NAME¶
Coro::State - first class continuations
SYNOPSIS¶
use Coro::State;
$new = new Coro::State sub {
print "in coro (called with @_), switching back\n";
$new->transfer ($main);
print "in coro again, switching back\n";
$new->transfer ($main);
}, 5;
$main = new Coro::State;
print "in main, switching to coro\n";
$main->transfer ($new);
print "back in main, switch to coro again\n";
$main->transfer ($new);
print "back in main\n";
DESCRIPTION¶
This module implements coro objects. Coros, similar to threads and
continuations, allow you to run more than one "thread of execution"
in parallel. Unlike so-called "kernel" threads, there is no
parallelism and only voluntary switching is used so locking problems are
greatly reduced. The latter is called "cooperative" threading as
opposed to "preemptive" threading.
This can be used to implement non-local jumps, exception handling, continuation
objects and more.
This module provides only low-level functionality useful to build other
abstractions, such as threads, generators or coroutines. See Coro and related
modules for a higher level threads abstraction including a scheduler.
MODEL¶
Coro::State implements two different thread models: Perl and C. The C threads
(called cctx's) are basically simplified perl interpreters
running/interpreting the Perl threads. A single interpreter can run any number
of Perl threads, so usually there are very few C threads.
When Perl code calls a C function (e.g. in an extension module) and that C
function then calls back into Perl or transfers control to another thread, the
C thread can no longer execute other Perl threads, so it stays tied to the
specific thread until it returns to the original Perl caller, after which it
is again available to run other Perl threads.
The main program always has its own "C thread" (which really is *the*
Perl interpreter running the whole program), so there will always be at least
one additional C thread. You can use the debugger (see Coro::Debug) to find
out which threads are tied to their cctx and which aren't.
MEMORY CONSUMPTION¶
A newly created Coro::State that has not been used only allocates a relatively
small (a hundred bytes) structure. Only on the first "transfer" will
perl allocate stacks (a few kb, 64 bit architectures use twice as much, i.e. a
few kb :) and optionally a C stack/thread (cctx) for threads that recurse
through C functions. All this is very system-dependent. On my x86-pc-linux-gnu
system this amounts to about 2k per (non-trivial but simple) Coro::State.
You can view the actual memory consumption using Coro::Debug. Keep in mind that
a for loop or other block constructs can easily consume 100-200 bytes per
nesting level.
GLOBAL VARIABLES¶
- $Coro::State::DIEHOOK
- This works similarly to $SIG{__DIE__} and is used as the default die hook
for newly created Coro::States. This is useful if you want some generic
logging function that works for all threads that don't set their own hook.
When Coro::State is first loaded it will install these handlers for the main
program, too, unless they have been overwritten already.
The default handlers provided will behave like the built-in ones (as if they
weren't there).
If you don't want to exit your program on uncaught exceptions, you must not
return from your die hook - call "Coro::terminate" instead.
Note 1: You must store a valid code reference in these variables,
"undef" will not do.
Note 2: The value of this variable will be shared among all threads, so
changing its value will change it in all threads that don't have their own
die handler.
- $Coro::State::WARNHOOK
- Similar to above die hook, but augments $SIG{__WARN__}.
Coro::State METHODS¶
- $coro = new Coro::State [$coderef[, @args...]]
- Create a new Coro::State thread object and return it. The first
"transfer" call to this thread will start execution at the given
coderef, with the given arguments.
Note that the arguments will not be copied. Instead, as with normal function
calls, the thread receives passed arguments by reference, so make sure you
don't change them in unexpected ways.
Returning from such a thread is NOT supported. Neither is calling
"exit" or throwing an uncaught exception. The following
paragraphs describe what happens in current versions of Coro.
If the subroutine returns the program will be terminated as if execution of
the main program ended.
If it throws an exception the program will terminate unless the exception is
caught, exactly like in the main program.
Calling "exit" in a thread does the same as calling it in the main
program, but due to libc bugs on many BSDs, this doesn't work reliable
everywhere.
If the coderef is omitted this function will create a new "empty"
thread, i.e. a thread that cannot be transferred to but can be used to
save the current thread state in (note that this is dangerous, as no
reference is taken to ensure that the "current thread state"
survives, the caller is responsible to ensure that the cloned state does
not go away).
The returned object is an empty hash which can be used for any purpose
whatsoever, for example when subclassing Coro::State.
Certain variables are "localised" to each thread, that is, certain
"global" variables are actually per thread. Not everything that
would sensibly be localised currently is, and not everything that is
localised makes sense for every application, and the future might bring
changes.
The following global variables can have different values per thread, and
have the stated initial values:
Variable Initial Value
@_ whatever arguments were passed to the Coro
$_ undef
$@ undef
$/ "\n"
$SIG{__DIE__} aliased to $Coro::State::DIEHOOK(*)
$SIG{__WARN__} aliased to $Coro::State::WARNHOOK(*)
(default fh) *STDOUT
$^H, %^H zero/empty.
$1, $2... all regex results are initially undefined
(*) reading the value from %SIG is not supported, but local'ising is.
If you feel that something important is missing then tell me. Also remember
that every function call that might call "transfer" (such as
"Coro::Channel::put") might clobber any global and/or special
variables. Yes, this is by design ;) You can always create your own
process abstraction model that saves these variables.
The easiest way to do this is to create your own scheduling primitive like
in the code below, and use it in your threads:
sub my_cede {
local ($;, ...);
Coro::cede;
}
Another way is to use dynamic winders, see "Coro::on_enter" and
"Coro::on_leave" for this.
Yet another way that works only for variables is
"->swap_sv".
- $prev->transfer ($next)
- Save the state of the current subroutine in $prev and switch to the thread
saved in $next.
The "state" of a subroutine includes the scope, i.e. lexical
variables and the current execution state (subroutine, stack).
- $state->throw ([$scalar])
- $state->is_new
- $state->is_zombie
- See the corresponding method(s) for Coro objects.
- $state->cancel
- Forcefully destructs the given Coro::State. While you can keep the
reference, and some memory is still allocated, the Coro::State object is
effectively dead, destructors have been freed, it cannot be transferred to
anymore, it's pushing up the daisies.
- $state->call ($coderef)
- Try to call the given $coderef in the context of the given state. This
works even when the state is currently within an XS function, and can be
very dangerous. You can use it to acquire stack traces etc. (see the
Coro::Debug module for more details). The coderef MUST NOT EVER transfer
to another state.
- $state->eval ($string)
- Like "call", but eval's the string. Dangerous.
- $state->swap_defsv
- $state->swap_defav
- Swap the current $_ (swap_defsv) or @_ (swap_defav) with the equivalent in
the saved state of $state. This can be used to give the coro a defined
content for @_ and $_ before transfer'ing to it.
- $state->swap_sv (\$sv, \$swap_sv)
- This (very advanced) function can be used to make any variable
local to a thread.
It works by swapping the contents of $sv and $swap_sv each time the thread
is entered and left again, i.e. it is similar to:
$tmp = $sv; $sv = $swap_sv; $swap_sv = $tmp;
Except that it doesn't make an copies and works on hashes and even more
exotic values (code references!).
When called on the current thread (i.e. from within the thread that will
receive the swap_sv), then this method acts as if it was called from
another thread, i.e. after adding the two SV's to the threads swap list
their values will be swapped.
Needless to say, this function can be very very dangerous: you can easily
swap a hash with a reference (i.e. %hash becomes a reference), and
perl will not like this at all.
It will also swap "magicalness" - so when swapping a builtin perl
variable (such as $.), it will lose it's magicalness, which, again, perl
will not like, so don't do it.
Lastly, the $swap_sv itself will be used, not a copy, so make sure you give
each thread it's own $swap_sv instance.
It is, however, quite safe to swap some normal variable with another. For
example, PApp::SQL stores the default database handle in $PApp::SQL::DBH.
To make this a per-thread variable, use this:
my $private_dbh = ...;
$coro->swap_sv (\$PApp::SQL::DBH, \$private_dbh);
This results in $PApp::SQL::DBH having the value of $private_dbh while it
executes, and whatever other value it had when it doesn't execute.
You can also swap hashes and other values:
my %private_hash;
$coro->swap_sv (\%some_hash, \%private_hash);
- $bytes = $state->rss
- Returns the memory allocated by the coro (which includes static
structures, various perl stacks but NOT local variables, arguments or any
C context data). This is a rough indication of how much memory it might
use.
- ($real, $cpu) = $state->times
- Returns the real time and cpu times spent in the given $state. See
"Coro::State::enable_times" for more info.
- $state->trace ($flags)
- Internal function to control tracing. I just mention this so you can stay
away from abusing it.
METHODS FOR C CONTEXTS
Most coros only consist of some Perl data structures - transferring to a coro
just reconfigures the interpreter to continue somewhere else.
However. this is not always possible: For example, when Perl calls a C/XS
function (such as an event loop), and C then invokes a Perl callback,
reconfiguring the interpreter is not enough. Coro::State detects these cases
automatically, and attaches a C-level thread to each such Coro::State object,
for as long as necessary.
The C-level thread structure is called "C context" (or cctxt for
short), and can be quite big, which is why Coro::State only creates them as
needed and can run many Coro::State's on a single cctxt.
This is mostly transparent, so the following methods are rarely needed.
- $state->has_cctx
- Returns whether the state currently uses a cctx/C context. An active state
always has a cctx, as well as the main program. Other states only use a
cctxts when needed.
- Coro::State::force_cctx
- Forces the allocation of a private cctxt for the currently executing
Coro::State even though it would not normally ned one. Apart from
benchmarking or testing Coro itself, there is little point in doing so,
however.
- $ncctx = Coro::State::cctx_count
- Returns the number of C contexts allocated. If this number is very high
(more than a dozen) it might be beneficial to identify points of C-level
recursion (Perl calls C/XS, which calls Perl again which switches coros -
this forces an allocation of a C context) in your code and moving this
into a separate coro.
- $nidle = Coro::State::cctx_idle
- Returns the number of allocated but idle (currently unused and free for
reuse) C contexts.
- $old = Coro::State::cctx_max_idle [$new_count]
- Coro caches C contexts that are not in use currently, as creating them
from scratch has some overhead.
This function returns the current maximum number of idle C contexts and
optionally sets the new amount. The count must be at least 1, with the
default being 4.
- $old = Coro::State::cctx_stacksize [$new_stacksize]
- Returns the current C stack size and optionally sets the new
minimum stack size to $new_stacksize (in units of pointer sizes,
i.e. typically 4 on 32 bit and 8 on 64 bit hosts). Existing stacks will
not be changed, but Coro will try to replace smaller stacks as soon as
possible. Any Coro::State that starts to use a stack after this call is
guaranteed this minimum stack size.
Please note that coros will only need to use a C-level stack if the
interpreter recurses or calls a function in a module that calls back into
the interpreter, so use of this feature is usually never needed.
FUNCTIONS¶
- @states = Coro::State::list
- Returns a list of all Coro::State objects currently allocated. This
includes all derived objects (such as Coro threads).
- $was_enabled = Coro::State::enable_times [$enable]
- Enables/disables/queries the current state of per-thread real and cpu-time
gathering.
When enabled, the real time and the cpu time (user + system time) spent in
each thread is accumulated. If disabled, then the accumulated times will
stay as they are (they start at 0).
Currently, cpu time is only measured on GNU/Linux systems, all other systems
only gather real time.
Enabling time profiling slows down thread switching by a factor of 2 to 10,
depending on platform on hardware.
The times will be displayed when running "Coro::Debug::command
"ps"", and can be queried by calling
"$state->times".
CLONING
- $clone = $state->clone
- This exciting method takes a Coro::State object and clones it, i.e., it
creates a copy. This makes it possible to restore a state more than once,
and even return to states that have returned or have been terminated.
Since its only known purpose is for intellectual self-gratification, and
because it is a difficult piece of code, it is not enabled by default, and
not supported.
Here are a few little-known facts: First, coros *are* full/true/real
continuations. Secondly Coro::State objects (without clone) *are* first
class continuations. Thirdly, nobody has ever found a use for the full
power of call/cc that isn't better (faster, easier, more efficiently)
implemented differently, and nobody has yet found a useful control
construct that can't be implemented without it already, just much faster
and with fewer resources. And lastly, Scheme's call/cc doesn't support
using call/cc to implement threads.
Among the games you can play with this is implementing a scheme-like
call-with-current-continuation, as the following code does (well, with
small differences).
# perl disassociates from local lexicals on frame exit,
# so use a global variable for return values.
my @ret;
sub callcc($@) {
my ($func, @arg) = @_;
my $continuation = new Coro::State;
$continuation->transfer (new Coro::State sub {
my $escape = sub {
@ret = @_;
Coro::State->new->transfer ($continuation->clone);
};
$escape->($func->($escape, @arg));
});
my @ret_ = @ret; @ret = ();
wantarray ? @ret_ : pop @ret_
}
Which could be used to implement a loop like this:
async {
my $n;
my $l = callcc sub { $_[0] };
$n++;
print "iteration $n\n";
$l->($l) unless $n == 10;
};
If you find this confusing, then you already understand the coolness of
call/cc: It can turn anything into spaghetti code real fast.
Besides, call/cc is much less useful in a Perl-like dynamic language (with
references, and its scoping rules) then in, say, scheme.
Now, the known limitations of "clone":
It probably only works on perl 5.10; it cannot clone a coro inside the
substition operator (but windows perl can't fork from there either) and
some other contexts, and "abort ()" is the preferred mechanism
to signal errors. It cannot clone a state that has a c context attached
(implementing clone on the C level is too hard for me to even try), which
rules out calling call/cc from the main coro. It cannot clone a context
that hasn't even been started yet. It doesn't work with
"-DDEBUGGING" (but what does). It probably also leaks, and
sometimes triggers a few assertions inside Coro. Most of these limitations
*are* fixable with some effort, but that's pointless just to make a point
that it could be done.
The current implementation could without doubt be optimised to be a
constant-time operation by doing lazy stack copying, if somebody were
insane enough to invest the time.
BUGS¶
This module is not thread-safe. You must only ever use this module from the same
thread (this requirement might be removed in the future).
SEE ALSO¶
Coro.
AUTHOR¶
Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de/
