=head1 DESCRIPTION
B<NOTE>: this tutorial describes the new Perl threading flavour
-introduced in Perl 5.6.0 called interpreter threads, or ithreads
-for short. There is another older Perl threading flavour called
-the 5.005 model, unsurprisingly for 5.005 versions of Perl.
-The old model is 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.
+introduced in Perl 5.6.0 called interpreter threads, or B<ithreads>
+for short. In this model each thread runs in its own Perl interpreter,
+and any data sharing between threads must be explicit.
+
+There is another older Perl threading flavour called the 5.005 model,
+unsurprisingly for 5.005 versions of Perl. The old model is known to
+have problems, deprecated, and will probably be removed around release
+5.10. You are strongly encouraged to migrate any existing 5.005
+threads code to the new model as soon as possible.
You can see which (or neither) threading flavour you have by
running C<perl -V> and looking at the C<Platform> section.
do things unless your operating systems threads allows it. So if your
system blocks the entire process on sleep(), Perl usually will as well.
+Perl Threads Are Different.
+
=head1 Threadsafe Modules
The addition of threads has changed Perl's internals
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
+later in the article.
+
+See also L</"Threadsafety of System Libraries">.
=head1 Thread Basics
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.";
+ $Config{useithreads} or die "Recompile Perl with threads to run this program.";
A possibly-threaded program using a possibly-threaded module might
have code like this:
use Config;
use MyMod;
- if ($Config{useithreads}) {
- # We have threads
- require MyMod_threaded;
- import MyMod_threaded;
- } else {
- require MyMod_unthreaded;
- import MyMod_unthreaded;
+ 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
the same subroutine, but in a separate thread with a separate
environment and potentially separate arguments.
-C<create()> is a synonym for C<new()>
+C<create()> is a synonym for C<new()>.
=head2 Giving up control
this. yield() is pretty straightforward, and works like this:
use threads;
-
+
sub loop {
my $thread = shift;
my $foo = 50;
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
}
}
-
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.
my $foo : shared = 1;
my $bar = 1;
threads->new(sub { $foo++; $bar++ })->join;
-
+
print "$foo\n"; #prints 2 since $foo is shared
print "$bar\n"; #prints 1 since $bar is not shared
a shared hash, all the keys and values are shared. This places
restrictions on what may be assigned to shared array and hash elements: only
simple values or references to shared variables are allowed - this is
-so that a private variable can't accidently become shared. A bad
+so that a private variable can't accidentally become shared. A bad
assignment will cause the thread to die. For example:
use threads;
data inconsistency and race conditions. Note that Perl will protect its
internals from your race conditions, but it won't protect you from you.
-=head1 Synchonisation and control
+=head1 Synchronization and control
Perl provides a number of mechanisms to coordinate the interactions
between themselves and their data, to avoid race conditions and the like.
Some of these are designed to resemble the common techniques used in thread
libraries such as C<pthreads>; others are Perl-specific. Often, the
-standard techniques are clumsly and difficult to get right (such as
+standard techniques are clumsily and difficult to get right (such as
condition waits). Where possible, it is usually easier to use Perlish
techniques such as queues, which remove some of the hard work involved.
by the thread holding the lock. Unlocking happens automatically
when the locking thread exists the outermost block that contains
C<lock()> function. Using lock() is straightforward: this example has
-several threads doing some calculations in parallel, and occasionaly
+several threads doing some calculations in parallel, and occasionally
updating a running total:
use threads;
{
lock($total); # block until we obtain the lock
$total += $result
- } # lock implicity released at end of scope
+ } # lock implicitly released at end of scope
last if $result == 0;
}
}
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, espacially when multiple locks are involved.
+without their dangers, especially when multiple locks are involved.
Consider the following code:
use threads;
$a before $b, and $b before $c. It's also best to hold on to locks for
as short a period of time to minimize the risks of deadlock.
-The other syncronisation primitives described below can suffer from
+The other synchronization primitives described below can suffer from
similar problems.
=head2 Queues: Passing Data Around
Also note that under the current implementation, shared variables
use a little more memory and are a little slower than ordinary variables.
+=head1 Threadsafety of System Libraries
+
+Whether various library calls are threadsafe is outside the control
+of Perl. Calls often suffering from not being threadsafe include:
+localtime(), gmtime(), get{gr,host,net,proto,serv,pw}*(), readdir(),
+rand(), and srand() -- in general, calls that depend on some external
+state.
+
+If the system Perl is compiled in has threadsafe variants of such
+calls, they will be used. Beyond that, Perl is at the mercy of
+the threadsafety or unsafety of the calls. Please consult your
+C library call documentation.
+
+In some platforms the threadsafe interfaces may fail if the result
+buffer is too small (for example getgrent() may return quite large
+group member lists). Perl will retry growing the result buffer
+a few times, but only up to 64k (for safety reasons).
+
=head1 Conclusion
A complete thread tutorial could fill a book (and has, many times),
=head1 AUTHOR
-Dan Sugalski E<lt>sugalskd@ous.eduE<gt>
+Dan Sugalski E<lt>dan@sidhe.org<gt>
Slightly modified by Arthur Bergman to fit the new thread model/module.
projects / p5sagit/p5-mst-13.2.git / blobdiff