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BEROV AERO

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1 non-PAUSE user.

Marc A. Lehmann

NAME

Coro::Multicore - make coro threads on multiple cores with specially supported modules

SYNOPSIS

 # when you DO control the main event loop, e.g. in the main program

 use Coro::Multicore; # enable by default

 Coro::Multicore::scoped_disable;
 AE::cv->recv; # or EV::run, AnyEvent::Loop::run, Event::loop, ...

 # when you DO NOT control the event loop, e.g. in a module on CPAN
 # do nothing (see HOW TO USE IT) or something like this:

 use Coro::Multicore (); # disable by default

 async {
    Coro::Multicore::scoped_enable;

    # blocking is safe in your own threads
    ...
 };

DESCRIPTION

While Coro threads (unlike ithreads) provide real threads similar to pthreads, python threads and so on, they do not run in parallel to each other even on machines with multiple CPUs or multiple CPU cores.

This module lifts this restriction under two very specific but useful conditions: firstly, the coro thread executes in XS code and does not touch any perl data structures, and secondly, the XS code is specially prepared to allow this.

This means that, when you call an XS function of a module prepared for it, this XS function can execute in parallel to any other Coro threads. This is useful for both CPU bound tasks (such as cryptography) as well as I/O bound tasks (such as loading an image from disk). It can also be used to do stuff in parallel via APIs that were not meant for this, such as database accesses via DBI.

The mechanism to support this is easily added to existing modules and is independent of Coro or Coro::Multicore, and therefore could be used, without changes, with other, similar, modules, or even the perl core, should it gain real thread support anytime soon. See http://perlmulticore.schmorp.de/ for more info on how to prepare a module to allow parallel execution. Preparing an existing module is easy, doesn't add much overhead and no dependencies.

This module is an AnyEvent user (and also, if not obvious, uses Coro).

HOW TO USE IT

Quick explanation: decide whether you control the main program/the event loop and choose one of the two styles from the SYNOPSIS.

Longer explanation: There are two major modes this module can used in - supported operations run asynchronously either by default, or only when requested. The reason you might not want to enable this module for all operations by default is compatibility with existing code:

Since this module integrates into an event loop and you must not normally block and wait for something in an event loop callbacks. Now imagine somebody patches your favourite module (e.g. Digest::MD5) to take advantage of of the Perl Multicore API.

Then code that runs in an event loop callback and executes Digest::MD5::md5 would work fine without Coro::Multicore - it would simply calculate the MD5 digest and block execution of anything else. But with Coro::Multicore enabled, the same operation would try to run other threads. And when those wait for events, there is no event loop anymore, as the event loop thread is busy doing the MD5 calculation, leading to a deadlock.

USE IT IN THE MAIN PROGRAM

One way to avoid this is to not run perlmulticore enabled functions in any callbacks. A simpler way to ensure it works is to disable Coro::Multicore thread switching in event loop callbacks, and enable it everywhere else.

Therefore, if you control the event loop, as is usually the case when you write program and not a module, then you can enable Coro::Multicore by default, and disable it in your event loop thread:

   # example 1, separate thread for event loop

   use EV;
   use Coro;
   use Coro::Multicore;

   async {
      Coro::Multicore::scoped_disable;
      EV::run;
   };

   # do something else

   # example 2, run event loop as main program

   use EV;
   use Coro;
   use Coro::Multicore;

   Coro::Multicore::scoped_disable;

   ... initialisation

   EV::run;

The latter form is usually better and more idiomatic - the main thread is the best place to run the event loop.

Often you want to do some initialisation before running the event loop. The most efficient way to do that is to put your intialisation code (and main program) into its own thread and run the event loop in your main program:

   use AnyEvent::Loop;
   use Coro::Multicore; # enable by default

   async {
      load_data;
      do_other_init;
      bind_socket;
      ...
   };

   Coro::Multicore::scoped_disable;
   AnyEvent::Loop::run;

This has the effect of running the event loop first, so the initialisation code can block if it wants to.

If this is too cumbersome but you still want to make sure you can call blocking functions before entering the event loop, you can keep Coro::Multicore disabled till you cna run the event loop:

   use AnyEvent::Loop;
   use Coro::Multicore (); # disable by default

   load_data;
   do_other_init;
   bind_socket;
   ...

   Coro::Multicore::scoped_disable; # disable for event loop
   Coro::Multicore::enable 1; # enable for the rest of the program
   AnyEvent::Loop::run;

USE IT IN A MODULE

When you do not control the event loop, for example, because you want to use this from a module you published on CPAN, then the previous method doesn't work.

However, this is not normally a problem in practise - most modules only do work at request of the caller. In that case, you might not care whether it does block other threads or not, as this would be the callers responsibility (or decision), and by extension, a decision for the main program.

So unless you use XS and want your XS functions to run asynchronously, you don't have to worry about Coro::Multicore at all - if you happen to call XS functions that are multicore-enabled and your caller has configured things correctly, they will automatically run asynchronously. Or in other words: nothing needs to be done at all, which also means that this method works fine for existing pure-perl modules, without having to change them at all.

Only if your module runs it's own Coro threads could it be an issue - maybe your module implements some kind of job pool and relies on certain operations to run asynchronously. Then you can still use Coro::Multicore by not enabling it be default and only enabling it in your own threads:

   use Coro;
   use Coro::Multicore (); # note the () to disable by default

   async {
      Coro::Multicore::scoped_enable;

      # do things asynchronously by calling perlmulticore-enabled functions
   };

EXPORTS

This module does not (at the moment) export any symbols. It does, however, export "behaviour" - if you use the default import, then Coro::Multicore will be enabled for all threads and all callers in the whole program:

   use Coro::Multicore;

In a module where you don't control what else might be loaded and run, you might want to be more conservative, and not import anything. This has the effect of not enabling the functionality by default, so you have to enable it per scope:

   use Coro::Multicore ();

   sub myfunc {
      Coro::Multicore::scoped_enable;

      # from here to the end of this function, and in any functions
      # called from this function, tasks will be executed asynchronously.
   }

API FUNCTIONS

$previous = Coro::Multicore::enable [$enable]

This function enables (if $enable is true) or disables (if $enable is false) the multicore functionality globally. By default, it is enabled.

This can be used to effectively disable this module's functionality by default, and enable it only for selected threads or scopes, by calling Coro::Multicore::scoped_enable.

The function returns the previous value of the enable flag.

Coro::Multicore::scoped_enable

This function instructs Coro::Multicore to handle all requests executed in the current coro thread, from the call to the end of the current scope.

Calls to scoped_enable and scoped_disable don't nest very well at the moment, so don't nest them.

Coro::Multicore::scoped_disable

The opposite of Coro::Multicore::scope_disable: instructs Coro::Multicore to not handle the next multicore-enabled request.

THREAD SAFETY OF SUPPORTING XS MODULES

Just because an XS module supports perlmulticore might not immediately make it reentrant. For example, while you can (try to) call execute on the same database handle for the patched DBD::mysql (see the registry), this will almost certainly not work, despite DBD::mysql and libmysqlclient being thread safe and reentrant - just not on the same database handle.

Many modules have limitations such as these - some can only be called concurrently from a single thread as they use global variables, some can only be called concurrently on different handles (e.g. database connections for DBD modules, or digest objects for Digest modules), and some can be called at any time (such as the md5 function in Digest::MD5).

Generally, you only have to be careful with the very few modules that use global variables or rely on C libraries that aren't thread-safe, which should be documented clearly in the module documentation.

Most modules are either perfectly reentrant, or at least reentrant as long as you give every thread it's own handle object.

EXCEPTIONS AND THREAD CANCELLATION

Coro allows you to cancel threads even when they execute within an XS function (cancel vs. cancel methods). Similarly, Coro allows you to send exceptions (e.g. via the throw method) to threads executing inside an XS function.

While doing this is questionable and dangerous with normal Coro threads already, they are both supported in this module, although with potentially unwanted effects. The following describes the current implementation and is subject to change. It is described primarily so you can understand what went wrong, if things go wrong.

EXCEPTIONS

When a thread that has currently released the perl interpreter (e.g. because it is executing a perlmulticore enabled XS function) receives an exception, it will at first continue normally.

After acquiring the perl interpreter again, it will throw the exception it previously received. More specifically, when a thread calls perlinterp_acquire () and has received an exception, then perlinterp_acquire () will not return but instead die.

Most code that has been updated for perlmulticore support will not expect this, and might leave internal state corrupted to some extent.

CANCELLATION

Unsafe cancellation on a thread that has released the perl interpreter frees its resources, but let's the XS code continue at first. This should not lead to corruption on the perl level, as the code isn't allowed to touch perl data structures until it reacquires the interpreter.

The call to perlinterp_acquire () will then block indefinitely, leaking the (OS level) thread.

Safe cancellation will simply fail in this case, so is still "safe" to call.

INTERACTION WITH OTHER SOFTWARE

This module is very similar to other environments where perl interpreters are moved between threads, such as mod_perl2, and the same caveats apply.

I want to spell out the most important ones:

pthreads usage

Any creation of pthreads make it impossible to fork portably from a perl program, as forking from within a threaded program will leave the program in a state similar to a signal handler. While it might work on some platforms (as an extension), this might also result in silent data corruption. It also seems to work most of the time, so it's hard to test for this.

I recommend using something like AnyEvent::Fork, which can create subprocesses safely (via Proc::FastSpawn).

Similar issues exist for signal handlers, although this module works hard to keep safe perl signals safe.

module support

This module moves the same perl interpreter between different threads. Some modules might get confused by that (although this can usually be considered a bug). This is a rare case though.

event loop reliance

To be able to wake up programs waiting for results, this module relies on an active event loop (via AnyEvent). This is used to notify the perl interpreter when the asynchronous task is done.

Since event loops typically fail to work properly after a fork, this means that some operations that were formerly working will now hang after fork.

A workaround is to call Coro::Multicore::enable 0 after a fork to disable the module.

Future versions of this module might do this automatically.

BUGS

(OS-) threads are never released

At the moment, threads that were created once will never be freed. They will be reused for asynchronous requests, though, so as long as you limit the maximum number of concurrent asynchronous tasks, this will also limit the maximum number of threads created.

The idle threads are not necessarily using a lot of resources: on GNU/Linux + glibc, each thread takes about 8KiB of userspace memory + whatever the kernel needs (probably less than 8KiB).

Future versions will likely lift this limitation.

AnyEvent is initalised at module load time

AnyEvent is initialised on module load, as opposed to at a later time.

Future versions will likely change this.

AUTHOR

 Marc Lehmann <schmorp@schmorp.de>
 http://software.schmorp.de/pkg/AnyEvent-XSThreadPool.html

Additional thanks to Zsbán Ambrus, who gave considerable desing input for this module and the perl multicore specification.

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