you kill it. The only way it
first. A guaranteed-to-hang version is more complicated, but the
principle is the same.
-The first thread will grab a lock on $a, then, after a pause during which
+The first thread will grab a lock on C<$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 $a. The second lock attempt for both threads will
+lock on C<$b>. Meanwhile, the second thread grabs a lock on C<$b>, then later
+tries to grab a lock on C<$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
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 $c. It's also best to hold on to locks for
+order. If, for example, you lock variables C<$a>, C<$b>, and C<$c>, always lock
+C<$a> before C<$b>, and C<$b> before C<$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
synchronization issues. They're pretty straightforward, and look like
this:
- use threads;
+ use threads;
use Thread::Queue;
- 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;
+ my $DataQueue = Thread::Queue->new();
+ my $thr = threads->create(sub {
+ while (my $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;
+ $thr->join();
-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
+You create the queue with C<Thread::Queue-E<gt>new()>. Then you can
+add lists of scalars onto the end with C<enqueue()>, and pop scalars off
+the front of it with C<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
+If a queue is empty, C<dequeue()> blocks until another thread enqueues
something. This makes queues ideal for event loops and other
communications between threads.
=head2 Semaphores: Synchronizing Data Access
Semaphores are a kind of generic locking mechanism. In their most basic
-form, they behave very much like lockable scalars, except that thay
+form, they behave very much like lockable scalars, except that they
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.
+threads to have the I<lock> at any one time.
=head2 Basic semaphores
-Semaphores have two methods, down() and up(): down() decrements the resource
-count, while up increments it. Calls to down() will block if the
+Semaphores have two methods, C<down()> and C<up()>: C<down()> decrements the resource
+count, while up() increments it. Calls to C<down()> will block if the
semaphore's current count would decrement below zero. This program
gives a quick demonstration:
- use threads qw(yield);
- use Thread::Semaphore;
-
- my $semaphore = new Thread::Semaphore;
- my $GlobalVariable : shared = 0;
-
- $thr1 = new threads \&sample_sub, 1;
- $thr2 = new threads \&sample_sub, 2;
- $thr3 = new threads \&sample_sub, 3;
-
- 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;
- }
+ use threads;
+ use Thread::Semaphore;
+
+ my $semaphore = Thread::Semaphore->new();
+ my $GlobalVariable :shared = 0;
+
+ $thr1 = threads->create(\&sample_sub, 1);
+ $thr2 = threads->create(\&sample_sub, 2);
+ $thr3 = threads->create(\&sample_sub, 3);
+
+ 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");
+ sleep(2);
+ $LocalCopy++;
+ $GlobalVariable = $LocalCopy;
+ $semaphore->up();
+ }
}
- $thr1->join;
- $thr2->join;
- $thr3->join;
+ $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
=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.
+C<down()> them at a time. However, there are other uses for semaphores.
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
+created with the counter set to one, C<down()> decrements the counter by
+one, and C<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);
+ my $thr1 = threads->create(\&sub1);
+ my $thr2 = threads->create(\&sub1);
sub sub1 {
$semaphore->down(5); # Decrements the counter by five
$semaphore->up(5); # Increment the counter by five
}
- $thr1->detach;
- $thr2->detach;
+ $thr1->detach();
+ $thr2->detach();
-If down() attempts to decrement the counter below zero, it blocks until
+If C<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).
+with a starting count of zero, any C<up()> or C<down()> always changes the
+counter by at least one, and so C<$semaphore->down(0)> is the same as
+C<$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
+decrement or 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.
for most purposes, queues are simpler to use and more intuitive. See
L<threads::shared> for more details.
+=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. 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 C<yield()> function that does
+this. C<yield()> is pretty straightforward, and works like this:
+
+ 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 $thr1 = threads->create(\&loop, 'first');
+ my $thr2 = threads->create(\&loop, 'second');
+ my $thr3 = threads->create(\&loop, 'third');
+
+It is important to remember that C<yield()> is only a hint to give up the CPU,
+it depends on your hardware, OS and threading libraries what actually happens.
+B<On many operating systems, yield() is a no-op.> Therefore it is important
+to note that one should not build the scheduling of the threads around
+C<yield()> calls. It might work on your platform but it won't work on another
+platform.
+
=head1 General Thread Utility Routines
We've covered the workhorse parts of Perl's threading package, and
=head2 What Thread Am I In?
-The C<< threads->self >> class method provides your program with a way to
+The C<threads-E<gt>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
-tid() is a thread object method that returns the thread ID of the
+C<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
+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.
+thread that's created. When used as a class method, C<threads-E<gt>tid()>
+can be used by a thread to get its own TID.
=head2 Are These Threads The Same?
-The equal() method takes two thread objects and returns true
+The C<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
+Thread objects also have an overloaded C<==> comparison so that you can do
comparison on them as you would with normal objects.
=head2 What Threads Are Running?
-C<< threads->list >> returns a list of thread objects, one for each thread
+C<threads-E<gt>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 $thr (threads->list) {
- # Don't join the main thread or ourselves
- if ($thr->tid && !threads::equal($thr, threads->self)) {
- $thr->join;
- }
+including cleaning up at the end of your program (from the main Perl thread,
+of course):
+
+ # Loop through all the threads
+ foreach my $thr (threads->list()) {
+ $thr->join();
}
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
+to clean up itself while other threads are running.
+
+NOTE: The main Perl thread (thread 0) is in a I<detached> state, and so
+does not appear in the list returned by C<threads-E<gt>list()>.
=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 threads;
- 7 use Thread::Queue;
- 8
- 9 my $stream = new Thread::Queue;
- 10 my $kid = new threads(\&check_num, $stream, 2);
- 11
- 12 for my $i ( 3 .. 1000 ) {
- 13 $stream->enqueue($i);
- 14 }
+ 1 #!/usr/bin/perl
+ 2 # prime-pthread, courtesy of Tom Christiansen
+ 3
+ 4 use strict;
+ 5 use warnings;
+ 6
+ 7 use threads;
+ 8 use Thread::Queue;
+ 9
+ 10 my $stream = Thread::Queue->new();
+ 11 for my $i ( 3 .. 1000 ) {
+ 12 $stream->enqueue($i);
+ 13 }
+ 14 $stream->enqueue(undef);
15
- 16 $stream->enqueue(undef);
- 17 $kid->join;
+ 16 threads->create(\&check_num, $stream, 2);
+ 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;
+ 22 my $downstream = Thread::Queue->new();
+ 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 threads(\&check_num, $downstream, $num);
+ 26 $downstream->enqueue($num);
+ 27 } else {
+ 28 print("Found prime $num\n");
+ 29 $kid = threads->create(\&check_num, $downstream, $num);
30 }
- 31 }
- 32 $downstream->enqueue(undef) if $kid;
- 33 $kid->join if $kid;
- 34 }
+ 31 }
+ 32 if ($kid) {
+ 33 $downstream->enqueue(undef);
+ 34 $kid->join();
+ 35 }
+ 36 }
This program uses the pipeline model to generate prime numbers. Each
thread in the pipeline has an input queue that feeds numbers to be
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)
+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
+The bulk of the work is done by the C<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)
+the subroutine is 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
+for. Line 24 checks to see if there's a remainder when we divide the
+number to be checked by 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
+Finally, once the loop terminates (because we got a 0 or C<undef> in the
+queue, which serves as a note to terminate), we pass on the notice to our
child and wait for it to exit if we've created a child (lines 32 and
-37).
+35).
-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.
+Meanwhile, back in the main thread, we first create a queue (line 10) and
+queue up all the numbers from 3 to 1000 for checking (lines 11-13),
+plus a termination notice (line 14). Then we create the initial child
+threads (line 16), passing it the queue and the first prime: 2. Finally,
+we wait for the first child thread to terminate (line 17). Because a
+child won't terminate until its child has terminated, we know that we're
+done once we return from the C<join()>.
That's how it works. It's pretty simple; as with many Perl programs,
the explanation is much longer than the program.
+=head1 Different implementations of threads
+
+Some background on thread implementations from the operating system
+viewpoint. 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 C<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 = $a + 2> can behave
+unpredictably with kernel threads if C<$a> is visible to other
+threads, as another thread may have changed C<$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.)
+
+Most modern operating systems support preemptive multitasking nowadays.
+
=head1 Performance considerations
-The main thing to bear in mind when comparing ithreads to other threading
+The main thing to bear in mind when comparing Perl's I<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
+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
+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.
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 using C<chdir()>. One thread calls C<chdir()>, and the working
directory of all the threads changes.
-Even more drastic example of a process-scope change is chroot():
+Even more drastic example of a process-scope change is C<chroot()>:
the root directory of all the threads changes, and no thread can
-undo it (as opposed to chdir()).
+undo it (as opposed to C<chdir()>).
-Further examples of process-scope changes include umask() and
-changing uids/gids.
+Further examples of process-scope changes include C<umask()> and
+changing uids and 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.)
+Thinking of mixing C<fork()> and threads? Please lie down and wait
+until the feeling passes. Be aware that the semantics of C<fork()> vary
+between platforms. For example, some UNIX systems copy all the current
+threads into the child process, while others only copy the thread that
+called C<fork()>. You have been warned!
-Similarly, mixing signals and threads should not be attempted.
+Similarly, mixing signals and threads may be problematic.
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).
+give you the full POSIX API). For example, there is no way to
+guarantee that a signal sent to a multi-threaded Perl application
+will get intercepted by any particular thread. (However, a recently
+added feature does provide the capability to send signals between
+threads. See L<threads/"THREAD SIGNALLING> for more details.)
=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
+C<localtime()>, C<gmtime()>, C<get{gr,host,net,proto,serv,pw}*()>, C<readdir()>,
+C<rand()>, and C<srand()> -- in general, calls that depend on some global
external state.
If the system Perl is compiled in has thread-safe variants of such
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
+C<PERL_REENTRANT_MAXSIZE> defined to the maximum number of bytes you will
allow.
=head1 Conclusion
but with what we've covered in this introduction, you should be well
on your way to becoming a threaded Perl expert.
+=head1 SEE ALSO
+
+Annotated POD for L<threads>:
+L<http://annocpan.org/?mode=search&field=Module&name=threads>
+
+Lastest version of L<threads> on CPAN:
+L<http://search.cpan.org/search?module=threads>
+
+Annotated POD for L<threads::shared>:
+L<http://annocpan.org/?mode=search&field=Module&name=threads%3A%3Ashared>
+
+Lastest version of L<threads::shared> on CPAN:
+L<http://search.cpan.org/search?module=threads%3A%3Ashared>
+
+Perl threads mailing list:
+L<http://lists.cpan.org/showlist.cgi?name=iThreads>
+
=head1 Bibliography
Here's a short bibliography courtesy of Jürgen Christoffel:
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.
+about thread-safety of Perl code.
+
+Rearranged slightly by Elizabeth Mattijsen E<lt>liz@dijkmat.nl<gt> to put
+less emphasis on yield().
=head1 Copyrights
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>.
-
+=cut
https://perl5.git.perl.org / perl5.git / blobdiff