--- cvsroot/Coro/README 2006/12/03 21:59:53 1.4 +++ cvsroot/Coro/README 2008/11/05 15:38:10 1.17 @@ -2,78 +2,185 @@ Coro - coroutine process abstraction SYNOPSIS - use Coro; - - async { - # some asynchronous thread of execution - }; - - # alternatively create an async coroutine like this: - - sub some_func : Coro { - # some more async code - } - - cede; + 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 coroutine + print "3\n"; + cede; # and again + + # use locking + use Coro::Semaphore; + my $lock = new Coro::Semaphore; + my $locked; + + $lock->down; + $locked = 1; + $lock->up; DESCRIPTION This module collection manages coroutines. Coroutines are similar to - threads but don't run in parallel. + threads but don't (in general) run in parallel at the same time even on + SMP machines. The specific flavor of coroutine used in this module also + guarantees you that it will not switch between coroutines unless + necessary, at easily-identified points in your program, so locking and + parallel access are rarely an issue, making coroutine programming much + safer and easier than threads programming. + + Unlike a normal perl program, however, coroutines allow you to have + 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 Coro::AnyEvent to learn more. + + Coroutines are also useful because Perl has no support for threads (the + so called "threads" that perl offers are nothing more than the (bad) + process emulation coming from the Windows platform: On standard + operating systems they serve no purpose whatsoever, except by making + your programs slow and making them use a lot of memory. Best disable + them when building perl, or aks your software vendor/distributor to do + it for you). In this module, coroutines are defined as "callchain + lexical variables - + @_ + $_ + $@ + $^W + C stack), that is, a coroutine has it's own - callchain, it's own set of lexicals and it's own set of perl's most - important global variables. - - $main - This coroutine represents the main program. - - $current (or as function: current) - The current coroutine (the last coroutine switched to). The initial - value is $main (of course). - - This variable is strictly *read-only*. It is provided for - performance reasons. If performance is not essentiel you are - encouraged to use the "Coro::current" function instead. - - $idle - A callback that is called whenever the scheduler finds no ready - coroutines to run. The default implementation prints "FATAL: - deadlock detected" and exits, because the program has no other way - to continue. + + @_ + $_ + $@ + $/ + C stack), that is, a coroutine has its own + callchain, its own set of lexicals and its own set of perls most + important global variables (see Coro::State for more configuration). + + $Coro::main + This variable stores the coroutine object that represents the main + program. While you cna "ready" it and do most other things you can + do to coroutines, it is mainly useful to compare again + $Coro::current, to see whether you are running in the main program + or not. + + $Coro::current + The coroutine object representing the current coroutine (the last + coroutine that the Coro scheduler switched to). The initial value is + $main (of course). + + This variable is strictly *read-only*. You can take copies of the + value stored in it and use it as any other coroutine object, but you + must not otherwise modify the variable itself. + + $Coro::idle + This variable is mainly useful to integrate Coro into event loops. + It is usually better to rely on Coro::AnyEvent or L"Coro::EV", as + this is pretty low-level functionality. + + This variable stores a callback that is called whenever the + scheduler finds no ready coroutines to run. The default + implementation prints "FATAL: deadlock detected" and exits, because + the program has no other way to continue. This hook is overwritten by modules such as "Coro::Timer" and - "Coro::Event" to wait on an external event that hopefully wake up a - coroutine so the scheduler can run it. + "Coro::AnyEvent" to wait on an external event that hopefully wake up + a coroutine so the scheduler can run it. - Please note that if your callback recursively invokes perl (e.g. for - event handlers), then it must be prepared to be called recursively. + Note that the callback *must not*, under any circumstances, block + the current coroutine. Normally, this is achieved by having an "idle + coroutine" that calls the event loop and then blocks again, and then + readying that coroutine in the idle handler. - STATIC METHODS - Static methods are actually functions that operate on the current - coroutine only. + See Coro::Event or Coro::AnyEvent for examples of using this + technique. + + Please note that if your callback recursively invokes perl (e.g. for + event handlers), then it must be prepared to be called recursively + itself. + SIMPLE COROUTINE CREATION async { ... } [@args...] - Create a new asynchronous coroutine and return it's coroutine object - (usually unused). When the sub returns the new coroutine is + Create a new coroutine and return it's coroutine object (usually + unused). The coroutine will be put into the ready queue, so it will + start running automatically on the next scheduler run. + + The first argument is a codeblock/closure that should be executed in + the coroutine. When it returns argument returns the coroutine is automatically terminated. - Calling "exit" in a coroutine will not work correctly, so do not do - that. + The remaining arguments are passed as arguments to the closure. + + See the "Coro::State::new" constructor for info about the coroutine + environment in which coroutines are executed. + + Calling "exit" in a coroutine will do the same as calling exit + outside the coroutine. Likewise, when the coroutine dies, the + program will exit, just as it would in the main program. - When the coroutine dies, the program will exit, just as in the main - program. + If you do not want that, you can provide a default "die" handler, or + simply avoid dieing (by use of "eval"). + + Example: Create a new coroutine that just prints its arguments. - # create a new coroutine that just prints its arguments async { print "@_\n"; } 1,2,3,4; + async_pool { ... } [@args...] + Similar to "async", but uses a coroutine pool, so you should not + call terminate or join on it (although you are allowed to), and you + get a coroutine that might have executed other code already (which + can be good or bad :). + + On the plus side, this function is faster than creating (and + destroying) a completly new coroutine, so if you need a lot of + generic coroutines in quick successsion, use "async_pool", not + "async". + + The code block is executed in an "eval" context and a warning will + be issued in case of an exception instead of terminating the + program, as "async" does. As the coroutine is being reused, stuff + like "on_destroy" will not work in the expected way, unless you call + terminate or cancel, which somehow defeats the purpose of pooling + (but is fine in the exceptional case). + + The priority will be reset to 0 after each run, tracing will be + disabled, the description will be reset and the default output + filehandle gets restored, so you can change all these. Otherwise the + coroutine will be re-used "as-is": most notably if you change other + per-coroutine global stuff such as $/ you *must needs* revert that + change, which is most simply done by using local as in: "local $/". + + The idle pool size is limited to 8 idle coroutines (this can be + adjusted by changing $Coro::POOL_SIZE), but there can be as many + non-idle coros as required. + + If you are concerned about pooled coroutines growing a lot because a + single "async_pool" used a lot of stackspace you can e.g. + "async_pool { terminate }" once per second or so to slowly replenish + the pool. In addition to that, when the stacks used by a handler + grows larger than 16kb (adjustable via $Coro::POOL_RSS) it will also + be destroyed. + + STATIC METHODS + Static methods are actually functions that operate on the current + coroutine. + schedule - Calls the scheduler. Please note that the current coroutine will 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 ready. + Calls the scheduler. The scheduler will find the next coroutine that + is to be run from the ready queue and switches to it. The next + coroutine to be run is simply the one with the highest priority that + is longest in its ready queue. If there is no coroutine ready, it + will clal the $Coro::idle hook. + + Please note that the current coroutine will *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 + "->ready", thus waking you up. + + This makes "schedule" *the* generic method to use to block the + current coroutine and wait for events: first you remember the + current coroutine in a variable, then arrange for some callback of + yours to call "->ready" on that once some event happens, and last + you call "schedule" to put yourself to sleep. Note that a lot of + things can wake your coroutine up, so you need to check whether the + event indeed happened, e.g. by storing the status in a variable. The canonical way to wait on external events is this: @@ -88,7 +195,7 @@ undef $current; }; - # call schedule until event occured. + # call schedule until event occurred. # in case we are woken up for other reasons # (current still defined), loop. Coro::schedule while $current; @@ -98,42 +205,90 @@ "Cede" to other coroutines. This function puts the current coroutine into the ready queue and calls "schedule", which has the effect of giving up the current "timeslice" to other coroutines of the same or - higher priority. + higher priority. Once your coroutine gets its turn again it will + automatically be resumed. + + This function is often called "yield" in other languages. + + Coro::cede_notself + Works like cede, but is not exported by default and will cede to + *any* coroutine, regardless of priority. This is useful sometimes to + ensure progress is made. terminate [arg...] Terminates the current coroutine with the given status values (see cancel). - # dynamic methods + killall + Kills/terminates/cancels all coroutines except the currently running + one. This is useful after a fork, either in the child or the parent, + as usually only one of them should inherit the running coroutines. + + Note that while this will try to free some of the main programs + resources, you cannot free all of them, so if a coroutine that is + not the main program calls this function, there will be some + one-time resource leak. COROUTINE METHODS - These are the methods you can call on coroutine objects. + These are the methods you can call on coroutine objects (or to create + them). new Coro \&sub [, @args...] - Create a new coroutine and return it. When the sub returns the + Create a new coroutine and return it. When the sub returns, the coroutine automatically terminates as if "terminate" with the returned values were called. To make the coroutine run you must first put it into the ready queue by calling the ready method. - Calling "exit" in a coroutine will not work correctly, so do not do - that. + See "async" and "Coro::State::new" for additional info about the + coroutine environment. $success = $coroutine->ready - Put the given coroutine into the ready queue (according to it's - priority) and return true. If the coroutine is already in the ready - queue, do nothing and return false. + Put the given coroutine into the end of its ready queue (there is + one queue for each priority) and return true. If the coroutine is + already in the ready queue, do nothing and return false. + + This ensures that the scheduler will resume this coroutine + automatically once all the coroutines of higher priority and all + coroutines of the same priority that were put into the ready queue + earlier have been resumed. $is_ready = $coroutine->is_ready - Return wether the coroutine is currently the ready queue or not, + Return whether the coroutine is currently the ready queue or not, $coroutine->cancel (arg...) Terminates the given coroutine and makes it return the given - arguments as status (default: the empty list). + arguments as status (default: the empty list). Never returns if the + coroutine is the current coroutine. + + $coroutine->throw ([$scalar]) + If $throw is specified and defined, it will be thrown as an + exception inside the coroutine at the next convenient point in time + (usually after it gains control at the next schedule/transfer/cede). + Otherwise clears the exception object. + + The exception object will be thrown "as is" with the specified + scalar in $@, i.e. if it is a string, no line number or newline will + be appended (unlike with "die"). + + This can be used as a softer means than "cancel" to ask a coroutine + to end itself, although there is no guarantee that the exception + will lead to termination, and if the exception isn't caught it might + well end the whole program. + + You might also think of "throw" as being the moral equivalent of + "kill"ing a coroutine with a signal (in this case, a scalar). $coroutine->join Wait until the coroutine terminates and return any values given to - the "terminate" or "cancel" functions. "join" can be called multiple - times from multiple coroutine. + the "terminate" or "cancel" functions. "join" can be called + concurrently from multiple coroutines, and all will be resumed and + given the status return once the $coroutine terminates. + + $coroutine->on_destroy (\&cb) + Registers a callback that is called when this coroutine gets + destroyed, but before it is joined. The callback gets passed the + terminate arguments, if any, and *must not* die, under any + circumstances. $oldprio = $coroutine->prio ($newprio) Sets (or gets, if the argument is missing) the priority of the @@ -166,46 +321,94 @@ this coroutine. This is just a free-form string you can associate with a coroutine. - UTILITY FUNCTIONS + This method simply sets the "$coroutine->{desc}" member to the given + string. You can modify this member directly if you wish. + + GLOBAL FUNCTIONS + Coro::nready + Returns the number of coroutines that are currently in the ready + state, i.e. that can be switched to by calling "schedule" directory + or indirectly. The value 0 means that the only runnable coroutine is + the currently running one, so "cede" would have no effect, and + "schedule" would cause a deadlock unless there is an idle handler + that wakes up some coroutines. + + my $guard = Coro::guard { ... } + This creates and returns a guard object. Nothing happens until the + object gets destroyed, in which case the codeblock given as argument + will be executed. This is useful to free locks or other resources in + case of a runtime error or when the coroutine gets canceled, as in + both cases the guard block will be executed. The guard object + supports only one method, "->cancel", which will keep the codeblock + from being executed. + + Example: set some flag and clear it again when the coroutine gets + canceled or the function returns: + + sub do_something { + my $guard = Coro::guard { $busy = 0 }; + $busy = 1; + + # do something that requires $busy to be true + } + unblock_sub { ... } This utility function takes a BLOCK or code reference and "unblocks" - it, returning the new coderef. This means that the new coderef will - return immediately without blocking, returning nothing, while the - original code ref will be called (with parameters) from within its - own coroutine. + 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 coroutine. - The reason this fucntion exists is that many event libraries (such + The reason this function exists is that many event libraries (such as the venerable Event module) are not coroutine-safe (a weaker form of thread-safety). This means you must not block within event - callbacks, otherwise you might suffer from crashes or worse. + callbacks, otherwise you might suffer from crashes or worse. The + only event library currently known that is safe to use without + "unblock_sub" is EV. This function allows your callbacks to block by executing them in another coroutine where it is safe to block. One example where blocking is handy is when you use the Coro::AIO functions to save - results to disk. + results to disk, for example. In short: simply use "unblock_sub { ... }" instead of "sub { ... }" when creating event callbacks that want to block. -BUGS/LIMITATIONS - - you must make very sure that no coro is still active on global - destruction. very bad things might happen otherwise (usually segfaults). + If your handler does not plan to block (e.g. simply sends a message + to another coroutine, or puts some other coroutine into the ready + queue), there is no reason to use "unblock_sub". + + Note that you also need to use "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 AnyEvent (and you use + Coro::AnyEvent) and it provides callbacks that are the result of + some event callback, then you must not block either, or use + "unblock_sub". - - this module is not thread-safe. You should only ever use this module - from the same thread (this requirement might be losened in the future - to allow per-thread schedulers, but Coro::State does not yet allow - this). +BUGS/LIMITATIONS + This module is not perl-pseudo-thread-safe. You should only ever use + this module from the same thread (this requirement might be removed in + the future to allow per-thread schedulers, but Coro::State does not yet + allow this). I recommend disabling thread support and using processes, + as this is much faster and uses less memory. SEE ALSO - Support/Utility: Coro::Cont, Coro::Specific, Coro::State, Coro::Util. + Event-Loop integration: Coro::AnyEvent, Coro::EV, Coro::Event. + + Debugging: Coro::Debug. + + Support/Utility: Coro::Specific, Coro::Util. Locking/IPC: Coro::Signal, Coro::Channel, Coro::Semaphore, Coro::SemaphoreSet, Coro::RWLock. - Event/IO: Coro::Timer, Coro::Event, Coro::Handle, Coro::Socket, - Coro::Select. + IO/Timers: Coro::Timer, Coro::Handle, Coro::Socket, Coro::AIO. + + Compatibility: Coro::LWP, Coro::BDB, Coro::Storable, Coro::Select. + + XS API: Coro::MakeMaker. - Embedding: + Low level Configuration, Coroutine Environment: Coro::State. AUTHOR Marc Lehmann