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EV::Loop::Async - run an EV event loop asynchronously


  use EV::Loop::Async;
  my $loop = EV::Loop::Async::default;
  my $timer;
  my $flag;
  # create a watcher, but make sure the loop is locked
     $loop->scope_lock; # lock the loop structures
     $timer = $loop->timer (5, 1, sub { $flag = 1 });
     $loop->notify; # tell loop to take note of the timer
  1 while $flag; # $flag will be set asynchronously
  # implement a critical section, uninterrupted by any callbacks
     # critical section, no watcher callback interruptions
  # stop the timer watcher again - locking is required once more
     $loop->scope_lock; # lock the loop structures
     # no need to notify


This module implements a rather specialised event loop - it takes a normal EV event loop and runs it in a separate thread. That means it will poll for events even while your foreground Perl interpreter is busy (you don't need to have perls pseudo-threads enabled for this either).

Whenever the event loop detecs new events, it will interrupt perl and ask it to invoke all the pending watcher callbacks. This invocation will be "synchronous" (in the perl thread), but it can happen at any time.

See the documentation for Async::Interrupt for details on when and how your perl program can be interrupted (and how to avoid it), and how to integrate background event loops into foreground ones.


Why on earth...???

Sometimes you need lower latency for specific events, but it's too heavy to continuously poll for events. And perl already does this for you anyways, so this module only uses this existing mechanism.

When do I have to lock?

When in doubt, lock. Do not start or stop a watcher, do not create a watcher (unless with the _ns methods) and do not DESTROY an active watcher without locking either.

Any other event loop modifications need to be done while locked as well. So when in doubt, lock (best using scope_lock).

Why explicit locking?

Because I was too lazy to wrap everything and there are probably only a few people on this world using this module.


$loop = EV::Loop::Async::default

Return the default loop, usable by all programs. The default loop will be created on the first call to default by calling , and should be used by all programs unless they have special requirements.

The associated Async::Interrupt object is stored in $EV::Loop::Async::AI, and can be used to lock critical sections etc.


The default async loop, available after the first call to EV::Loop::Async::default.


The default loop's Async::Interrupt object, for easy access.

Example: create a section of code where no callback invocations will interrupt:

      # no default loop callbacks will be executed here.
      # the loop will not be locked, however.

Example: embed the default EV::Async::Loop loop into the default EV loop (note that it could be any other event loop as well).

   my $async_w = EV::io
                    sub { };
$loop = new EV::Loop::Async $flags, [Async-Interrupt-Arguments...]

This constructor:

1. creates a new EV::Loop (similar new EV::Loop).
2. creates a new Async::Interrupt object and attaches itself to it.
3. creates a new background thread.
4. runs $loop->run in that thread.

The resulting loop will be running and unlocked when it is returned.

Example: create a new loop, block it's interrupt object and embed it into the foreground AnyEvent event loop. This basically runs the EV::Loop::Async loop in a synchronous way inside another loop.

   my $loop  = new EV::Loop::Async 0;
   my $async = $loop->interrupt;


   my $async_w = AnyEvent->io (
      fh => $async->pipe_fileno,
      poll => "r",
      cb => sub {
         # temporarily unblock to handle events

Wake up the asynchronous loop. This is useful after registering a new watcher, to ensure that the background event loop integrates the new watcher(s) (which only happens when it iterates, which you can force by calling this method).

Without calling this method, the event loop eventually takes notice of new watchers, bit when this happens is not well-defined (can be instantaneous, or take a few hours).

No locking is required.

Example: lock the loop, create a timer, nudge the loop so it takes notice of the new timer, then evily busy-wait till the timer fires.

   my $timer;
   my $flag;

      $timer = $loop->timer (1, 0, sub { $flag = 1 });

   1 until $flag;

Lock/unlock the loop data structures. Since the event loop runs in a separate thread, you have to lock the loop data structures before accessing them in any way. Since I was lazy, you have to do this manually.

You must lock under the same conditions as you would have to lock the underlying C library, e.g. when starting or stopping watchers (but not when creating or destroying them, but note that create and destroy often starts and stops for you, in which case you have to lock).

When in doubt, lock.

See also the next method, $loop->scope_lock for a more failsafe way to lock parts of your code.

Note that there must be exactly one call of "unblock" for every previous call to "block" (i.e. calls can nest).


Calls lock immediately, and unlock automatically whent he current scope is left.

$loop->set_max_foreground_loops ($max_loops)

The background loop will immediately stop polling for new events after it has collected at least one new event, regardless of how long it then takes to actually handle them.

When Perl finally handles the events, there could be many more ready file descriptors. To improve latency and performance, you can ask EV::Loop::Async to loop an additional number of times in the foreground after invoking the callbacks, effectively doing the polling in the foreground.

The default is 0, meaning that no foreground polling will be done. A value of 1 means that, after handling the pending events, it will call $loop->loop (EV::LOOP_NONBLOCK) and handle the resulting events, if any. A value of 2 means that this will be iterated twice.

When a foreground event poll does not yield any new events, then no further iterations will be made, so this is only a maximum value of additional loop runs.

Take also note of the standard EV set_io_collect_interval functionality, which can achieve a similar, but different, effect - YMMV.


EV, Async::Interrupt.


 Marc Lehmann <>