forks(3) User Contributed Perl Documentation forks(3)NAMEforks - drop-in replacement for Perl threads using fork()VERSION
This documentation describes version 0.33.
SYNOPSIS
use forks; #ALWAYS LOAD AS FIRST MODULE, if possible
use warnings;
my $thread = threads->new( sub { # or ->create or async()
print "Hello world from a thread\n";
} );
$thread->join;
$thread = threads->new( { 'context' => 'list' }, sub {
print "Thread is expected to return a list\n";
return (1, 'abc', 5);
}
my @result = $thread->join();
threads->detach;
$thread->detach;
my $tid = $thread->tid;
my $owntid = threads->tid;
my $self = threads->self;
my $threadx = threads->object( $tidx );
my @running = threads->list(threads::running);
$_->join() foreach (threads->list(threads::joinable));
$_->join foreach threads->list; #block until all threads done
unless (fork) {
threads->isthread; # could be used a child-init Apache handler
}
# Enable debugging
use forksqw(debug);
threads->debug( 1 );
# Stringify thread objects
use forksqw(stringify);
# Check state of a thread
my $thr = threads->new( ... );
if ($thr->is_running()) {
print "Thread $thr running\n"; #prints "Thread 1 running"
}
# Send a signal to a thread
$thr->kill('SIGUSR1');
# Manual deadlock detection
if ($thr->is_deadlocked()) {
print "Thread $thr is currently deadlocked!\n";
}
# Use forks as a drop-in replacement for an ithreads application
perl -Mforks -Mforks::shared threadapplication
See "SYNOPSIS" in threads for more examples.
DESCRIPTION
The "forks" pragma allows a developer to use threads without having to
have a threaded perl, or to even run 5.8.0 or higher.
Refer to the threads module for ithreads API documentation. Also, use
perl -Mforks -e 'print $threads::VERSION'
to see what version of threads you should refer to regarding supported
API features.
There were a number of goals that I am trying to reach with this
implementation.
Using this module only makes sense if you run on a system that has an
implementation of the "fork" function by the Operating System.
Windows is currently the only known system on which Perl runs which
does not have an implementation of "fork". Therefore, it doesn't
make any sense to use this module on a Windows system. And
therefore, a check is made during installation barring you from
installing on a Windows system.
module load order: forks first
Since forks overrides core Perl functions, you are *strongly*
encouraged to load the forks module before any other Perl modules.
This will insure the most consistent and stable system behavior. This
can be easily done without affecting existing code, like the following
examples:
perl -Mforks script.pl
perl -Mforks -Mforks::shared script.pl
memory usage
The standard Perl 5.8.0 threads implementation is very memory
consuming, which makes it basically impossible to use in a production
environment, particularly with mod_perl and Apache. Because of the use
of the standard Unix fork() capabilities, most operating systems will
be able to use the Copy-On-Write (COW) memory sharing capabilities
(whereas with the standard Perl 5.8.0 threads implementation, this is
thwarted by the Perl interpreter cloning process that is used to create
threads). The memory savings have been confirmed.
mod_perl / Apache
This threads implementation allows you to use a standard, pre-forking
Apache server and have the children act as threads (with the class
method "isthread").
same API as threads
You should be able to run threaded applications unchanged by simply
making sure that the "forks" and "forks::shared" modules are loaded,
e.g. by specifying them on the command line. Forks is currently API
compatible with CPAN threads version 1.53.
Additionally, you do not need to worry about upgrading to the latest
Perl maintenance release to insure that the (CPAN) release of threads
you wish to use is fully compatibly and stable. Forks code is
completely independent of the perl core, and thus will guarantee
reliable behavior on any release of Perl 5.8 or later. (Note that
there may be behavior variances if running under Perl 5.6.x, as that
version does not support safe signals and requires a source filter to
load forks).
using as a development tool
Because you do not need a threaded Perl to use forks.pm, you can start
prototyping threaded applications with the Perl executable that you are
used to. Just download and install the "forks" package from CPAN. So
the threshold for trying out threads in Perl has become much lower.
Even Perl 5.005 should, in principle, be able to support the forks.pm
module; however, some issues with regards to the availability of XS
features between different versions of Perl, it seems that 5.6.0
(unthreaded) is what you need at least.
Additionally, forks offers a full thread deadlock detection engine, to
help discover and optionally resolve locking issues in threaded
applications. See "Deadlock detection and resolution" in forks::shared
for more information.
using in production environments
This package has successfully been proven as stable and reliable in
production environments. I have personally used it in high-
availability, database-driven, message processing server applications
since 2004 with great success.
Also, unlike pure ithreads, forks.pm is fully compatible with all perl
modules, whether or not they have been updated to be ithread safe.
This means that you do not need to feel limited in what you can develop
as a threaded perl application, a problem that continues to plague the
acceptance of ithreads in production enviroments today. Just handle
these modules as you would when using a standard fork: be sure to
create new instances of, or connections to, resources where a single
instance can not be shared between multiple processes.
The only major concern is the potentially slow (relative to pure
ithreads) performance of shared data and locks. If your application
doesn't depend on extensive semaphore use, and reads/writes from shared
variables moderately (such as using them primarily to deliver data to a
child thread to process and the child thread uses a shared structure to
return the result), then this will likely not be an issue for your
application. See the TODO section regarding plans to tackle this
issue.
Also, you may wish to try forks::BerkeleyDB, which has shown
signifigant performance gains and consistent throughoutput in high-
concurrency shared variable applications.
Perl built without native ithreads
If your Perl release was not built with ithreads or does not support
ithreads, you will have a compile-time option of installing forks into
the threads and threads::shared namespaces. This is done as a
convenience to give users a reasonably seamless ithreads API experience
without having to rebuild their distribution with native threading (and
its slight performance overhead on all perl runtime, even if not using
threads).
Note: When using forks in this manner (e.g. "use threads;") for the
first time in your code, forks will attempt to behave identically to
threads relative to the current version of threads it supports (refer
to $threads::VERSION), even if the behavior is (or was) considered a
bug. At this time, this means that shared variables will lose their
pre-existing value at the time they are shared and that splice will die
if attempted on a shared scalar.
If you use forks for the first time as "use forks" and other loaded
code uses "use threads", then this threads behavior emulation does not
apply.
REQUIRED MODULES
Acme::Damn (any)
Attribute::Handlers (any)
Devel::Symdump (any)
File::Spec (any)
if (any)
IO::Socket (1.18)
List::MoreUtils (0.15)
Scalar::Util (1.11)
Storable (any)
Sys::SigAction (0.11)
Test::More (any)
Time::HiRes (any)
IMPLEMENTATION
This version is mostly written in Perl. Inter-process communication is
done by using sockets, with the process that stores the shared
variables as the server and all the processes that function as threads,
as clients.
why sockets?
The reason I chose sockets for inter-thread communication above using a
shared memory library, is that a blocking socket allows you to
elegantly solve the problem of a thread that is blocking for a certain
event. Any polling that might occur, is not occurring at the Perl
level, but at the level of the socket, which should be much better and
probably very optimized already.
EXTRA CLASS METHODS
Apart from the standard class methods, the following class methods are
supplied by the "forks" threads implementation.
isthread
unless (fork) {
threads->isthread; # this process is a detached thread now
exit; # can not return values, as thread is detached
}
The "isthread" class method attempt to make a connection with the
shared variables process. If it succeeds, then the process will
function as a detached thread and will allow all the threads methods to
operate.
This method is mainly intended to be used from within a child-init
handler in a pre-forking Apache server. All the children that handle
requests become threads as far as Perl is concerned, allowing you to
use shared variables between all of the Apache processes. See
Apache::forks for more information.
debug
threads->debug( 1 );
$debug = threads->debug;
The "debug" class method allows you to (re)set a flag which causes
extensive debugging output of the communication between threads to be
output to STDERR. The format is still subject to change and therefore
still undocumented.
Debugging can only be switched on by defining the environment variable
"THREADS_DEBUG". If the environment variable does not exist when the
forks.pm module is compiled, then all debugging code will be optimised
away to create a better performance. If the environment variable has a
true value, then debugging will also be enabled from the start.
EXTRA FEATURES
Native threads 'to-the-letter' emulation mode
By default, forks behaves slightly differently than native ithreads,
regarding shared variables. Specifically, native threads does not
support splice() on shared arrays, nor does it retain any pre-existing
values of arrays or hashes when they are shared; however, forks
supports all of these functions. These are behaviors are considered
limitations/bugs in the current native ithread implementation.
To allow for complete drop-in compatibility with scripts and modules
written for threads.pm, you may specify the environment variable
"THREADS_NATIVE_EMULATION" to a true value before running your script.
This will instruct forks to behave exactly as native ithreads would in
the above noted situations.
This mode may also be enabled by default (without requiring this
environment variable if you do not have a threaded Perl and wish to
install forks as a full drop-in replacement. See "Perl built without
native ithreads" for more information.
Deadlock detection
Forks also offers a full thread deadlock detection engine, to help
discover and optionally resolve locking issues in threaded
applications. See "Deadlock detection and resolution" in forks::shared
for more information.
Perl debugger support
Forks supports basic compabitility with the Perl debugger. By default,
only the main thread to the active terminal (TTY), allowing for
debugging of scripts where child threads are run as background tasks
without any extra steps.
If you wish to debug code executed in child threads, you may need to
perform a few steps to prepare your environment for multi-threaded
debugging.
The simplest option is run your script in xterm, as Perl will
automatically create additional xterm windows for each child thread
that encounters a debugger breakpoint.
Otherwise, you will need to manually tell Perl how to map a control of
thread to a TTY. Two undocumented features exist in the Perl debugger:
1. Define global variable $DB::fork_TTY as the first stem in the
subroutine for a thread. The value must be a valid TTY name, such as
'/dev/pts/1' or '/dev/ttys001'; valid names may vary across platforms.
For example:
threads->new(sub {
$DB::fork_TTY = '/dev/tty003'; #tie thread to TTY 3
...
});
Also, the TTY must be active and idle prior to the thread executing.
This normally is accomplished by opening a new local or remote session
to your machine, identifying the TTY via `tty`, and then typing `sleep
10000000` to prevent user input from being passed to the command line
while you are debugging.
When the debugger halts at a breakpoint in your code in a child thread,
all output and user input will be managed via this TTY.
2. Define subroutine DB::get_fork_TTY()
This subroutine will execute once each child thread as soon as it has
spawned. Thus, you can create a new TTY, or simply bind to an existng,
active TTY. In this subroutine, you should define a unique, valid TTY
name for the global variable $DB::fork_TTY.
For example, to dynamically spawn a new xterm session and bind a new
thread to it, you could do the following:
sub DB::get_fork_TTY {
open XT, q[3>&1 xterm -title 'Forked Perl debugger' -e sh -c
'tty1>&3;\ sleep 10000000' |];
$DB::fork_TTY = <XT>;
chomp $DB::fork_TTY; }
For more information and tips, refer to this excellent Perl Monks
thread: "/www.perlmonks.org/?node_id=128283"" in <a
href="http:Debugging Several Proccesses at Same Time</a>>.
INET socket IP mask
For security, inter-thread communication INET sockets only will allow
connections from the default local machine IPv4 loopback address (e.g
127.0.0.1). However, this filter may be modified by defining the
environment variable "THREADS_IP_MASK" with a standard perl regular
expression (or with no value, which would disable the filter).
UNIX socket support
For users who do not wish to (or can not) use TCP sockets, UNIX socket
support is available. This can be only switched on by defining the
environment variable "THREADS_SOCKET_UNIX". If the environment
variable has a true value, then UNIX sockets will be used instead of
the default TCP sockets. Socket descriptors are currently written to
/var/tmp and given a+rw access by default (for cleanest functional
support on multi-user systems).
This feature is excellent for applications that require extra security,
as it does not expose forks.pm to any INET vunerabilities your system
may be subject to (i.e. systems not protected by a firewall). It also
may provide an additional performance boost, as there is less system
overhead necessary to handle UNIX vs INET socket communication.
Co-existance with fork-aware modules and environments
For modules that actively monitor and clean up after defunct child
processes like POE, forks has added support to switch the methodology
used to maintain thraad group state. This feature is switched on by
defining the environment variable "THREADS_DAEMON_MODEL". An example
use might be:
THREADS_DAEMON_MODEL=1 perl -Mforks -Mforks::shared -MPOE threadapplication
This function essentially reverses the parent-child relationship
between the main thread and the thread state process that forks.pm
uses. Extra care has gone into retaining full system signal support
and compatibility when using this mode, so it should be quite stable.
NOTES
Some important items you should be aware of.
Signal behavior
Unlike ithreads, signals being sent are standard OS signals, so you
should program defensively if you plan to use inter-thread signals.
Also, be aware that certain signals may untrappable depending on the
target platform, such as SIGKILL and SIGSTOP. Thus, it is recommended
you only use normal signals (such as TERM, INT, HUP, USR1, USR2) for
inter-thread signal handling.
exit() behavior
If you call exit() in a thread other than the main thread and exit
behavior is configured to cause entire application to exit (default
behavior), be aware that all other threads will be agressively
terminated using SIGKILL. This will cause END blocks and global
destruction to be ignored in those threads.
This behavior conforms to the expected behavior of native Perl threads.
The only subtle difference is that the main thread will be signaled
using SIGABRT to immediately exit.
If you call "fork()" but do not call <threads->isthread()>, then the
child process will default to the pre-existing CORE::GLOBAL::exit() or
CORE::exit() behavior. Note that such processes are exempt from
application global termination if exit() is called in a thread, so you
must manually clean up child processes created in this manner before
exiting your threaded application.
END block behavior
In native ithreads, END blocks are only executed in the thread in which
the code was loaded/evaluated. However, in forks, END blocks are
processed in all threads that are aware of such code segments (i.e.
threads started after modules with END blocks are loaded). This may be
considered a bug or a feature depending on what your END blocks are
doing, such as closing important external resources for which each
thread may have it's own handle.
In general, it is a good defensive programming practice to add the
following to your END blocks when you want to insure sure they only are
evaluated in the thread that they were created in:
{
my $tid = threads->tid if exists $INC{'threads.pm'};
END {
return if defined($tid) && $tid != threads->tid;
# standard end block code goes here
}
}
This code is completely compatible with native ithreads. Note that
this behavior may change in the future (at least with
THREADS_NATIVE_EMULATION mode).
Modifying signals
Since the threads API provides a method to send signals between threads
(processes), untrapped normal and error signals are defined by forks
with a basic exit() shutdown function to provide safe termination.
Thus, if you (or any modules you use) modify signal handlers, it is
important that the signal handlers at least remain defined and are not
undefined (for whatever reason). The system signal handler default,
usually abnormal process termination which skips END blocks, may cause
undesired behavior if a thread exits due to an unhandled signal.
In general, the following signals are considered "safe" to trap and use
in threads (depending on your system behavior when such signals are
trapped):
HUP INT PIPE TERM USR1 USR2 ABRT EMT QUIT TRAP
Modules that modify %SIG or use POSIX::sigaction()
To insure highest stability, forks ties some hooks into the global %SIG
hash to co-exist as peacefully as possible with user-defined signals.
This has a few subtle, but important implications:
- As long as you modify signals using %SIG, you should never encounter any
unexpected issues.
- If you use POSIX::sigaction, it may subvert protections that forks has
added to the signal handling system. In normal circumstances, this will not
create any run-time issues; however, if you also attempt to access shared
variables in signal handlers or END blocks, you may encounter unexpected
results. Note: if you do use sigaction, please avoid overloading the ABRT
signal in the main thread, as it is used for process group flow control.
Modules that modify $SIG{CHLD}
In order to be compatible with perl's core system() function on all
platforms, extra care has gone into implementing a smarter $SIG{CHLD}
in forks.pm. The only functional effect is that you will never need to
(or be able to) reap threads (processes) if you define your own CHLD
handler.
You may define the environment variable THREADS_SIGCHLD_IGNORE to to
force forks to use 'IGNORE' on systems where a custom CHLD signal
handler has been automatically installed to support correct exit code
of perl core system() function. Note that this should *not* be
necessary unless you encounter specific issues with the forks.pm CHLD
signal handler.
$thr->wantarray() returns void after $thr->join or $thr->detach
Be aware that thread return context is purged and $thr->wantarray will
return void context after a thread is detached or joined. This is done
to minimize memory in programs that spawn many (millions of) threads.
This differs from default threads.pm behavior, but should be acceptable
as the context no longer serves a functional purpose after a join or
detach. Thus, if you still require thread context information after a
join, be sure to request and store the value of $thr->wantarray first.
$thr->get_stack_size() returns default after $thr->join or $thr->detach
Thread stack size information is purged and $thr->get_stack_size will
return the current threads default after a thread is detached or
joined. This is done to minimize memory in programs that spawn many
(millions of) threads. This differs from default threads.pm behavior,
which retains per-thread stack size information indefinitely. Thus, if
you require individual thread stack size information after a join or
detach, be sure to request and store the value of $thr->get_stack_size
first.
Modules that modify CORE::GLOBAL::fork()
This modules goes to great lengths to insure that normal fork behavior
is seamlessly integrated into the threaded environment by overloading
CORE::GLOBAL::fork. Thus, please refrain from overloading this
function unless absolutely necessary. In such a case, forks.pm
provides a set of four functions:
_fork_pre
_fork
_fork_post_parent
_fork_post_child
that represent all possible functional states before and after a fork
occurs. These states must be called to insure that fork() works for
both threads and normal fork calls.
Refer to forks.pm source code, *CORE::GLOBAL::fork = sub { ... }
definition as an example usage. Please contact the author if you have
any questions regarding this.
CAVEATS
Some caveats that you need to be aware of.
Greater latency
Because of the use of sockets for inter-thread communication, there is
an inherent larger latency with the interaction between threads.
However, the fact that TCP sockets are used, may open up the
possibility to share threads over more than one physical machine.
You may decrease some latency by using UNIX sockets (see "UNIX socket
support").
Also, you may wish to try forks::BerkeleyDB, which has shown
signifigant performance gains and consistent throughoutput in
applications requiring high-concurrency shared variable access.
Module CLONE & CLONE_SKIP functions and threads
In rare cases, module CLONE functions may have issues when being auto-
executed by a new thread (forked process). This only affects modules
that use XS data (objects or struts) created by to external C
libraries. If a module attempts to CLONE non-fork safe XS data, at
worst it may core dump only the newly created thread (process).
If CLONE_SKIP function is defined in a package and it returns a true
value, all objects of this class type will be undefined in new threads.
This is generally the same behavior as native threads with Perl 5.8.7
and later. See <<a
href="http://perldoc.perl.org/perlmod.html#Making-your-module-threadsafe-threadsafe-thread-safe-module%2c-threadsafe-module%2c-thread-safe-CLONE-CLONE_SKIP-thread-threads-ithread">perlmod</a>>
for more information.
However, two subtle behavior variances exist relative to native Perl
threads:
1. The actual undefining of variables occurs in the child thread. This should
be portable with all non-perl modules, as long as those module datastructures can be
safely garbage collected in the child thread (note that DESTROY will not be called).
2. Arrays and hashes will be emptied and unblessed, but value will not be converted
to an undef scalar ref. This differs from native threads, where all references
become an undef scalar ref. This should be generally harmless, as long as you are
careful with variable state checks (e.g. check whether reference is still blessed,
not whether the reftype has changed, to determine if it is still a valid object
in a new thread).
Overall, if you treat potentially sensitive resources (such as DBI
driver instances) as non-thread-safe by default and close these
resources prior to creating a new thread, you should never encounter
any portability issues.
Can't return unshared filehandles from threads
Currently, it is not possible to return a file handle from a thread to
the thread that is joining it. Attempting to do so will throw a
terminal error. However, if you share the filehandle first with
forks::shared, you can safely return the shared filehandle.
Signals and safe-signal enabled Perl
In order to use signals, you must be using perl 5.8 compiled with safe
signal support. Otherwise, you'll get a terminal error like "Cannot
signal threads without safe signals" if you try to use signal
functions.
Source filter
To get forks.pm working on Perl 5.6.x, it was necessary to use a source
filter to ensure a smooth upgrade path from using forks under Perl
5.6.x to Perl 5.8.x and higher. The source filter used is pretty
simple and may prove to be too simple. Please report any problems that
you may find when running under 5.6.x.
TODO
See the TODO file in the distribution.
KNOWN PROBLEMS
These problems are known and will hopefully be fixed in the future:
test-suite exits in a weird way
Although there are no errors in the test-suite, the test harness
sometimes thinks there is something wrong because of an unexpected
exit() value. This is an issue with Test::More's END block, which
wasn't designed to co-exist with a threads environment and forked
processes. Hopefully, that module will be patched in the future, but
for now, the warnings are harmless and may be safely ignored.
And of course, there might be other, undiscovered issues. Patches
are welcome!
CREDITS
Refer to the "CREDITS" file included in the distribution.
CURRENT AUTHOR AND MAINTAINER
Eric Rybski <rybskej@yahoo.com>. Please send all module inquries to
me.
ORIGINAL AUTHOR
Elizabeth Mattijsen, <liz@dijkmat.nl>.
COPYRIGHT
Copyright (c)
2005-2009 Eric Rybski <rybskej@yahoo.com>,
2002-2004 Elizabeth Mattijsen <liz@dijkmat.nl>. All rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
SEE ALSO
threads, forks::BerkeleyDB, Apache::forks.
perl v5.10.1 2009-04-07 forks(3)
