PERLTHRTUT(1) Perl Programmers Reference Guide PERLTHRTUT(1)
NAME
perlthrtut - Tutorial on threads in Perl
DESCRIPTION
This tutorial describes the use of Perl interpreter threads (sometimes
referred to as ithreads). 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 ithreads uses the threads
class.
NOTE: There was another older Perl threading flavor called the 5.005
model that used the threads class. This old model was known to have
problems, is deprecated, and was removed for 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
"perl -V" and looking at the "Platform" section. If you have
"useithreads=define" you have ithreads, if you have
"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 threads and 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.
What Is A Thread Anyway?
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 5.8, though, you can create extra threads. We're going
to show you how, when, and why.
Threaded Program Models
There are three basic ways that you can structure a threaded program.
Which model you choose depends on what you need your program to do.
For many non-trivial threaded programs, you'll need to choose different
models for different pieces of your program.
Boss/Worker
The boss/worker model usually has one boss thread and one or more
worker threads. The boss thread gathers or generates tasks that need
to be done, then parcels those tasks out to the appropriate worker
thread.
This model is common in GUI and server programs, where a main thread
waits for some event and then passes that event to the appropriate
worker threads for processing. Once the event has been passed on, the
boss thread goes back to waiting for another event.
The boss thread does relatively little work. While tasks aren't
necessarily performed faster than with any other method, it tends to
have the best user-response times.
Work Crew
In the work crew model, several threads are created that do essentially
the same thing to different pieces of data. It closely mirrors
classical parallel processing and vector processors, where a large
array of processors do the exact same thing to many pieces of data.
This model is particularly useful if the system running the program
will distribute multiple threads across different processors. It can
also be useful in ray tracing or rendering engines, where the
individual threads can pass on interim results to give the user visual
feedback.
Pipeline
The pipeline model divides up a task into a series of steps, and passes
the results of one step on to the thread processing the next. Each
thread does one thing to each piece of data and passes the results to
the next thread in line.
This model makes the most sense if you have multiple processors so two
or more threads will be executing in parallel, though it can often make
sense in other contexts as well. It tends to keep the individual tasks
small and simple, as well as allowing some parts of the pipeline to
block (on I/O or system calls, for example) while other parts keep
going. If you're running different parts of the pipeline on different
processors you may also take advantage of the caches on each processor.
This model is also handy for a form of recursive programming where,
rather than having a subroutine call itself, it instead creates another
thread. Prime and Fibonacci generators both map well to this form of
the pipeline model. (A version of a prime number generator is presented
later on.)
What kind of threads are Perl threads?
If you have experience with other thread implementations, you might
find that things aren't quite what you expect. It's very important to
remember when dealing with Perl threads that Perl Threads Are Not X
Threads for all values of X. They aren't POSIX threads, or DecThreads,
or Java's Green threads, or Win32 threads. There are similarities, and
the broad concepts are the same, but if you start looking for
implementation details you're going to be either disappointed or
confused. Possibly both.
This is not to say that Perl threads are completely different from
everything that's ever come before. They're not. Perl's threading
model owes a lot to other thread models, especially POSIX. Just as
Perl is not C, though, Perl threads are not POSIX threads. So if you
find yourself looking for mutexes, or thread priorities, it's time to
step back a bit and think about what you want to do and how Perl can do
it.
However, it is important to remember that Perl threads cannot magically
do things unless your operating system's threads allow it. So if your
system blocks the entire process on "sleep()", Perl usually will, as
well.
Perl Threads Are Different.
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 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 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 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 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 "Basic semaphores".
See also "Thread-Safety of System Libraries".
Thread Basics
The 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 threads module that
make threaded programming easier.
Basic Thread Support
Thread support is a Perl compile-time option. It's something that's
turned on or off when Perl is built at your site, rather than when your
programs are compiled. If your Perl wasn't compiled with thread support
enabled, then any attempt to use threads will fail.
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:
use Config;
$Config{useithreads} or
die('Recompile Perl with threads to run this program.');
A possibly-threaded program using a possibly-threaded module might have
code like this:
use Config;
use MyMod;
BEGIN {
if ($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.
A Note about the Examples
In a real situation, care should be taken that all threads are finished
executing before the program exits. That care has 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.
Creating Threads
The 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;
"use threads;" imports all the pieces you need to create basic threads.
The simplest, most straightforward way to create a thread is with
"create()":
use threads;
my $thr = threads->create(\&sub1);
sub sub1 {
print("In the thread\n");
}
The "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 "threads->create()" call, like this:
use threads;
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 $thr3 = threads->create(\&sub1, qw(Param1 Param2 Param3));
sub sub1 {
my @InboundParameters = @_;
print("In the thread\n");
print('Got parameters >', join('<>',@InboundParameters), "<\n");
}
The last example illustrates another feature of threads. You can spawn
off several threads using the same subroutine. Each thread executes
the same subroutine, but in a separate thread with a separate
environment and potentially separate arguments.
"new()" is a synonym for "create()".
Waiting For A Thread To Exit
Since threads are also subroutines, they can return values. To wait
for a thread to exit and extract any values it might return, you can
use the "join()" method:
use threads;
my ($thr) = threads->create(\&sub1);
my @ReturnData = $thr->join();
print('Thread returned ', join(', ', @ReturnData), "\n");
sub sub1 { return ('Fifty-six', 'foo', 2); }
In the example above, the "join()" method returns as soon as the thread
ends. In addition to waiting for a thread to finish and gathering up
any values that the thread might have returned, "join()" also performs
any OS cleanup necessary for the thread. That cleanup might be
important, especially for long-running programs that spawn lots of
threads. If you don't want the return values and don't want to wait
for the thread to finish, you should call the "detach()" method
instead, as described next.
NOTE: In the example above, the thread returns a list, thus
necessitating that the thread creation call be made in list context
(i.e., "my ($thr)"). See "$thr->join()" in threads and "THREAD
CONTEXT" in threads for more details on thread context and return
values.
Ignoring A Thread
"join()" does three things: it waits for a thread to exit, cleans up
after it, and returns any data the thread may have produced. But what
if you're not interested in the thread's return values, and you don't
really care when the thread finishes? All you want is for the thread to
get cleaned up after when it's done.
In this case, you use the "detach()" method. Once a thread is
detached, it'll run until it's finished; then Perl will clean up after
it automatically.
use threads;
my $thr = threads->create(\&sub1); # Spawn the thread
$thr->detach(); # Now we officially don't care any more
sleep(15); # Let thread run for awhile
sub sub1 {
my $count = 0;
while (1) {
$count++;
print("\$count is $count\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.
"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
}
Process and Thread Termination
With threads one must be careful to make sure they all have a chance to
run to completion, assuming that is what you want.
An action that terminates a process will terminate all running threads.
die() and exit() have this property, and perl does an exit when the
main thread exits, perhaps implicitly by falling off the end of your
code, even if that's not what you want.
As an example of this case, this code prints the message "Perl exited
with active threads: 2 running and unjoined":
use threads;
my $thr1 = threads->new(\&thrsub, "test1");
my $thr2 = threads->new(\&thrsub, "test2");
sub thrsub {
my ($message) = @_;
sleep 1;
print "thread $message\n";
}
But when the following lines are added at the end:
$thr1->join();
$thr2->join();
it prints two lines of output, a perhaps more useful outcome.
Threads And Data
Now that we've covered the basics of threads, it's time for our next
topic: Data. Threading introduces a couple of complications to data
access that non-threaded programs never need to worry about.
Shared And Unshared Data
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
threads::shared module and the ":shared" attribute:
use threads;
use threads::shared;
my $foo :shared = 1;
my $bar = 1;
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
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;
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})
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.
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 threads;
use threads::shared;
my $x :shared = 1;
my $thr1 = threads->create(\&sub1);
my $thr2 = threads->create(\&sub2);
$thr1->join();
$thr2->join();
print("$x\n");
sub sub1 { my $foo = $x; $x = $foo + 1; }
sub sub2 { my $bar = $x; $x = $bar + 1; }
What do you think $x will be? The answer, unfortunately, is it depends.
Both "sub1()" and "sub2()" access the global variable $x, once to read
and once to write. Depending on factors ranging from your thread
implementation's scheduling algorithm to the phase of the moon, $x 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 threads;
my $x :shared = 2;
my $y :shared;
my $z :shared;
my $thr1 = threads->create(sub { $y = $x; $x = $y + 1; });
my $thr2 = threads->create(sub { $z = $x; $x = $z + 1; });
$thr1->join();
$thr2->join();
Two threads both access $x. Each thread can potentially be interrupted
at any point, or be executed in any order. At the end, $x could be 3
or 4, and both $y and $z could be 2 or 3.
Even "$x += 5" or "$x++" 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
inconsistency and race conditions. Note that Perl will protect its
internals from your race conditions, but it won't protect you from you.
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 "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.
Controlling access: lock()
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 block that contains the call to the "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 {
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->create(\&calc);
my $thr2 = threads->create(\&calc);
my $thr3 = threads->create(\&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 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.
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.
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:
my $x :shared;
doit();
sub doit {
{
{
lock($x); # Wait for lock
lock($x); # NOOP - we already have the lock
{
lock($x); # NOOP
{
lock($x); # NOOP
lockit_some_more();
}
}
} # *** Implicit unlock here ***
}
}
sub lockit_some_more {
lock($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.
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.
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, especially when multiple locks are involved.
Consider the following code:
use threads;
my $x :shared = 4;
my $y :shared = 'foo';
my $thr1 = threads->create(sub {
lock($x);
sleep(20);
lock($y);
});
my $thr2 = threads->create(sub {
lock($y);
sleep(20);
lock($x);
});
This program will probably hang until you kill it. The only way it
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 will grab a lock on $x, then, after a pause during
which the second thread has probably had time to do some work, try to
grab a lock on $y. Meanwhile, the second thread grabs a lock on $y,
then later tries to grab a lock on $x. The second lock attempt for
both threads will block, each waiting for the other to release its
lock.
This condition is called a deadlock, and it occurs whenever two or more
threads are trying to get locks on resources that the others own. Each
thread will block, waiting for the other to release a lock on a
resource. That never happens, though, since the thread with the
resource is itself waiting for a lock to be released.
There are a number of ways to handle this sort of problem. The best
way is to always have all threads acquire locks in the exact same
order. If, for example, you lock variables $x, $y, and $z, always lock
$x before $y, and $y before $z. 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.
Queues: Passing Data Around
A queue is a special thread-safe object that lets you put data in one
end and take it out the other without having to worry about
synchronization issues. They're pretty straightforward, and look like
this:
use threads;
use Thread::Queue;
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");
sleep(10);
$DataQueue->enqueue(undef);
$thr->join();
You create the queue with "Thread::Queue->new()". 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.
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 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.
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 semaphore's current count would decrement below zero.
This program gives a quick demonstration:
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();
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.
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. 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
my $thr1 = threads->create(\&sub1);
my $thr2 = threads->create(\&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
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 example, let's take a GUI driven program. It has a semaphore that
it uses to synchronize access to the display, so only one thread is
ever drawing at once. Handy, but of course you don't want any thread
to start drawing until things are properly set up. In this case, you
can create a semaphore with a counter set to zero, and up it when
things are ready for drawing.
Semaphores with counters greater than one are also useful for
establishing quotas. Say, for example, that you have a number of
threads that can do I/O at once. You don't want all the threads
reading or writing at once though, since that can potentially swamp
your I/O channels, or deplete your process's quota of filehandles. You
can use a semaphore initialized to the number of concurrent I/O
requests (or open files) that you want at any one time, and have your
threads quietly block and unblock themselves.
Larger increments or decrements are handy in those cases where a thread
needs to check out or return a number of resources at once.
Waiting for a Condition
The functions "cond_wait()" and "cond_signal()" 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 "pthreads". However for most purposes, queues are simpler to
use and more intuitive. See threads::shared for more details.
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 "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 $thr1 = threads->create(\&loop, 'first');
my $thr2 = threads->create(\&loop, 'second');
my $thr3 = threads->create(\&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. 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 "yield()" calls. It might work on your
platform but it won't work on another platform.
General Thread Utility Routines
We've covered the workhorse parts of Perl's threading package, and with
these tools you should be well on your way to writing threaded code and
packages. There are a few useful little pieces that didn't really fit
in anyplace else.
What Thread Am I In?
The "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.
Thread IDs
"tid()" is a thread object method that returns the thread ID of the
thread the object represents. Thread IDs are integers, with the main
thread in a program being 0. Currently Perl assigns a unique TID to
every thread ever created in your program, assigning the first thread
to be created a TID of 1, and increasing the TID by 1 for each new
thread that's created. When used as a class method, "threads->tid()"
can be used by a thread to get its own TID.
Are These Threads The Same?
The "equal()" method takes two thread objects and returns true if the
objects represent the same thread, and false if they don't.
Thread objects also have an overloaded "==" comparison so that you can
do comparison on them as you would with normal objects.
What Threads Are Running?
"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 (from the main
Perl thread, of course):
# Loop through all the threads
foreach my $thr (threads->list()) {
$thr->join();
}
If some threads have not finished running when the main Perl thread
ends, Perl will warn you about it and die, since it is impossible for
Perl to clean up itself while other threads are running.
NOTE: The main Perl thread (thread 0) is in a detached state, and so
does not appear in the list returned by "threads->list()".
A Complete Example
Confused yet? It's time for an example program to show some of the
things we've covered. This program finds prime numbers using threads.
1 #!/usr/bin/perl
2 # prime-pthread, courtesy of Tom Christiansen
3
4 use strict;
5 use warnings;
6
7 use threads;
8 use Thread::Queue;
9
10 sub check_num {
11 my ($upstream, $cur_prime) = @_;
12 my $kid;
13 my $downstream = Thread::Queue->new();
14 while (my $num = $upstream->dequeue()) {
15 next unless ($num % $cur_prime);
16 if ($kid) {
17 $downstream->enqueue($num);
18 } else {
19 print("Found prime: $num\n");
20 $kid = threads->create(\&check_num, $downstream, $num);
21 if (! $kid) {
22 warn("Sorry. Ran out of threads.\n");
23 last;
24 }
25 }
26 }
27 if ($kid) {
28 $downstream->enqueue(undef);
29 $kid->join();
30 }
31 }
32
33 my $stream = Thread::Queue->new(3..1000, undef);
34 check_num($stream, 2);
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
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 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.)
The bulk of the work is done by the "check_num()" subroutine, which
takes a reference to its input queue and a prime number that it's
responsible for. After pulling in the input queue and the prime that
the subroutine is checking (line 11), we create a new queue (line 13)
and reserve a scalar for the thread that we're likely to create later
(line 12).
The while loop from line 14 to line 26 grabs a scalar off the input
queue and checks against the prime this thread is responsible for.
Line 15 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 17) or create a new thread if we
haven't.
The new thread creation is line 20. We pass on to it a reference to
the queue we've created, and the prime number we've found. In lines 21
through 24, we check to make sure that our new thread got created, and
if not, we stop checking any remaining numbers in the queue.
Finally, once the loop terminates (because we got a 0 or "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 27
and 30).
Meanwhile, back in the main thread, we first create a queue (line 33)
and queue up all the numbers from 3 to 1000 for checking, plus a
termination notice. Then all we have to do to get the ball rolling is
pass the queue and the first prime to the "check_num()" subroutine
(line 34).
That's how it works. It's pretty simple; as with many Perl programs,
the explanation is much longer than the program.
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 "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 "$x = $x + 2" can behave
unpredictably with kernel threads if $x is visible to other threads, as
another thread may have changed $x 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.
Performance considerations
The main thing to bear in mind when comparing Perl's 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.
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 and gids.
Thinking of mixing "fork()" and threads? Please lie down and wait
until the feeling passes. Be aware that the semantics of "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 "fork()". You have been warned!
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). 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 "THREAD SIGNALLING" in
threads for more details.)
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()", functions fetching user, group and network
information (such as "getgrent()", "gethostent()", "getnetent()" and so
on), "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.
Conclusion
A complete thread tutorial could fill a book (and has, many times), but
with what we've covered in this introduction, you should be well on
your way to becoming a threaded Perl expert.
SEE ALSO
Annotated POD for threads:
<http://annocpan.org/?mode=search&field=Module&name=threads>;
Latest version of threads on CPAN:
<http://search.cpan.org/search?module=threads>;
Annotated POD for threads::shared:
<http://annocpan.org/?mode=search&field=Module&name=threads%3A%3Ashared>
Latest version of threads::shared on CPAN:
<http://search.cpan.org/search?module=threads%3A%3Ashared>
Perl threads mailing list: <http://lists.perl.org/list/ithreads.html>;
Bibliography
Here's a short bibliography courtesy of Juergen Christoffel:
Introductory Texts
Birrell, Andrew D. An Introduction to Programming with Threads. Digital
Equipment Corporation, 1989, DEC-SRC Research Report #35 online as
<ftp://ftp.dec.com/pub/DEC/SRC/research-reports/SRC-035.pdf>; (highly
recommended)
Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A
Guide to Concurrency, Communication, and Multithreading. Prentice-Hall,
1996.
Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with
Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written
introduction to threads).
Nelson, Greg (editor). Systems Programming with Modula-3. Prentice
Hall, 1991, ISBN 0-13-590464-1.
Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell.
Pthreads Programming. O'Reilly & Associates, 1996, ISBN 156592-115-1
(covers POSIX threads).
OS-Related References
Boykin, Joseph, David Kirschen, Alan Langerman, and Susan LoVerso.
Programming under Mach. Addison-Wesley, 1994, ISBN 0-201-52739-1.
Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall,
1995, ISBN 0-13-219908-4 (great textbook).
Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts,
4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4
Other References
Arnold, Ken and James Gosling. The Java Programming Language, 2nd ed.
Addison-Wesley, 1998, ISBN 0-201-31006-6.
comp.programming.threads FAQ,
<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,
<http://www.perl.com/pub/a/2002/06/11/threads.html>;
Acknowledgements
Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy
Sarathy, Ilya Zakharevich, Benjamin Sugars, Juergen Christoffel, Joshua
Pritikin, and Alan Burlison, for their help in reality-checking and
polishing this article. Big thanks to Tom Christiansen for his rewrite
of the prime number generator.
AUTHOR
Dan Sugalski <dan AT sidhe.org<gt>
Slightly modified by Arthur Bergman to fit the new thread model/module.
Reworked slightly by Joerg Walter <jwalt AT cpan.org<gt> to be more
concise about thread-safety of Perl code.
Rearranged slightly by Elizabeth Mattijsen <liz AT dijkmat.nl<gt> to put
less emphasis on yield().
Copyrights
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.
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