Coro
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=head1 NAME
Coro - the only real threads in perl
=head1 SYNOPSIS
use Coro;
async {
# some asynchronous thread of execution
print "2\n";
cede; # yield back to main
print "4\n";
};
print "1\n";
cede; # yield to coro
print "3\n";
cede; # and again
# use locking
my $lock = new Coro::Semaphore;
my $locked;
$lock->down;
$locked = 1;
$lock->up;
=head1 DESCRIPTION
For a tutorial-style introduction, please read the L<Coro::Intro>
manpage. This manpage mainly contains reference information.
This module collection manages continuations in general, most often in
the form of cooperative threads (also called coros, or simply "coro"
in the documentation). They are similar to kernel threads but don't (in
general) run in parallel at the same time even on SMP machines. The
specific flavor of thread offered by this module also guarantees you that
it will not switch between threads unless necessary, at easily-identified
points in your program, so locking and parallel access are rarely an
issue, making thread programming much safer and easier than using other
thread models.
Unlike the so-called "Perl threads" (which are not actually real threads
but only the windows process emulation (see section of same name for
more details) ported to UNIX, and as such act as processes), Coro
provides a full shared address space, which makes communication between
threads very easy. And coro threads are fast, too: disabling the Windows
process emulation code in your perl and using Coro can easily result in
a two to four times speed increase for your programs. A parallel matrix
multiplication benchmark (very communication-intensive) runs over 300
times faster on a single core than perls pseudo-threads on a quad core
using all four cores.
Coro achieves that by supporting multiple running interpreters that share
data, which is especially useful to code pseudo-parallel processes and
for event-based programming, such as multiple HTTP-GET requests running
concurrently. See L<Coro::AnyEvent> to learn more on how to integrate Coro
into an event-based environment.
In this module, a thread is defined as "callchain + lexical variables +
some package variables + C stack), that is, a thread has its own callchain,
its own set of lexicals and its own set of perls most important global
variables (see L<Coro::State> for more configuration and background info).
See also the C<SEE ALSO> section at the end of this document - the Coro
module family is quite large.
=head1 CORO THREAD LIFE CYCLE
During the long and exciting (or not) life of a coro thread, it goes
through a number of states:
=over 4
=item 1. Creation
The first thing in the life of a coro thread is it's creation -
obviously. The typical way to create a thread is to call the C<async
BLOCK> function:
async {
# thread code goes here
};
You can also pass arguments, which are put in C<@_>:
}
=back
=head1 STATIC METHODS
Static methods are actually functions that implicitly operate on the
current coro.
=over 4
=item schedule
Calls the scheduler. The scheduler will find the next coro that is
to be run from the ready queue and switches to it. The next coro
to be run is simply the one with the highest priority that is longest
in its ready queue. If there is no coro ready, it will call the
C<$Coro::idle> hook.
Please note that the current coro will I<not> be put into the ready
queue, so calling this function usually means you will never be called
again unless something else (e.g. an event handler) calls C<< ->ready >>,
thus waking you up.
This makes C<schedule> I<the> generic method to use to block the current
coro and wait for events: first you remember the current coro in
a variable, then arrange for some callback of yours to call C<< ->ready
>> on that once some event happens, and last you call C<schedule> to put
yourself to sleep. Note that a lot of things can wake your coro up,
so you need to check whether the event indeed happened, e.g. by storing the
status in a variable.
See B<HOW TO WAIT FOR A CALLBACK>, below, for some ways to wait for callbacks.
=item cede
"Cede" to other coros. This function puts the current coro into
the ready queue and calls C<schedule>, which has the effect of giving
up the current "timeslice" to other coros of the same or higher
priority. Once your coro gets its turn again it will automatically be
resumed.
This function is often called C<yield> in other languages.
=item Coro::cede_notself
Works like cede, but is not exported by default and will cede to I<any>
coro, regardless of priority. This is useful sometimes to ensure
progress is made.
=item terminate [arg...]
Terminates the current coro with the given status values (see
L<cancel>). The values will not be copied, but referenced directly.
=item Coro::on_enter BLOCK, Coro::on_leave BLOCK
These function install enter and leave winders in the current scope. The
enter block will be executed when on_enter is called and whenever the
current coro is re-entered by the scheduler, while the leave block is
executed whenever the current coro is blocked by the scheduler, and
also when the containing scope is exited (by whatever means, be it exit,
die, last etc.).
I<Neither invoking the scheduler, nor exceptions, are allowed within those
BLOCKs>. That means: do not even think about calling C<die> without an
eval, and do not even think of entering the scheduler in any way.
Since both BLOCKs are tied to the current scope, they will automatically
be removed when the current scope exits.
These functions implement the same concept as C<dynamic-wind> in scheme
does, and are useful when you want to localise some resource to a specific
coro.
They slow down thread switching considerably for coros that use them
(about 40% for a BLOCK with a single assignment, so thread switching is
still reasonably fast if the handlers are fast).
These functions are best understood by an example: The following function
will change the current timezone to "Antarctica/South_Pole", which
requires a call to C<tzset>, but by using C<on_enter> and C<on_leave>,
which remember/change the current timezone and restore the previous
value, respectively, the timezone is only changed for the coro that
installed those handlers.
use POSIX qw(tzset);
async {
my $old_tz; # store outside TZ value here
Coro::on_enter {
$old_tz = $ENV{TZ}; # remember the old value
$ENV{TZ} = "Antarctica/South_Pole";
tzset; # enable new value
};
Coro::on_leave {
$ENV{TZ} = $old_tz;
tzset; # restore old value
};
# at this place, the timezone is Antarctica/South_Pole,
# without disturbing the TZ of any other coro.
};
This can be used to localise about any resource (locale, uid, current
working directory etc.) to a block, despite the existence of other
coros.
Another interesting example implements time-sliced multitasking using
interval timers (this could obviously be optimised, but does the job):
# "timeslice" the given block
sub timeslice(&) {
use Time::HiRes ();
Coro::on_enter {
# on entering the thread, we set an VTALRM handler to cede
$SIG{VTALRM} = sub { cede };
sub my_long_function {
local $Coro::current->{desc} = "now in my_long_function";
...
$Coro::current->{desc} = "my_long_function: phase 1";
...
$Coro::current->{desc} = "my_long_function: phase 2";
...
}
=cut
sub desc {
my $old = $_[0]{desc};
$_[0]{desc} = $_[1] if @_ > 1;
$old;
}
sub transfer {
require Carp;
Carp::croak ("You must not call ->transfer on Coro objects. Use Coro::State objects or the ->schedule_to method. Caught");
}
=back
=head1 GLOBAL FUNCTIONS
=over 4
=item Coro::nready
Returns the number of coro that are currently in the ready state,
i.e. that can be switched to by calling C<schedule> directory or
indirectly. The value C<0> means that the only runnable coro is the
currently running one, so C<cede> would have no effect, and C<schedule>
would cause a deadlock unless there is an idle handler that wakes up some
coro.
=item my $guard = Coro::guard { ... }
This function still exists, but is deprecated. Please use the
C<Guard::guard> function instead.
=cut
BEGIN { *guard = \&Guard::guard }
=item unblock_sub { ... }
This utility function takes a BLOCK or code reference and "unblocks" it,
returning a new coderef. Unblocking means that calling the new coderef
will return immediately without blocking, returning nothing, while the
original code ref will be called (with parameters) from within another
coro.
The reason this function exists is that many event libraries (such as
the venerable L<Event|Event> module) are not thread-safe (a weaker form
of reentrancy). This means you must not block within event callbacks,
otherwise you might suffer from crashes or worse. The only event library
currently known that is safe to use without C<unblock_sub> is L<EV> (but
you might still run into deadlocks if all event loops are blocked).
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.
This function allows your callbacks to block by executing them in another
coro where it is safe to block. One example where blocking is handy
is when you use the L<Coro::AIO|Coro::AIO> functions to save results to
disk, for example.
In short: simply use C<unblock_sub { ... }> instead of C<sub { ... }> when
creating event callbacks that want to block.
If your handler does not plan to block (e.g. simply sends a message to
another coro, or puts some other coro into the ready queue), there is
no reason to use C<unblock_sub>.
Note that you also need to use C<unblock_sub> for any other callbacks that
are indirectly executed by any C-based event loop. For example, when you
use a module that uses L<AnyEvent> (and you use L<Coro::AnyEvent>) and it
provides callbacks that are the result of some event callback, then you
must not block either, or use C<unblock_sub>.
=cut
our @unblock_queue;
# we create a special coro because we want to cede,
# to reduce pressure on the coro pool (because most callbacks
# return immediately and can be reused) and because we cannot cede
# inside an event callback.
our $unblock_scheduler = new Coro sub {
while () {
while (my $cb = pop @unblock_queue) {
&async_pool (@$cb);
# for short-lived callbacks, this reduces pressure on the coro pool
# as the chance is very high that the async_poll coro will be back
# in the idle state when cede returns
cede;
}
schedule; # sleep well
}
};
$unblock_scheduler->{desc} = "[unblock_sub scheduler]";
sub unblock_sub(&) {
my $cb = shift;
sub {
unshift @unblock_queue, [$cb, @_];
$unblock_scheduler->ready;
}
}
=item $cb = rouse_cb
Create and return a "rouse callback". That's a code reference that,
when called, will remember a copy of its arguments and notify the owner
coro of the callback.
See the next function.
( run in 2.372 seconds using v1.01-cache-2.11-cpan-98e64b0badf )