NAME
AnyEvent::Fork - everything you wanted to use fork() for, but couldn't
SYNOPSIS
use AnyEvent::Fork;
AnyEvent::Fork
->new
->require ("MyModule")
->run ("MyModule::server", my $cv = AE::cv);
my $fh = $cv->recv;
DESCRIPTION
This module allows you to create new processes, without actually forking
them from your current process (avoiding the problems of forking), but
preserving most of the advantages of fork.
It can be used to create new worker processes or new independent
subprocesses for short- and long-running jobs, process pools (e.g. for
use in pre-forked servers) but also to spawn new external processes
(such as CGI scripts from a web server), which can be faster (and more
well behaved) than using fork+exec in big processes.
Special care has been taken to make this module useful from other
modules, while still supporting specialised environments such as
App::Staticperl or PAR::Packer.
WHAT THIS MODULE IS NOT
This module only creates processes and lets you pass file handles and
strings to it, and run perl code. It does not implement any kind of RPC
- there is no back channel from the process back to you, and there is no
RPC or message passing going on.
If you need some form of RPC, you could use the AnyEvent::Fork::RPC
companion module, which adds simple RPC/job queueing to a process
created by this module.
And if you need some automatic process pool management on top of
AnyEvent::Fork::RPC, you can look at the AnyEvent::Fork::Pool companion
module.
Or you can implement it yourself in whatever way you like: use some
message-passing module such as AnyEvent::MP, some pipe such as
AnyEvent::ZeroMQ, use AnyEvent::Handle on both sides to send e.g. JSON
or Storable messages, and so on.
COMPARISON TO OTHER MODULES
There is an abundance of modules on CPAN that do "something fork", such
as Parallel::ForkManager, AnyEvent::ForkManager, AnyEvent::Worker or
AnyEvent::Subprocess. There are modules that implement their own process
management, such as AnyEvent::DBI.
The problems that all these modules try to solve are real, however, none
of them (from what I have seen) tackle the very real problems of
unwanted memory sharing, efficiency, not being able to use event
processing or similar modules in the processes they create.
This module doesn't try to replace any of them - instead it tries to
solve the problem of creating processes with a minimum of fuss and
overhead (and also luxury). Ideally, most of these would use
AnyEvent::Fork internally, except they were written before AnyEvent:Fork
was available, so obviously had to roll their own.
PROBLEM STATEMENT
There are two traditional ways to implement parallel processing on UNIX
like operating systems - fork and process, and fork+exec and process.
They have different advantages and disadvantages that I describe below,
together with how this module tries to mitigate the disadvantages.
Forking from a big process can be very slow.
A 5GB process needs 0.05s to fork on my 3.6GHz amd64 GNU/Linux box.
This overhead is often shared with exec (because you have to fork
first), but in some circumstances (e.g. when vfork is used),
fork+exec can be much faster.
This module can help here by telling a small(er) helper process to
fork, which is faster then forking the main process, and also uses
vfork where possible. This gives the speed of vfork, with the
flexibility of fork.
Forking usually creates a copy-on-write copy of the parent process.
For example, modules or data files that are loaded will not use
additional memory after a fork. When exec'ing a new process, modules
and data files might need to be loaded again, at extra CPU and
memory cost. But when forking, literally all data structures are
copied - if the program frees them and replaces them by new data,
the child processes will retain the old version even if it isn't
used, which can suddenly and unexpectedly increase memory usage when
freeing memory.
The trade-off is between more sharing with fork (which can be good
or bad), and no sharing with exec.
This module allows the main program to do a controlled fork, and
allows modules to exec processes safely at any time. When creating a
custom process pool you can take advantage of data sharing via fork
without risking to share large dynamic data structures that will
blow up child memory usage.
In other words, this module puts you into control over what is being
shared and what isn't, at all times.
Exec'ing a new perl process might be difficult.
For example, it is not easy to find the correct path to the perl
interpreter - $^X might not be a perl interpreter at all.
This module tries hard to identify the correct path to the perl
interpreter. With a cooperative main program, exec'ing the
interpreter might not even be necessary, but even without help from
the main program, it will still work when used from a module.
Exec'ing a new perl process might be slow, as all necessary modules have
to be loaded from disk again, with no guarantees of success.
Long running processes might run into problems when perl is upgraded
and modules are no longer loadable because they refer to a different
perl version, or parts of a distribution are newer than the ones
already loaded.
This module supports creating pre-initialised perl processes to be
used as a template for new processes.
Forking might be impossible when a program is running.
For example, POSIX makes it almost impossible to fork from a
multi-threaded program while doing anything useful in the child - in
fact, if your perl program uses POSIX threads (even indirectly via
e.g. IO::AIO or threads), you cannot call fork on the perl level
anymore without risking corruption issues on a number of operating
systems.
This module can safely fork helper processes at any time, by calling
fork+exec in C, in a POSIX-compatible way (via Proc::FastSpawn).
Parallel processing with fork might be inconvenient or difficult to
implement. Modules might not work in both parent and child.
For example, when a program uses an event loop and creates watchers
it becomes very hard to use the event loop from a child program, as
the watchers already exist but are only meaningful in the parent.
Worse, a module might want to use such a module, not knowing whether
another module or the main program also does, leading to problems.
Apart from event loops, graphical toolkits also commonly fall into
the "unsafe module" category, or just about anything that
communicates with the external world, such as network libraries and
file I/O modules, which usually don't like being copied and then
allowed to continue in two processes.
With this module only the main program is allowed to create new
processes by forking (because only the main program can know when it
is still safe to do so) - all other processes are created via
fork+exec, which makes it possible to use modules such as event
loops or window interfaces safely.
EXAMPLES
Create a single new process, tell it to run your worker function.
AnyEvent::Fork
->new
->require ("MyModule")
->run ("MyModule::worker, sub {
my ($master_filehandle) = @_;
# now $master_filehandle is connected to the
# $slave_filehandle in the new process.
});
"MyModule" might look like this:
package MyModule;
sub worker {
my ($slave_filehandle) = @_;
# now $slave_filehandle is connected to the $master_filehandle
# in the original prorcess. have fun!
}
Create a pool of server processes all accepting on the same socket.
# create listener socket
my $listener = ...;
# create a pool template, initialise it and give it the socket
my $pool = AnyEvent::Fork
->new
->require ("Some::Stuff", "My::Server")
->send_fh ($listener);
# now create 10 identical workers
for my $id (1..10) {
$pool
->fork
->send_arg ($id)
->run ("My::Server::run");
}
# now do other things - maybe use the filehandle provided by run
# to wait for the processes to die. or whatever.
"My::Server" might look like this:
package My::Server;
sub run {
my ($slave, $listener, $id) = @_;
close $slave; # we do not use the socket, so close it to save resources
# we could go ballistic and use e.g. AnyEvent here, or IO::AIO,
# or anything we usually couldn't do in a process forked normally.
while (my $socket = $listener->accept) {
# do sth. with new socket
}
}
use AnyEvent::Fork as a faster fork+exec
This runs "/bin/echo hi", with standard output redirected to /tmp/log
and standard error redirected to the communications socket. It is
usually faster than fork+exec, but still lets you prepare the
environment.
open my $output, ">/tmp/log" or die "$!";
AnyEvent::Fork
->new
->eval ('
# compile a helper function for later use
sub run {
my ($fh, $output, @cmd) = @_;
# perl will clear close-on-exec on STDOUT/STDERR
open STDOUT, ">&", $output or die;
open STDERR, ">&", $fh or die;
exec @cmd;
}
')
->send_fh ($output)
->send_arg ("/bin/echo", "hi")
->run ("run", my $cv = AE::cv);
my $stderr = $cv->recv;
For stingy users: put the worker code into a "DATA" section.
When you want to be stingy with files, you cna put your code into the
"DATA" section of your module (or program):
use AnyEvent::Fork;
AnyEvent::Fork
->new
->eval (do { local $/; <DATA> })
->run ("doit", sub { ... });
__DATA__
sub doit {
... do something!
}
For stingy standalone programs: do not rely on external files at
all.
For single-file scripts it can be inconvenient to rely on external files
- even when using < "DATA" section, you still need to "exec" an external
perl interpreter, which might not be available when using
App::Staticperl, Urlader or PAR::Packer for example.
Two modules help here - AnyEvent::Fork::Early forks a template process
for all further calls to "new_exec", and AnyEvent::Fork::Template forks
the main program as a template process.
Here is how your main program should look like:
#! perl
# optional, as the very first thing.
# in case modules want to create their own processes.
use AnyEvent::Fork::Early;
# next, load all modules you need in your template process
use Example::My::Module
use Example::Whatever;
# next, put your run function definition and anything else you
# need, but do not use code outside of BEGIN blocks.
sub worker_run {
my ($fh, @args) = @_;
...
}
# now preserve everything so far as AnyEvent::Fork object
# in §TEMPLATE.
use AnyEvent::Fork::Template;
# do not put code outside of BEGIN blocks until here
# now use the $TEMPLATE process in any way you like
# for example: create 10 worker processes
my @worker;
my $cv = AE::cv;
for (1..10) {
$cv->begin;
$TEMPLATE->fork->send_arg ($_)->run ("worker_run", sub {
push @worker, shift;
$cv->end;
});
}
$cv->recv;
CONCEPTS
This module can create new processes either by executing a new perl
process, or by forking from an existing "template" process.
All these processes are called "child processes" (whether they are
direct children or not), while the process that manages them is called
the "parent process".
Each such process comes with its own file handle that can be used to
communicate with it (it's actually a socket - one end in the new
process, one end in the main process), and among the things you can do
in it are load modules, fork new processes, send file handles to it, and
execute functions.
There are multiple ways to create additional processes to execute some
jobs:
fork a new process from the "default" template process, load code, run
it
This module has a "default" template process which it executes when
it is needed the first time. Forking from this process shares the
memory used for the perl interpreter with the new process, but
loading modules takes time, and the memory is not shared with
anything else.
This is ideal for when you only need one extra process of a kind,
with the option of starting and stopping it on demand.
Example:
AnyEvent::Fork
->new
->require ("Some::Module")
->run ("Some::Module::run", sub {
my ($fork_fh) = @_;
});
fork a new template process, load code, then fork processes off of it
and run the code
When you need to have a bunch of processes that all execute the same
(or very similar) tasks, then a good way is to create a new template
process for them, loading all the modules you need, and then create
your worker processes from this new template process.
This way, all code (and data structures) that can be shared (e.g.
the modules you loaded) is shared between the processes, and each
new process consumes relatively little memory of its own.
The disadvantage of this approach is that you need to create a
template process for the sole purpose of forking new processes from
it, but if you only need a fixed number of processes you can create
them, and then destroy the template process.
Example:
my $template = AnyEvent::Fork->new->require ("Some::Module");
for (1..10) {
$template->fork->run ("Some::Module::run", sub {
my ($fork_fh) = @_;
});
}
# at this point, you can keep $template around to fork new processes
# later, or you can destroy it, which causes it to vanish.
execute a new perl interpreter, load some code, run it
This is relatively slow, and doesn't allow you to share memory
between multiple processes.
The only advantage is that you don't have to have a template process
hanging around all the time to fork off some new processes, which
might be an advantage when there are long time spans where no extra
processes are needed.
Example:
AnyEvent::Fork
->new_exec
->require ("Some::Module")
->run ("Some::Module::run", sub {
my ($fork_fh) = @_;
});
THE "AnyEvent::Fork" CLASS
This module exports nothing, and only implements a single class -
"AnyEvent::Fork".
There are two class constructors that both create new processes - "new"
and "new_exec". The "fork" method creates a new process by forking an
existing one and could be considered a third constructor.
Most of the remaining methods deal with preparing the new process, by
loading code, evaluating code and sending data to the new process. They
usually return the process object, so you can chain method calls.
If a process object is destroyed before calling its "run" method, then
the process simply exits. After "run" is called, all responsibility is
passed to the specified function.
As long as there is any outstanding work to be done, process objects
resist being destroyed, so there is no reason to store them unless you
need them later - configure and forget works just fine.
my $proc = new AnyEvent::Fork
Create a new "empty" perl interpreter process and returns its
process object for further manipulation.
The new process is forked from a template process that is kept
around for this purpose. When it doesn't exist yet, it is created by
a call to "new_exec" first and then stays around for future calls.
$new_proc = $proc->fork
Forks $proc, creating a new process, and returns the process object
of the new process.
If any of the "send_" functions have been called before fork, then
they will be cloned in the child. For example, in a pre-forked
server, you might "send_fh" the listening socket into the template
process, and then keep calling "fork" and "run".
my $proc = new_exec AnyEvent::Fork
Create a new "empty" perl interpreter process and returns its
process object for further manipulation.
Unlike the "new" method, this method *always* spawns a new perl
process (except in some cases, see AnyEvent::Fork::Early for
details). This reduces the amount of memory sharing that is
possible, and is also slower.
You should use "new" whenever possible, except when having a
template process around is unacceptable.
The path to the perl interpreter is divined using various methods -
first $^X is investigated to see if the path ends with something
that looks as if it were the perl interpreter. Failing this, the
module falls back to using $Config::Config{perlpath}.
The path to perl can also be overriden by setting the global
variable $AnyEvent::Fork::PERL - it's value will be used for all
subsequent invocations.
$pid = $proc->pid
Returns the process id of the process *iff it is a direct child of
the process running AnyEvent::Fork*, and "undef" otherwise. As a
general rule (that you cannot rely upon), processes created via
"new_exec", AnyEvent::Fork::Early or AnyEvent::Fork::Template are
direct children, while all other processes are not.
Or in other words, you do not normally have to take care of zombies
for processes created via "new", but when in doubt, or zombies are a
problem, you need to check whether a process is a diretc child by
calling this method, and possibly creating a child watcher or reap
it manually.
$proc = $proc->eval ($perlcode, @args)
Evaluates the given $perlcode as ... Perl code, while setting @_ to
the strings specified by @args, in the "main" package.
This call is meant to do any custom initialisation that might be
required (for example, the "require" method uses it). It's not
supposed to be used to completely take over the process, use "run"
for that.
The code will usually be executed after this call returns, and there
is no way to pass anything back to the calling process. Any
evaluation errors will be reported to stderr and cause the process
to exit.
If you want to execute some code (that isn't in a module) to take
over the process, you should compile a function via "eval" first,
and then call it via "run". This also gives you access to any
arguments passed via the "send_xxx" methods, such as file handles.
See the "use AnyEvent::Fork as a faster fork+exec" example to see it
in action.
Returns the process object for easy chaining of method calls.
$proc = $proc->require ($module, ...)
Tries to load the given module(s) into the process
Returns the process object for easy chaining of method calls.
$proc = $proc->send_fh ($handle, ...)
Send one or more file handles (*not* file descriptors) to the
process, to prepare a call to "run".
The process object keeps a reference to the handles until they have
been passed over to the process, so you must not explicitly close
the handles. This is most easily accomplished by simply not storing
the file handles anywhere after passing them to this method - when
AnyEvent::Fork is finished using them, perl will automatically close
them.
Returns the process object for easy chaining of method calls.
Example: pass a file handle to a process, and release it without
closing. It will be closed automatically when it is no longer used.
$proc->send_fh ($my_fh);
undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT
$proc = $proc->send_arg ($string, ...)
Send one or more argument strings to the process, to prepare a call
to "run". The strings can be any octet strings.
The protocol is optimised to pass a moderate number of relatively
short strings - while you can pass up to 4GB of data in one go, this
is more meant to pass some ID information or other startup info, not
big chunks of data.
Returns the process object for easy chaining of method calls.
$proc->run ($func, $cb->($fh))
Enter the function specified by the function name in $func in the
process. The function is called with the communication socket as
first argument, followed by all file handles and string arguments
sent earlier via "send_fh" and "send_arg" methods, in the order they
were called.
The process object becomes unusable on return from this function -
any further method calls result in undefined behaviour.
The function name should be fully qualified, but if it isn't, it
will be looked up in the "main" package.
If the called function returns, doesn't exist, or any error occurs,
the process exits.
Preparing the process is done in the background - when all commands
have been sent, the callback is invoked with the local
communications socket as argument. At this point you can start using
the socket in any way you like.
If the communication socket isn't used, it should be closed on both
sides, to save on kernel memory.
The socket is non-blocking in the parent, and blocking in the newly
created process. The close-on-exec flag is set in both.
Even if not used otherwise, the socket can be a good indicator for
the existence of the process - if the other process exits, you get a
readable event on it, because exiting the process closes the socket
(if it didn't create any children using fork).
Compatibility to AnyEvent::Fork::Remote
If you want to write code that works with both this module and
AnyEvent::Fork::Remote, you need to write your code so that it
assumes there are two file handles for communications, which
might not be unix domain sockets. The "run" function should
start like this:
sub run {
my ($rfh, @args) = @_; # @args is your normal arguments
my $wfh = fileno $rfh ? $rfh : *STDOUT;
# now use $rfh for reading and $wfh for writing
}
This checks whether the passed file handle is, in fact, the
process "STDIN" handle. If it is, then the function was invoked
visa AnyEvent::Fork::Remote, so STDIN should be used for reading
and "STDOUT" should be used for writing.
In all other cases, the function was called via this module, and
there is only one file handle that should be sued for reading
and writing.
Example: create a template for a process pool, pass a few strings,
some file handles, then fork, pass one more string, and run some
code.
my $pool = AnyEvent::Fork
->new
->send_arg ("str1", "str2")
->send_fh ($fh1, $fh2);
for (1..2) {
$pool
->fork
->send_arg ("str3")
->run ("Some::function", sub {
my ($fh) = @_;
# fh is nonblocking, but we trust that the OS can accept these
# few octets anyway.
syswrite $fh, "hi #$_\n";
# $fh is being closed here, as we don't store it anywhere
});
}
# Some::function might look like this - all parameters passed before fork
# and after will be passed, in order, after the communications socket.
sub Some::function {
my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_;
print scalar <$fh>; # prints "hi #1\n" and "hi #2\n" in any order
}
EXPERIMENTAL METHODS
These methods might go away completely or change behaviour, at any time.
$proc->to_fh ($cb->($fh)) # EXPERIMENTAL, MIGHT BE REMOVED
Flushes all commands out to the process and then calls the callback
with the communications socket.
The process object becomes unusable on return from this function -
any further method calls result in undefined behaviour.
The point of this method is to give you a file handle that you can
pass to another process. In that other process, you can call
"new_from_fh AnyEvent::Fork $fh" to create a new "AnyEvent::Fork"
object from it, thereby effectively passing a fork object to another
process.
new_from_fh AnyEvent::Fork $fh # EXPERIMENTAL, MIGHT BE REMOVED
Takes a file handle originally rceeived by the "to_fh" method and
creates a new "AnyEvent:Fork" object. The child process itself will
not change in any way, i.e. it will keep all the modifications done
to it before calling "to_fh".
The new object is very much like the original object, except that
the "pid" method will return "undef" even if the process is a direct
child.
PERFORMANCE
Now for some unscientific benchmark numbers (all done on an amd64
GNU/Linux box). These are intended to give you an idea of the relative
performance you can expect, they are not meant to be absolute
performance numbers.
OK, so, I ran a simple benchmark that creates a socket pair, forks,
calls exit in the child and waits for the socket to close in the parent.
I did load AnyEvent, EV and AnyEvent::Fork, for a total process size of
5100kB.
2079 new processes per second, using manual socketpair + fork
Then I did the same thing, but instead of calling fork, I called
AnyEvent::Fork->new->run ("CORE::exit") and then again waited for the
socket from the child to close on exit. This does the same thing as
manual socket pair + fork, except that what is forked is the template
process (2440kB), and the socket needs to be passed to the server at the
other end of the socket first.
2307 new processes per second, using AnyEvent::Fork->new
And finally, using "new_exec" instead "new", using vforks+execs to exec
a new perl interpreter and compile the small server each time, I get:
479 vfork+execs per second, using AnyEvent::Fork->new_exec
So how can "AnyEvent->new" be faster than a standard fork, even though
it uses the same operations, but adds a lot of overhead?
The difference is simply the process size: forking the 5MB process takes
so much longer than forking the 2.5MB template process that the extra
overhead is canceled out.
If the benchmark process grows, the normal fork becomes even slower:
1340 new processes, manual fork of a 20MB process
731 new processes, manual fork of a 200MB process
235 new processes, manual fork of a 2000MB process
What that means (to me) is that I can use this module without having a
bad conscience because of the extra overhead required to start new
processes.
TYPICAL PROBLEMS
This section lists typical problems that remain. I hope by recognising
them, most can be avoided.
leaked file descriptors for exec'ed processes
POSIX systems inherit file descriptors by default when exec'ing a
new process. While perl itself laudably sets the close-on-exec flags
on new file handles, most C libraries don't care, and even if all
cared, it's often not possible to set the flag in a race-free
manner.
That means some file descriptors can leak through. And since it
isn't possible to know which file descriptors are "good" and
"necessary" (or even to know which file descriptors are open), there
is no good way to close the ones that might harm.
As an example of what "harm" can be done consider a web server that
accepts connections and afterwards some module uses AnyEvent::Fork
for the first time, causing it to fork and exec a new process, which
might inherit the network socket. When the server closes the socket,
it is still open in the child (which doesn't even know that) and the
client might conclude that the connection is still fine.
For the main program, there are multiple remedies available -
AnyEvent::Fork::Early is one, creating a process early and not using
"new_exec" is another, as in both cases, the first process can be
exec'ed well before many random file descriptors are open.
In general, the solution for these kind of problems is to fix the
libraries or the code that leaks those file descriptors.
Fortunately, most of these leaked descriptors do no harm, other than
sitting on some resources.
leaked file descriptors for fork'ed processes
Normally, AnyEvent::Fork does start new processes by exec'ing them,
which closes file descriptors not marked for being inherited.
However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a
way to create these processes by forking, and this leaks more file
descriptors than exec'ing them, as there is no way to mark
descriptors as "close on fork".
An example would be modules like EV, IO::AIO or Gtk2. Both create
pipes for internal uses, and Gtk2 might open a connection to the X
server. EV and IO::AIO can deal with fork, but Gtk2 might have
trouble with a fork.
The solution is to either not load these modules before use'ing
AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay
initialising them, for example, by calling "init Gtk2" manually.
exiting calls object destructors
This only applies to users of AnyEvent::Fork:Early and
AnyEvent::Fork::Template, or when initialising code creates objects
that reference external resources.
When a process created by AnyEvent::Fork exits, it might do so by
calling exit, or simply letting perl reach the end of the program.
At which point Perl runs all destructors.
Not all destructors are fork-safe - for example, an object that
represents the connection to an X display might tell the X server to
free resources, which is inconvenient when the "real" object in the
parent still needs to use them.
This is obviously not a problem for AnyEvent::Fork::Early, as you
used it as the very first thing, right?
It is a problem for AnyEvent::Fork::Template though - and the
solution is to not create objects with nontrivial destructors that
might have an effect outside of Perl.
PORTABILITY NOTES
Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a
nop, and ::Template is not going to work), and it cost a lot of blood
and sweat to make it so, mostly due to the bloody broken perl that
nobody seems to care about. The fork emulation is a bad joke - I have
yet to see something useful that you can do with it without running into
memory corruption issues or other braindamage. Hrrrr.
Since fork is endlessly broken on win32 perls (it doesn't even remotely
work within it's documented limits) and quite obviously it's not getting
improved any time soon, the best way to proceed on windows would be to
always use "new_exec" and thus never rely on perl's fork "emulation".
Cygwin perl is not supported at the moment due to some hilarious
shortcomings of its API - see IO::FDPoll for more details. If you never
use "send_fh" and always use "new_exec" to create processes, it should
work though.
SEE ALSO
AnyEvent::Fork::Early, to avoid executing a perl interpreter at all
(part of this distribution).
AnyEvent::Fork::Template, to create a process by forking the main
program at a convenient time (part of this distribution).
AnyEvent::Fork::Remote, for another way to create processes that is
mostly compatible to this module and modules building on top of it, but
works better with remote processes.
AnyEvent::Fork::RPC, for simple RPC to child processes (on CPAN).
AnyEvent::Fork::Pool, for simple worker process pool (on CPAN).
AUTHOR AND CONTACT INFORMATION
Marc Lehmann <schmorp@schmorp.de>
http://software.schmorp.de/pkg/AnyEvent-Fork