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Death to tasklets! Why killing tasklets is good practice.

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Presentation on theme: "Death to tasklets! Why killing tasklets is good practice."— Presentation transcript:

1 Death to tasklets! Why killing tasklets is good practice

2 History Multiprocessing – Master process spawns off multiple processes – Waits for them to finish Multi-threading – Within a process, threads are spawned – main thread waits for their results

3 A typical problem 1.Main process starts a server process 2.Main process wants to stop server process Get request Process request Finish request StartStop

4 How?

5 Processes: – Unix signals: SIGINT, can cause system calls to fail with an EINTERRUPTED error code. SIGKILL or SIGTERM just kills them outright Files are closed, memory is released. Buffers are not flushed – Windows: TerminateProcess() kills a process outright Files, sockets, are closed, memory released, but buffers are not flushed

6 Threads: – pthreads pthread_cancel() on cancellation points. – Win32 TerminateThread() kills the thread dead. – Python Threading.Thread has no kill() method – Python resources (references) would not be cleaned up by a hard kill – Pthread_cancel() could be used to implement a soft kill but is not available on all platforms. – CPython thread switching is synchronized (via GIL) so sending exceptions to threads is technically possible » CPython devs havent drunk the Svali yet.

7 Killing processes/threads: – If at all possible, it is messy (portability) – C++ exception handling is usually not invoked – Resources are released, but cleanup not executed – Sockets are aborted, causing errors – Locks are left un-released – Buffers are left un-flushed – Karma is left un-balanced

8 Tasklets? – Tasklet.kill() Sends a TaskletExit exception to the tasklet Immediatelly awakes the target – Optional pending delivery mode of exception – TaskletExit is BaseException Not caught by regular exception filters which catch Exception objects Normally causes the exception to bubble to the top – Finally clauses and context managers run as expected

9 Killing tasklets is synchronous – It is like sending a message to a blocking method Killing tasklets is not killing – It is merely raising an exception It can even be caught, logged, handled, etc. Killing tasklet is not a system operation – It invokes the languages native exception handling (unlike, e.g. C++)

10 Now we have an out-of-band way of sending messages to workers! Code written with proper try/finally scopes works as expected. Other apis and other exceptions can be used: – Tasklet.kill(pending=True) # new feature – Tasklet.throw(exc, val, tb, pending=False)

11 Examples: def serve_forever(self, poll_interval=0.5 ): """Handle one request at a time until shutdown. Polls for shutdown every poll_interval seconds. Ignores self.timeout. If you need to do periodic tasks, do them in another thread. """ self.__is_shut_down.clear() try: while not self.__shutdown_request: # XXX: Consider using another file descriptor or # connecting to the socket to wake this up instead of # polling. Polling reduces our responsiveness to a # shutdown request and wastes cpu at all other times. r, w, e =[self], [], [], poll_interval ) if self in r: self._handle_request_noblock() finally: self.__shutdown_request = False self.__is_shut_down.set()

12 Examples: while True: self.LogInfo("RunContactUpdates sleeping for", CONTACT_UPDATE_SLEEPTIME, "minutes") blue.pyos.synchro.SleepWallclock(1000 * 60 * CONTACT_UPDATE_SLEEPTIME) if not self.running: return …

13 UI code can be many tasklets doing things, e.g. Waiting for input. Complex UI state must be shut down e.g. When disconnection occurs UI tasklets register with the UI manager UI code carefully uses try:/finally: or context managers to release UI elements. UI manager can kill all registered tasklets, causing UI to be shut down gracefully.

14 def UIDialogBoxWorker(): with UIManager.register_tasklet(): with UI.DialogueBox() as box: input = box.GetResult() # blocks here class UIManager(object): def OnClear(self): for worker in self.GetWorkers(): worker.kill()

15 Gated access to a service def StartProcess(self): self.servitor = sake.servitor.DirectServitor() def ServitorCall(self, function, args): """ Wrap the call. This can involve a servitor, for example. The purpose of this is to allow Crust to cancel any calls that are in progress. """ # Use the DirectServitor. This causes TaskletExit calls to result # in a stacklesslib.errors.CancelledError to be raised. return, args) def Disconnect(self, *reasons): # We have decided to disconnect. Kill all calls currently in action self.servitor.kill_servitors(args=reasons)

16 Clients is aware of CancelledError (or not): def GetThrused(): svc = app.GetService('ThrusingService') try: return svc.DoTheThrus('I am the bethrused. Thrus me!') except stacklesslib.errors.CancelledError: # handle this error here: return None

17 Server wraps its api: class ThrusingService(service): def StartService(self): self.servitor = servitor.SimpleServitor() def GetAPI(self): return servitor.WrapAPI(self) def CancelClients(self): self.servitor.cancel_clients() def DoTheThrus(self, subject): return, subject)

18 Tasklet kill is a soft kill A gentle nudge of a rocking boat on a summer evening Requests a tasklet to politely stop what it is doing TaskletExit is a special exception that is silently ignored at the top level. – Like a gentle rustle in the autumn leaves

19 What to be aware of: – Every stackless switching function can raise an exception. – Running under watchdog, every instruction can raise an exception. (see: atomic) – Must be aware of this if writing low-level stackless code, such as when using channels. – stacklesslib.locks classes are exception-aware So are uthread.locks and others in EVE, I believe.

20 Best practices: – Use context managers to tear down resources Acquire locks, open files, etc. – Know that blocking calls can raise exceptions But everyone knew that, right? – Use with stacklesslib.util.atomic(): to guard critical stackless framework code. Because you just love to write your own synchronization primitives

21 Never again write polling loops with exit request states Grow a moustache or plant an herb garden. Go forth and procreate.

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