Coro::AnyEvent - integrate threads into AnyEvent
use Coro;
use AnyEvent;
# using both Coro and AnyEvent will automatically load Coro::AnyEvent
# or load it manually for its utility functions:
use Coro::AnyEvent;
Coro::AnyEvent::sleep 5; # block current thread for 5s
Coro::AnyEvent::poll; # poll for new events once
Coro::AnyEvent::idle; # block until process no longer busy
Coro::AnyEvent::idle_upto 5; # same, but only up to 5 seconds
Coro::AnyEvent::readable $fh, 60
or die "fh didn't become readable within 60 seconds\n";
When one naively starts to use threads in Perl, one will quickly run into the
problem that threads which block on a syscall (sleeping, reading from a socket
etc.) will block all threads.
If one then uses an event loop, the problem is that the event loop has no
knowledge of threads and will not run them before it polls for new events,
again blocking the whole process.
This module integrates threads into any event loop supported by AnyEvent,
combining event-based programming with coroutine-based programming in a
natural way.
As of Coro 5.21 and newer, this module gets loaded automatically when AnyEvent
initialises itself and Coro is used in the same process, thus there is no need
to load it manually if you just want your threads to coexist with AnyEvent.
If you want to use any functions from this module, you of course still need to
"use Coro::AnyEvent", just as with other perl modules.
Also, this module autodetects the event loop used (by relying on AnyEvent) and
will either automatically defer to the high-performance Coro::EV or
Coro::Event modules, or will use a generic integration method that should work
with any event loop supported by AnyEvent.
For performance reasons, it is recommended that the main program or something
else run the event loop of the event model you use, i.e.
use Gtk2; # <- the event model
use AnyEvent;
use Coro:
# initialise stuff
async { ... };
# now run mainloop of Gtk2
main Gtk2;
You can move the event loop into a thread as well, although this tends to get
confusing:
use Gtk2;
use AnyEvent;
use Coro:
async { main Gtk2 };
# do other things...
while () {
use Coro::AnyEvent;
Coro::AnyEvent::sleep 1;
print "ping...\n";
}
You can also do nothing, in which case Coro::AnyEvent will invoke the event loop
as needed, which is less efficient, but sometimes very convenient.
What you
MUST NOT EVER DO is to block inside an event loop callback. The
reason is that most event loops are not reentrant and this can cause a
deadlock at best and corrupt memory at worst.
Coro will try to catch you when you block in the event loop ("FATAL:
$Coro::IDLE blocked itself"), but this is just best effort and only works
when you do not run your own event loop.
To avoid this problem, start a new thread (e.g. with
"Coro::async_pool") or use "Coro::unblock_sub" to run
blocking tasks.
If you need to wait for a single event, the rouse functions will come in handy
(see the Coro manpage for details):
# wait for single SIGINT
{
my $int_w = AnyEvent->signal (signal => "INT", cb => Coro::rouse_cb);
Coro::rouse_wait;
}
Keep in mind that, as shipped, Coro and Coro::AnyEvent only work with AnyEvent,
and only when AnyEvent is actually used (i.e. initialised), so this will not
work:
# does not work: EV without AnyEvent is not recognised
use EV;
use Coro;
EV::loop;
And neither does this, unless you actually
use AnyEvent for something:
# does not work: AnyEvent must be initialised (e.g. by creating watchers)
use EV;
use AnyEvent;
use Coro;
EV::loop;
This does work, however, because you create a watcher (condvars work, too), thus
forcing AnyEvent to initialise itself:
# does work: AnyEvent is actually used
use EV;
use AnyEvent;
use Coro;
my $timer = AE::timer 1, 1, sub { };
EV::loop;
And if you want to use AnyEvent just to bridge between Coro and your event model
of choice, you can simply force it to initialise itself, like this:
# does work: AnyEvent is initialised manually
use POE;
use AnyEvent;
use Coro;
AnyEvent::detect; # force AnyEvent to integrate Coro into POE
POE::Kernel->run;
Coro::AnyEvent also offers a few functions that might be useful.
- Coro::AnyEvent::poll
- This call will block the current thread until the event
loop has polled for potential new events and instructs the event loop to
poll for new events once, without blocking.
Note that this call will not actually execute the poll, nor will it wait
until there are some events, just block until the event loop has polled
for new events, so other threads will have a chance to run.
This is useful when you have a thread that does some computations, but you
still want to poll for new events from time to time. Simply call
"poll" from time to time:
my $long_calc = async {
for (1..10000) {
Coro::AnyEvent::poll;
# do some stuff, make sure it takes at least 0.001s or so
}
}
Although you should also consider "idle" or "idle_upto"
in such cases.
- Coro::AnyEvent::sleep $seconds
- This blocks the current thread for at least the given
number of seconds.
- Coro::AnyEvent::idle
- This call is similar to "poll" in that it will
also poll for events. Unlike "poll", it will only resume the
thread once there are no events to handle anymore, i.e. when the process
is otherwise idle.
This is good for background threads that shouldn't use CPU time when
foreground jobs are ready to run.
- Coro::AnyEvent::idle_upto $seconds
- Like "idle", but with a maximum waiting time.
If your process is busy handling events, calling "idle" can mean
that your thread will never be resumed. To avoid this, you can use
"idle_upto" and specify a timeout, after which your thread will
be resumed even if the process is completely busy.
- Coro::AnyEvent::readable $fh_or_fileno[, $timeout]
- Coro::AnyEvent::writable $fh_or_fileno[, $timeout]
- Blocks the current thread until the given file handle (or
file descriptor) becomes readable (or writable), or the given timeout has
elapsed, whichever happens first. No timeout counts as infinite timeout.
Returns true when the file handle became ready, false when a timeout
occurred.
Note that these functions are quite inefficient as compared to using a
single watcher (they recreate watchers on every invocation) or compared to
using Coro::Handle.
Note also that they only work for sources that have reasonable non-blocking
behaviour (e.g. not files).
Example: wait until STDIN becomes readable, then quit the program.
use Coro::AnyEvent;
print "press enter to quit...\n";
Coro::AnyEvent::readable *STDIN;
exit 0;
Unfortunately, few event loops (basically only EV and Event) support the kind of
integration required for smooth operations well, and consequently, AnyEvent
cannot completely offer the functionality required by this module, so we need
to improvise.
Here is what this module does when it has to work with other event loops:
- •
- run ready threads before blocking the process
Each time a thread is put into the ready queue (and there are no other
threads in the ready queue), a timer with an "after" value of 0
is registered with AnyEvent.
This creates something similar to an idle watcher, i.e. a watcher
that keeps the event loop from blocking but still polls for new events.
(Unfortunately, some badly designed event loops (e.g. Event::Lib) don't
support a timeout of 0 and will always block for a bit).
The callback for that timer will "cede" to other threads of the
same or higher priority for as long as such threads exists. This has the
effect of running all threads that have work to do until all threads block
to wait for external events.
If no threads of equal or higher priority are ready, it will cede to any
thread, but only once. This has the effect of running lower-priority
threads as well, but it will not keep higher priority threads from
receiving new events.
The priority used is simply the priority of the thread that runs the event
loop, usually the main program, which usually has a priority of 0. Note
that Coro::AnyEvent does not run an event loop for you, so unless
the main program runs one, there will simply be no event loop to
"cede" to (event handling will still work, somewhat
inefficiently, but any thread will have a higher priority than event
handling in that case).
- •
- provide a suitable idle callback.
In addition to hooking into "ready", this module will also provide
a $Coro::idle handler that runs the event loop. It is best not to take
advantage of this too often, as this is rather inefficient, but it should
work perfectly fine.
- •
- provide overrides for AnyEvent's condvars
This module installs overrides for AnyEvent's condvars. That is, when the
module is loaded it will provide its own condition variables. This makes
them coroutine-safe, i.e. you can safely block on them from within a
coroutine.
- •
- lead to data corruption or worse
As "unblock_sub" cannot be used by this module (as it is the
module that implements it, basically), you must not call into the event
loop recursively from any coroutine. This is not usually a difficult
restriction to live with, just use condvars, "unblock_sub" or
other means of inter-coroutine-communications.
If you use a module that supports AnyEvent (or uses the same event loop as
AnyEvent, making it implicitly compatible), and it offers callbacks of any
kind, then you must not block in them, either (or use e.g.
"unblock_sub"), see the description of "unblock_sub"
in the Coro module.
This also means that you should load the module as early as possible, as
only condvars created after this module has been loaded will work
correctly.
AnyEvent, to see which event loops are supported, Coro::EV and Coro::Event for
more efficient and more correct solutions (they will be used automatically if
applicable).
Marc A. Lehmann <[email protected]>
http://software.schmorp.de/pkg/Coro.html