perlothrtut.pod

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=head2 Attributes: Restricting Access To Subroutines

In addition to synchronizing access to data or resources, you might
find it useful to synchronize access to subroutines.  You may be
accessing a singular machine resource (perhaps a vector processor), or
find it easier to serialize calls to a particular subroutine than to
have a set of locks and semaphores.

One of the additions to Perl 5.005 is subroutine attributes.  The
Thread package uses these to provide several flavors of
serialization.  It's important to remember that these attributes are
used in the compilation phase of your program so you can't change a
subroutine's behavior while your program is actually running.

=head2 Subroutine Locks

The basic subroutine lock looks like this:

    sub test_sub :locked { 
    }

This ensures that only one thread will be executing this subroutine at
any one time.  Once a thread calls this subroutine, any other thread
that calls it will block until the thread in the subroutine exits
it.  A more elaborate example looks like this:

    use Thread qw(yield); 

    new Thread \&thread_sub, 1; 
    new Thread \&thread_sub, 2; 
    new Thread \&thread_sub, 3; 
    new Thread \&thread_sub, 4;

    sub sync_sub :locked { 
        my $CallingThread = shift @_; 
        print "In sync_sub for thread $CallingThread\n";
        yield; 
        sleep 3; 
        print "Leaving sync_sub for thread $CallingThread\n"; 
    }

    sub thread_sub { 
        my $ThreadID = shift @_; 
        print "Thread $ThreadID calling sync_sub\n";
        sync_sub($ThreadID); 
        print "$ThreadID is done with sync_sub\n"; 
    }

The C<locked> attribute tells perl to lock sync_sub(), and if you run
this, you can see that only one thread is in it at any one time.

=head2 Methods

Locking an entire subroutine can sometimes be overkill, especially
when dealing with Perl objects.  When calling a method for an object,
for example, you want to serialize calls to a method, so that only one
thread will be in the subroutine for a particular object, but threads
calling that subroutine for a different object aren't blocked.  The
method attribute indicates whether the subroutine is really a method.

    use Thread;

    sub tester { 
        my $thrnum = shift @_; 
        my $bar = Foo->new();
        foreach (1..10) { 	
            print "$thrnum calling per_object\n"; 
            $bar->per_object($thrnum); 	
            print "$thrnum out of per_object\n"; 
            yield; 
            print "$thrnum calling one_at_a_time\n";
            $bar->one_at_a_time($thrnum); 	
            print "$thrnum out of one_at_a_time\n"; 
            yield; 
        } 
    }

    foreach my $thrnum (1..10) { 
        new Thread \&tester, $thrnum; 
    }

    package Foo; 
    sub new { 
        my $class = shift @_; 
        return bless [@_], $class; 
    }

    sub per_object :locked :method { 
        my ($class, $thrnum) = @_; 
        print "In per_object for thread $thrnum\n"; 
        yield; 
        sleep 2; 
        print "Exiting per_object for thread $thrnum\n"; 
    }

    sub one_at_a_time :locked { 
        my ($class, $thrnum) = @_; 
        print "In one_at_a_time for thread $thrnum\n";     
        yield; 
        sleep 2; 
        print "Exiting one_at_a_time for thread $thrnum\n"; 
    }

As you can see from the output (omitted for brevity; it's 800 lines)
all the threads can be in per_object() simultaneously, but only one
thread is ever in one_at_a_time() at once.

=head2 Locking A Subroutine

You can lock a subroutine as you would lock a variable.  Subroutine locks
work the same as specifying a C<locked> attribute for the subroutine,
and block all access to the subroutine for other threads until the
lock goes out of scope.  When the subroutine isn't locked, any number
of threads can be in it at once, and getting a lock on a subroutine
doesn't affect threads already in the subroutine.  Getting a lock on a
subroutine looks like this:

    lock(\&sub_to_lock);

Simple enough.  Unlike the C<locked> attribute, which is a compile time
option, locking and unlocking a subroutine can be done at runtime at your
discretion.  There is some runtime penalty to using lock(\&sub) instead
of the C<locked> attribute, so make sure you're choosing the proper
method to do the locking.

You'd choose lock(\&sub) when writing modules and code to run on both
threaded and unthreaded Perl, especially for code that will run on
5.004 or earlier Perls.  In that case, it's useful to have subroutines
that should be serialized lock themselves if they're running threaded,
like so:

    package Foo; 
    use Config; 
    $Running_Threaded = 0;

    BEGIN { $Running_Threaded = $Config{'usethreads'} }

    sub sub1 { lock(\&sub1) if $Running_Threaded }


This way you can ensure single-threadedness regardless of which
version of Perl you're running.

=head1 General Thread Utility Routines

We've covered the workhorse parts of Perl's threading package, and
with these tools you should be well on your way to writing threaded
code and packages.  There are a few useful little pieces that didn't
really fit in anyplace else.

=head2 What Thread Am I In?

The Thread->self method provides your program with a way to get an
object representing the thread it's currently in.  You can use this
object in the same way as the ones returned from the thread creation.

=head2 Thread IDs

tid() is a thread object method that returns the thread ID of the
thread the object represents.  Thread IDs are integers, with the main
thread in a program being 0.  Currently Perl assigns a unique tid to
every thread ever created in your program, assigning the first thread
to be created a tid of 1, and increasing the tid by 1 for each new
thread that's created.

=head2 Are These Threads The Same?

The equal() method takes two thread objects and returns true 
if the objects represent the same thread, and false if they don't.

=head2 What Threads Are Running?

Thread->list returns a list of thread objects, one for each thread
that's currently running.  Handy for a number of things, including
cleaning up at the end of your program:

    # Loop through all the threads 
    foreach $thr (Thread->list) { 
        # Don't join the main thread or ourselves 
        if ($thr->tid && !Thread::equal($thr, Thread->self)) { 
            $thr->join; 
        } 
    }

The example above is just for illustration.  It isn't strictly
necessary to join all the threads you create, since Perl detaches all
the threads before it exits.

=head1 A Complete Example

Confused yet? It's time for an example program to show some of the
things we've covered.  This program finds prime numbers using threads.

    1  #!/usr/bin/perl -w
    2  # prime-pthread, courtesy of Tom Christiansen
    3
    4  use strict;
    5
    6  use Thread;
    7  use Thread::Queue;
    8
    9  my $stream = Thread::Queue->new();
    10 my $kid    = Thread->new(\&check_num, $stream, 2);
    11
    12 for my $i ( 3 .. 1000 ) {
    13     $stream->enqueue($i);
    14 } 
    15
    16 $stream->enqueue(undef);
    17 $kid->join();
    18
    19 sub check_num {
    20     my ($upstream, $cur_prime) = @_;
    21     my $kid;
    22     my $downstream = 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 = Thread->new(\&check_num, $downstream, $num);
    30         }
    31     } 
    32     $downstream->enqueue(undef) if $kid;
    33     $kid->join()		if $kid;
    34 }

This program uses the pipeline model to generate prime numbers.  Each
thread in the pipeline has an input queue that feeds numbers to be
checked, a prime number that it's responsible for, and an output queue
that it funnels numbers that have failed the check into.  If the thread
has a number that's failed its check and there's no child thread, then
the thread must have found a new prime number.  In that case, a new
child thread is created for that prime and stuck on the end of the
pipeline.

This probably sounds a bit more confusing than it really is, so lets
go through this program piece by piece and see what it does.  (For
those of you who might be trying to remember exactly what a prime
number is, it's a number that's only evenly divisible by itself and 1)

The bulk of the work is done by the check_num() subroutine, which
takes a reference to its input queue and a prime number that it's
responsible for.  After pulling in the input queue and the prime that
the subroutine's checking (line 20), we create a new queue (line 22)
and reserve a scalar for the thread that we're likely to create later
(line 21).

The while loop from lines 23 to line 31 grabs a scalar off the input
queue and checks against the prime this thread is responsible
for.  Line 24 checks to see if there's a remainder when we modulo the
number to be checked against our prime.  If there is one, the number
must not be evenly divisible by our prime, so we need to either pass
it on to the next thread if we've created one (line 26) or create a
new thread if we haven't.

The new thread creation is line 29.  We pass on to it a reference to
the queue we've created, and the prime number we've found.

Finally, once the loop terminates (because we got a 0 or undef in the
queue, which serves as a note to die), we pass on the notice to our
child and wait for it to exit if we've created a child (Lines 32 and
37).

Meanwhile, back in the main thread, we create a queue (line 9) and the
initial child thread (line 10), and pre-seed it with the first prime:
2.  Then we queue all the numbers from 3 to 1000 for checking (lines
12-14), then queue a die notice (line 16) and wait for the first child
thread to terminate (line 17).  Because a child won't die until its
child has died, we know that we're done once we return from the join.

That's how it works.  It's pretty simple; as with many Perl programs,
the explanation is much longer than the program.

=head1 Conclusion

A complete thread tutorial could fill a book (and has, many times),
but this should get you well on your way.  The final authority on how
Perl's threads behave is the documentation bundled with the Perl
distribution, but with what we've covered in this article, you should
be well on your way to becoming a threaded Perl expert.

=head1 Bibliography

Here's a short bibliography courtesy of J黵gen Christoffel:

=head2 Introductory Texts

Birrell, Andrew D. An Introduction to Programming with
Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report
#35 online as
http://www.research.digital.com/SRC/staff/birrell/bib.html (highly
recommended)

Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A
Guide to Concurrency, Communication, and
Multithreading. Prentice-Hall, 1996.

Lewis, Bill, and Daniel J. Berg. Multithreaded Programming with
Pthreads. Prentice Hall, 1997, ISBN 0-13-443698-9 (a well-written
introduction to threads).

Nelson, Greg (editor). Systems Programming with Modula-3.  Prentice
Hall, 1991, ISBN 0-13-590464-1.

Nichols, Bradford, Dick Buttlar, and Jacqueline Proulx Farrell.
Pthreads Programming. O'Reilly & Associates, 1996, ISBN 156592-115-1
(covers POSIX threads).

=head2 OS-Related References

Boykin, Joseph, David Kirschen, Alan Langerman, and Susan
LoVerso. Programming under Mach. Addison-Wesley, 1994, ISBN
0-201-52739-1.

Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall,
1995, ISBN 0-13-219908-4 (great textbook).

Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts,
4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4

=head2 Other References

Arnold, Ken and James Gosling. The Java Programming Language, 2nd
ed. Addison-Wesley, 1998, ISBN 0-201-31006-6.

Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage
Collection on Virtually Shared Memory Architectures" in Memory
Management: Proc. of the International Workshop IWMM 92, St. Malo,
France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer,
1992, ISBN 3540-55940-X (real-life thread applications).

=head1 Acknowledgements

Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy
Sarathy, Ilya Zakharevich, Benjamin Sugars, J黵gen Christoffel, Joshua
Pritikin, and Alan Burlison, for their help in reality-checking and
polishing this article.  Big thanks to Tom Christiansen for his rewrite
of the prime number generator.

=head1 AUTHOR

Dan Sugalski E<lt>[e-mail]<gt>

=head1 Copyrights

This article originally appeared in The Perl Journal #10, and is
copyright 1998 The Perl Journal. It appears courtesy of Jon Orwant and
The Perl Journal.  This document may be distributed under the same terms
as Perl itself.