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perlothrtut - old tutorial on threads in Perl
WARNING:
This tutorial describes the old-style thread model that was introduced in
release 5.005. This model is now deprecated, and will be removed, probably
in version 5.10. The interfaces described here were considered
experimental, and are likely to be buggy.
For information about the new interpreter threads ("ithreads") model, see
the perlthrtut tutorial, and the the threads manpage and the threads::shared manpage
modules.
You are strongly encouraged to migrate any existing threads code to the
new model as soon as possible.
A thread is a flow of control through a program with a single
execution point.
Sounds an awful lot like a process, doesn't it? Well, it should.
Threads are one of the pieces of a process. Every process has at least
one thread and, up until now, every process running Perl had only one
thread. With 5.005, though, you can create extra threads. We're going
to show you how, when, and why.
There are three basic ways that you can structure a threaded
program. Which model you choose depends on what you need your program
to do. For many non-trivial threaded programs you'll need to choose
different models for different pieces of your program.
The boss/worker model usually has one `boss' thread and one or more
`worker' threads. The boss thread gathers or generates tasks that need
to be done, then parcels those tasks out to the appropriate worker
thread.
This model is common in GUI and server programs, where a main thread
waits for some event and then passes that event to the appropriate
worker threads for processing. Once the event has been passed on, the
boss thread goes back to waiting for another event.
The boss thread does relatively little work. While tasks aren't
necessarily performed faster than with any other method, it tends to
have the best user-response times.
In the work crew model, several threads are created that do
essentially the same thing to different pieces of data. It closely
mirrors classical parallel processing and vector processors, where a
large array of processors do the exact same thing to many pieces of
data.
This model is particularly useful if the system running the program
will distribute multiple threads across different processors. It can
also be useful in ray tracing or rendering engines, where the
individual threads can pass on interim results to give the user visual
feedback.
The pipeline model divides up a task into a series of steps, and
passes the results of one step on to the thread processing the
next. Each thread does one thing to each piece of data and passes the
results to the next thread in line.
This model makes the most sense if you have multiple processors so two
or more threads will be executing in parallel, though it can often
make sense in other contexts as well. It tends to keep the individual
tasks small and simple, as well as allowing some parts of the pipeline
to block (on I/O or system calls, for example) while other parts keep
going. If you're running different parts of the pipeline on different
processors you may also take advantage of the caches on each
processor.
This model is also handy for a form of recursive programming where,
rather than having a subroutine call itself, it instead creates
another thread. Prime and Fibonacci generators both map well to this
form of the pipeline model. (A version of a prime number generator is
presented later on.)
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 $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.)
If you have experience with other thread implementations, you might
find that things aren't quite what you expect. It's very important to
remember when dealing with Perl threads that Perl Threads Are Not X
Threads, for all values of X. They aren't POSIX threads, or
DecThreads, or Java's Green threads, or Win32 threads. There are
similarities, and the broad concepts are the same, but if you start
looking for implementation details you're going to be either
disappointed or confused. Possibly both.
This is not to say that Perl threads are completely different from
everything that's ever come before--they're not. Perl's threading
model owes a lot to other thread models, especially POSIX. Just as
Perl is not C, though, Perl threads are not POSIX threads. So if you
find yourself looking for mutexes, or thread priorities, it's time to
step back a bit and think about what you want to do and how Perl can
do it.
The addition of threads has changed Perl's internals
substantially. There are implications for people who write
modules--especially modules with XS code or external libraries. While
most modules won't encounter any problems, modules that aren't
explicitly tagged as thread-safe should be tested before being used in
production code.
Not all modules that you might use are thread-safe, and you should
always assume a module is unsafe unless the documentation says
otherwise. This includes modules that are distributed as part of the
core. Threads are a beta feature, and even some of the standard
modules aren't thread-safe.
If you're using a module that's not thread-safe for some reason, you
can protect yourself by using semaphores and lots of programming
discipline to control access to the module. Semaphores are covered
later in the article. Perl Threads Are Different
The core Thread module provides the basic functions you need to write
threaded programs. In the following sections we'll cover the basics,
showing you what you need to do to create a threaded program. After
that, we'll go over some of the features of the Thread module that
make threaded programming easier.
Thread support is a Perl compile-time option-it's something that's
turned on or off when Perl is built at your site, rather than when
your programs are compiled. If your Perl wasn't compiled with thread
support enabled, then any attempt to use threads will fail.
Remember that the threading support in 5.005 is in beta release, and
should be treated as such. You should expect that it may not function
entirely properly, and the thread interface may well change some
before it is a fully supported, production release. The beta version
shouldn't be used for mission-critical projects. Having said that,
threaded Perl is pretty nifty, and worth a look.
Your programs can use the Config module to check whether threads are
enabled. If your program can't run without them, you can say something
like:
$Config{usethreads} or die "Recompile Perl with threads to run this program.";
A possibly-threaded program using a possibly-threaded module might
have code like this:
use Config;
use MyMod;
if ($Config{usethreads}) {
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.
The Thread package provides the tools you need to create new
threads. Like any other module, you need to tell Perl you want to use
it; use Thread imports all the pieces you need to create basic
threads.
The simplest, straightforward way to create a thread is with new():
use Thread;
$thr = Thread->new( \&sub1 );
sub sub1 {
print "In the thread\n";
}
The new() method takes a reference to a subroutine and creates a new
thread, which starts executing in the referenced subroutine. Control
then passes both to the subroutine and the caller.
If you need to, your program can pass parameters to the subroutine as
part of the thread startup. Just include the list of parameters as
part of the Thread::new call, like this:
use Thread;
$Param3 = "foo";
$thr = Thread->new( \&sub1, "Param 1", "Param 2", $Param3 );
$thr = Thread->new( \&sub1, @ParamList );
$thr = Thread->new( \&sub1, qw(Param1 Param2 $Param3) );
sub sub1 {
my @InboundParameters = @_;
print "In the thread\n";
print "got parameters >", join("<>", @InboundParameters), "<\n";
}
The subroutine runs like a normal Perl subroutine, and the call to new
Thread returns whatever the subroutine returns.
The last example illustrates another feature of threads. You can spawn
off several threads using the same subroutine. Each thread executes
the same subroutine, but in a separate thread with a separate
environment and potentially separate arguments.
The other way to spawn a new thread is with async(), which is a way to
spin off a chunk of code like eval(), but into its own thread:
use Thread qw(async);
$LineCount = 0;
$thr = async {
while(<>) {$LineCount++}
print "Got $LineCount lines\n";
};
print "Waiting for the linecount to end\n";
$thr->join;
print "All done\n";
You'll notice we did a use Thread qw(async) in that example. async is
not exported by default, so if you want it, you'll either need to
import it before you use it or fully qualify it as
Thread::async. You'll also note that there's a semicolon after the
closing brace. That's because async() treats the following block as an
anonymous subroutine, so the semicolon is necessary.
Like eval(), the code executes in the same context as it would if it
weren't spun off. Since both the code inside and after the async start
executing, you need to be careful with any shared resources. Locking
and other synchronization techniques are covered later.
There are times when you may find it useful to have a thread
explicitly give up the CPU to another thread. Your threading package
might not support preemptive multitasking for threads, for example, or
you may be doing something compute-intensive and want to make sure
that the user-interface thread gets called frequently. Regardless,
there are times that you might want a thread to give up the processor.
Perl's threading package provides the yield() function that does
this. yield() is pretty straightforward, and works like this:
use Thread qw(yield async);
async {
my $foo = 50;
while ($foo--) { print "first async\n" }
yield;
$foo = 50;
while ($foo--) { print "first async\n" }
};
async {
my $foo = 50;
while ($foo--) { print "second async\n" }
yield;
$foo = 50;
while ($foo--) { print "second async\n" }
};
Since threads are also subroutines, they can return values. To wait
for a thread to exit and extract any scalars it might return, you can
use the join() method.
use Thread;
$thr = Thread->new( \&sub1 );
@ReturnData = $thr->join;
print "Thread returned @ReturnData";
sub sub1 { return "Fifty-six", "foo", 2; }
In the example above, the join() method returns as soon as the thread
ends. In addition to waiting for a thread to finish and gathering up
any values that the thread might have returned, join() also performs
any OS cleanup necessary for the thread. That cleanup might be
important, especially for long-running programs that spawn lots of
threads. If you don't want the return values and don't want to wait
for the thread to finish, you should call the detach() method
instead. detach() is covered later in the article.
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};
$foo = eval {$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.
join() does three things: it waits for a thread to exit, cleans up
after it, and returns any data the thread may have produced. But what
if you're not interested in the thread's return values, and you don't
really care when the thread finishes? All you want is for the thread
to get cleaned up after when it's done.
In this case, you use the detach() method. Once a thread is detached,
it'll run until it's finished, then Perl will clean up after it
automatically.
use Thread;
$thr = Thread->new( \&sub1 );
$thr->detach;
sub sub1 {
$a = 0;
while (1) {
$a++;
print "\$a is $a\n";
sleep 1;
}
}
Once a thread is detached, it may not be joined, and any output that
it might have produced (if it was done and waiting for a join) is
lost.
Now that we've covered the basics of threads, it's time for our next
topic: data. Threading introduces a couple of complications to data
access that non-threaded programs never need to worry about.
The single most important thing to remember when using threads is that
all threads potentially have access to all the data anywhere in your
program. While this is true with a nonthreaded Perl program as well,
it's especially important to remember with a threaded program, since
more than one thread can be accessing this data at once.
Perl's scoping rules don't change because you're using threads. If a
subroutine (or block, in the case of async()) could see a variable if
you weren't running with threads, it can see it if you are. This is
especially important for the subroutines that create, and makes my
variables even more important. Remember--if your variables aren't
lexically scoped (declared with my) you're probably sharing them
between threads.
While threads bring a new set of useful tools, they also bring a
number of pitfalls. One pitfall is the race condition:
use Thread;
$a = 1;
$thr1 = Thread->new(\&sub1);
$thr2 = Thread->new(\&sub2);
sleep 10;
print "$a\n";
sub sub1 { $foo = $a; $a = $foo + 1; }
sub sub2 { $bar = $a; $a = $bar + 1; }
What do you think $a will be? The answer, unfortunately, is "it
depends." Both sub1() and sub2() access the global variable $a, once
to read and once to write. Depending on factors ranging from your
thread implementation's scheduling algorithm to the phase of the moon,
$a can be 2 or 3.
Race conditions are caused by unsynchronized access to shared
data. Without explicit synchronization, there's no way to be sure that
nothing has happened to the shared data between the time you access it
and the time you update it. Even this simple code fragment has the
possibility of error:
use Thread qw(async);
$a = 2;
async{ $b = $a; $a = $b + 1; };
async{ $c = $a; $a = $c + 1; };
Two threads both access $a. Each thread can potentially be interrupted
at any point, or be executed in any order. At the end, $a could be 3
or 4, and both $b and $c could be 2 or 3.
Whenever your program accesses data or resources that can be accessed
by other threads, you must take steps to coordinate access or risk
data corruption and race conditions.
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);
$b = $a;
$a = $b * $foo;
}
print "\$foo was $foo\n";
};
$thr2 = async {
$bar = 7;
{
lock ($a);
$c = $a;
$a = $c * $bar;
}
print "\$bar was $bar\n";
};
$thr1->join;
$thr2->join;
print "\$a is $a\n";
lock() blocks the thread until the variable being locked is
available. When lock() returns, your thread can be sure that no other
thread can lock that variable until the innermost block containing the
lock exits.
It's important to note that locks don't prevent access to the variable
in question, only lock attempts. This is in keeping with Perl's
longstanding tradition of courteous programming, and the advisory file
locking that flock() gives you. Locked subroutines behave differently,
however. We'll cover that later in the article.
You may lock arrays and hashes as well as scalars. Locking an array,
though, will not block subsequent locks on array elements, just lock
attempts on the array itself.
Finally, locks are recursive, which means it's okay for a thread to
lock a variable more than once. The lock will last until the outermost
lock() on the variable goes out of scope.
Locks are a handy tool to synchronize access to data. Using them
properly is the key to safe shared data. Unfortunately, locks aren't
without their dangers. Consider the following code:
use | 
