| 1 |
NAME |
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AnyEvent::Fork - everything you wanted to use fork() for, but couldn't |
| 3 |
|
| 4 |
SYNOPSIS |
| 5 |
use AnyEvent::Fork; |
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|
| 7 |
################################################################## |
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# create a single new process, tell it to run your worker function |
| 9 |
|
| 10 |
AnyEvent::Fork |
| 11 |
->new |
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->require ("MyModule") |
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->run ("MyModule::worker, sub { |
| 14 |
my ($master_filehandle) = @_; |
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|
| 16 |
# now $master_filehandle is connected to the |
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# $slave_filehandle in the new process. |
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}); |
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|
| 20 |
# MyModule::worker might look like this |
| 21 |
sub MyModule::worker { |
| 22 |
my ($slave_filehandle) = @_; |
| 23 |
|
| 24 |
# now $slave_filehandle is connected to the $master_filehandle |
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# in the original prorcess. have fun! |
| 26 |
} |
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|
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################################################################## |
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# create a pool of server processes all accepting on the same socket |
| 30 |
|
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# create listener socket |
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my $listener = ...; |
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|
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# create a pool template, initialise it and give it the socket |
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my $pool = AnyEvent::Fork |
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->new |
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->require ("Some::Stuff", "My::Server") |
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->send_fh ($listener); |
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|
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# now create 10 identical workers |
| 41 |
for my $id (1..10) { |
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$pool |
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->fork |
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->send_arg ($id) |
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->run ("My::Server::run"); |
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} |
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|
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# now do other things - maybe use the filehandle provided by run |
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# to wait for the processes to die. or whatever. |
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|
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# My::Server::run might look like this |
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sub My::Server::run { |
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my ($slave, $listener, $id) = @_; |
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|
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close $slave; # we do not use the socket, so close it to save resources |
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|
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# we could go ballistic and use e.g. AnyEvent here, or IO::AIO, |
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# or anything we usually couldn't do in a process forked normally. |
| 59 |
while (my $socket = $listener->accept) { |
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# do sth. with new socket |
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} |
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} |
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|
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DESCRIPTION |
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This module allows you to create new processes, without actually forking |
| 66 |
them from your current process (avoiding the problems of forking), but |
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preserving most of the advantages of fork. |
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|
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It can be used to create new worker processes or new independent |
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subprocesses for short- and long-running jobs, process pools (e.g. for |
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use in pre-forked servers) but also to spawn new external processes |
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(such as CGI scripts from a webserver), which can be faster (and more |
| 73 |
well behaved) than using fork+exec in big processes. |
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|
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Special care has been taken to make this module useful from other |
| 76 |
modules, while still supporting specialised environments such as |
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App::Staticperl or PAR::Packer. |
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|
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PROBLEM STATEMENT |
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There are two ways to implement parallel processing on UNIX like |
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operating systems - fork and process, and fork+exec and process. They |
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have different advantages and disadvantages that I describe below, |
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together with how this module tries to mitigate the disadvantages. |
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|
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Forking from a big process can be very slow (a 5GB process needs 0.05s |
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to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead is |
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often shared with exec (because you have to fork first), but in some |
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circumstances (e.g. when vfork is used), fork+exec can be much faster. |
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This module can help here by telling a small(er) helper process to |
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fork, or fork+exec instead. |
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|
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Forking usually creates a copy-on-write copy of the parent process. |
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Memory (for example, modules or data files that have been will not take |
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additional memory). When exec'ing a new process, modules and data files |
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might need to be loaded again, at extra cpu and memory cost. Likewise |
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when forking, all data structures are copied as well - if the program |
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frees them and replaces them by new data, the child processes will |
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retain the memory even if it isn't used. |
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This module allows the main program to do a controlled fork, and |
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allows modules to exec processes safely at any time. When creating a |
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custom process pool you can take advantage of data sharing via fork |
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without risking to share large dynamic data structures that will |
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blow up child memory usage. |
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|
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Exec'ing a new perl process might be difficult and slow. For example, it |
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is not easy to find the correct path to the perl interpreter, and all |
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modules have to be loaded from disk again. Long running processes might |
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run into problems when perl is upgraded for example. |
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This module supports creating pre-initialised perl processes to be |
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used as template, and also tries hard to identify the correct path |
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to the perl interpreter. With a cooperative main program, exec'ing |
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the interpreter might not even be necessary. |
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|
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Forking might be impossible when a program is running. For example, |
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POSIX makes it almost impossible to fork from a multithreaded program |
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and do anything useful in the child - strictly speaking, if your perl |
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program uses posix threads (even indirectly via e.g. IO::AIO or |
| 118 |
threads), you cannot call fork on the perl level anymore, at all. |
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This module can safely fork helper processes at any time, by caling |
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fork+exec in C, in a POSIX-compatible way. |
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|
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Parallel processing with fork might be inconvenient or difficult to |
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implement. For example, when a program uses an event loop and creates |
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watchers it becomes very hard to use the event loop from a child |
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program, as the watchers already exist but are only meaningful in the |
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parent. Worse, a module might want to use such a system, not knowing |
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whether another module or the main program also does, leading to |
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problems. |
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This module only lets the main program create pools by forking |
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(because only the main program can know when it is still safe to do |
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so) - all other pools are created by fork+exec, after which such |
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modules can again be loaded. |
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|
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CONCEPTS |
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This module can create new processes either by executing a new perl |
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process, or by forking from an existing "template" process. |
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|
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Each such process comes with its own file handle that can be used to |
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communicate with it (it's actually a socket - one end in the new |
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process, one end in the main process), and among the things you can do |
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in it are load modules, fork new processes, send file handles to it, and |
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execute functions. |
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|
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There are multiple ways to create additional processes to execute some |
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jobs: |
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|
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fork a new process from the "default" template process, load code, run |
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it |
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This module has a "default" template process which it executes when |
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it is needed the first time. Forking from this process shares the |
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memory used for the perl interpreter with the new process, but |
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loading modules takes time, and the memory is not shared with |
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anything else. |
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|
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This is ideal for when you only need one extra process of a kind, |
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with the option of starting and stipping it on demand. |
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|
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Example: |
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|
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AnyEvent::Fork |
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->new |
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->require ("Some::Module") |
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->run ("Some::Module::run", sub { |
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my ($fork_fh) = @_; |
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}); |
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|
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fork a new template process, load code, then fork processes off of it |
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and run the code |
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When you need to have a bunch of processes that all execute the same |
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(or very similar) tasks, then a good way is to create a new template |
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process for them, loading all the modules you need, and then create |
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your worker processes from this new template process. |
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|
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This way, all code (and data structures) that can be shared (e.g. |
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the modules you loaded) is shared between the processes, and each |
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new process consumes relatively little memory of its own. |
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|
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The disadvantage of this approach is that you need to create a |
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template process for the sole purpose of forking new processes from |
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it, but if you only need a fixed number of proceses you can create |
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them, and then destroy the template process. |
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|
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Example: |
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|
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my $template = AnyEvent::Fork->new->require ("Some::Module"); |
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|
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for (1..10) { |
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$template->fork->run ("Some::Module::run", sub { |
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my ($fork_fh) = @_; |
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}); |
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} |
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|
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# at this point, you can keep $template around to fork new processes |
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# later, or you can destroy it, which causes it to vanish. |
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|
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execute a new perl interpreter, load some code, run it |
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This is relatively slow, and doesn't allow you to share memory |
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between multiple processes. |
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|
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The only advantage is that you don't have to have a template process |
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hanging around all the time to fork off some new processes, which |
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might be an advantage when there are long time spans where no extra |
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processes are needed. |
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|
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Example: |
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|
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AnyEvent::Fork |
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->new_exec |
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->require ("Some::Module") |
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->run ("Some::Module::run", sub { |
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my ($fork_fh) = @_; |
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}); |
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|
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FUNCTIONS |
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my $pool = new AnyEvent::Fork key => value... |
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Create a new process pool. The following named parameters are |
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supported: |
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|
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my $proc = new AnyEvent::Fork |
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Create a new "empty" perl interpreter process and returns its |
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process object for further manipulation. |
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|
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The new process is forked from a template process that is kept |
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around for this purpose. When it doesn't exist yet, it is created by |
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a call to "new_exec" and kept around for future calls. |
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|
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When the process object is destroyed, it will release the file |
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handle that connects it with the new process. When the new process |
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has not yet called "run", then the process will exit. Otherwise, |
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what happens depends entirely on the code that is executed. |
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|
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$new_proc = $proc->fork |
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Forks $proc, creating a new process, and returns the process object |
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of the new process. |
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|
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If any of the "send_" functions have been called before fork, then |
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they will be cloned in the child. For example, in a pre-forked |
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server, you might "send_fh" the listening socket into the template |
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process, and then keep calling "fork" and "run". |
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|
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my $proc = new_exec AnyEvent::Fork |
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Create a new "empty" perl interpreter process and returns its |
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process object for further manipulation. |
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|
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Unlike the "new" method, this method *always* spawns a new perl |
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process (except in some cases, see AnyEvent::Fork::Early for |
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details). This reduces the amount of memory sharing that is |
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possible, and is also slower. |
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|
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You should use "new" whenever possible, except when having a |
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template process around is unacceptable. |
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|
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The path to the perl interpreter is divined usign various methods - |
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first $^X is investigated to see if the path ends with something |
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that sounds as if it were the perl interpreter. Failing this, the |
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module falls back to using $Config::Config{perlpath}. |
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|
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$proc = $proc->eval ($perlcode, @args) |
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Evaluates the given $perlcode as ... perl code, while setting @_ to |
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the strings specified by @args. |
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|
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This call is meant to do any custom initialisation that might be |
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required (for example, the "require" method uses it). It's not |
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supposed to be used to completely take over the process, use "run" |
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for that. |
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|
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The code will usually be executed after this call returns, and there |
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is no way to pass anything back to the calling process. Any |
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evaluation errors will be reported to stderr and cause the process |
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to exit. |
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|
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Returns the process object for easy chaining of method calls. |
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|
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$proc = $proc->require ($module, ...) |
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Tries to load the given module(s) into the process |
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|
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Returns the process object for easy chaining of method calls. |
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|
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$proc = $proc->send_fh ($handle, ...) |
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Send one or more file handles (*not* file descriptors) to the |
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process, to prepare a call to "run". |
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|
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The process object keeps a reference to the handles until this is |
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done, so you must not explicitly close the handles. This is most |
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easily accomplished by simply not storing the file handles anywhere |
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after passing them to this method. |
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|
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Returns the process object for easy chaining of method calls. |
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|
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Example: pass an fh to a process, and release it without closing. it |
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will be closed automatically when it is no longer used. |
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|
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$proc->send_fh ($my_fh); |
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undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT |
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|
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$proc = $proc->send_arg ($string, ...) |
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Send one or more argument strings to the process, to prepare a call |
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to "run". The strings can be any octet string. |
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|
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Returns the process object for easy chaining of emthod calls. |
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|
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$proc->run ($func, $cb->($fh)) |
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Enter the function specified by the fully qualified name in $func in |
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the process. The function is called with the communication socket as |
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first argument, followed by all file handles and string arguments |
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sent earlier via "send_fh" and "send_arg" methods, in the order they |
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were called. |
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|
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If the called function returns, the process exits. |
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|
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Preparing the process can take time - when the process is ready, the |
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callback is invoked with the local communications socket as |
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argument. |
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|
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The process object becomes unusable on return from this function. |
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|
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If the communication socket isn't used, it should be closed on both |
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sides, to save on kernel memory. |
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|
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The socket is non-blocking in the parent, and blocking in the newly |
| 321 |
created process. The close-on-exec flag is set on both. Even if not |
| 322 |
used otherwise, the socket can be a good indicator for the existance |
| 323 |
of the process - if the other process exits, you get a readable |
| 324 |
event on it, because exiting the process closes the socket (if it |
| 325 |
didn't create any children using fork). |
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|
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Example: create a template for a process pool, pass a few strings, |
| 328 |
some file handles, then fork, pass one more string, and run some |
| 329 |
code. |
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|
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my $pool = AnyEvent::Fork |
| 332 |
->new |
| 333 |
->send_arg ("str1", "str2") |
| 334 |
->send_fh ($fh1, $fh2); |
| 335 |
|
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for (1..2) { |
| 337 |
$pool |
| 338 |
->fork |
| 339 |
->send_arg ("str3") |
| 340 |
->run ("Some::function", sub { |
| 341 |
my ($fh) = @_; |
| 342 |
|
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# fh is nonblocking, but we trust that the OS can accept these |
| 344 |
# extra 3 octets anyway. |
| 345 |
syswrite $fh, "hi #$_\n"; |
| 346 |
|
| 347 |
# $fh is being closed here, as we don't store it anywhere |
| 348 |
}); |
| 349 |
} |
| 350 |
|
| 351 |
# Some::function might look like this - all parameters passed before fork |
| 352 |
# and after will be passed, in order, after the communications socket. |
| 353 |
sub Some::function { |
| 354 |
my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; |
| 355 |
|
| 356 |
print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" |
| 357 |
} |
| 358 |
|
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TYPICAL PROBLEMS |
| 360 |
This section lists typical problems that remain. I hope by recognising |
| 361 |
them, most can be avoided. |
| 362 |
|
| 363 |
"leaked" file descriptors for exec'ed processes |
| 364 |
POSIX systems inherit file descriptors by default when exec'ing a |
| 365 |
new process. While perl itself laudably sets the close-on-exec flags |
| 366 |
on new file handles, most C libraries don't care, and even if all |
| 367 |
cared, it's often not possible to set the flag in a race-free |
| 368 |
manner. |
| 369 |
|
| 370 |
That means some file descriptors can leak through. And since it |
| 371 |
isn't possible to know which file descriptors are "good" and |
| 372 |
"neccessary" (or even to know which file descreiptors are open), |
| 373 |
there is no good way to close the ones that might harm. |
| 374 |
|
| 375 |
As an example of what "harm" can be done consider a web server that |
| 376 |
accepts connections and afterwards some module uses AnyEvent::Fork |
| 377 |
for the first time, causing it to fork and exec a new process, which |
| 378 |
might inherit the network socket. When the server closes the socket, |
| 379 |
it is still open in the child (which doesn't even know that) and the |
| 380 |
client might conclude that the connection is still fine. |
| 381 |
|
| 382 |
For the main program, there are multiple remedies available - |
| 383 |
AnyEvent::Fork::Early is one, creating a process early and not using |
| 384 |
"new_exec" is another, as in both cases, the first process can be |
| 385 |
exec'ed well before many random file descriptors are open. |
| 386 |
|
| 387 |
In general, the solution for these kind of problems is to fix the |
| 388 |
libraries or the code that leaks those file descriptors. |
| 389 |
|
| 390 |
Fortunately, most of these lekaed descriptors do no harm, other than |
| 391 |
sitting on some resources. |
| 392 |
|
| 393 |
"leaked" file descriptors for fork'ed processes |
| 394 |
Normally, AnyEvent::Fork does start new processes by exec'ing them, |
| 395 |
which closes file descriptors not marked for being inherited. |
| 396 |
|
| 397 |
However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a |
| 398 |
way to create these processes by forking, and this leaks more file |
| 399 |
descriptors than exec'ing them, as there is no way to mark |
| 400 |
descriptors as "close on fork". |
| 401 |
|
| 402 |
An example would be modules like EV, IO::AIO or Gtk2. Both create |
| 403 |
pipes for internal uses, and Gtk2 might open a connection to the X |
| 404 |
server. EV and IO::AIO can deal with fork, but Gtk2 might have |
| 405 |
trouble with a fork. |
| 406 |
|
| 407 |
The solution is to either not load these modules before use'ing |
| 408 |
AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay |
| 409 |
initialising them, for example, by calling "init Gtk2" manually. |
| 410 |
|
| 411 |
PORTABILITY NOTES |
| 412 |
Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a |
| 413 |
nop, and ::Template is not going to work), and it cost a lot of blood |
| 414 |
and sweat to make it so, mostly due to the bloody broken perl that |
| 415 |
nobody seems to care about. The fork emulation is a bad joke - I have |
| 416 |
yet to see something useful that you cna do with it without running into |
| 417 |
memory corruption issues or other braindamage. Hrrrr. |
| 418 |
|
| 419 |
Cygwin perl is not supported at the moment, as it should implement fd |
| 420 |
passing, but doesn't, and rolling my own is hard, as cygwin doesn't |
| 421 |
support enough functionality to do it. |
| 422 |
|
| 423 |
SEE ALSO |
| 424 |
AnyEvent::Fork::Early (to avoid executing a perl interpreter), |
| 425 |
AnyEvent::Fork::Template (to create a process by forking the main |
| 426 |
program at a convenient time). |
| 427 |
|
| 428 |
AUTHOR |
| 429 |
Marc Lehmann <schmorp@schmorp.de> |
| 430 |
http://home.schmorp.de/ |
| 431 |
|
