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# Have a task wait for writing on a file descriptor def writewait(self,task,fd): selfwrite_waiting[fd] = task # Main scheduler loop def mainloop(self,count=-1,timeout=None): while selfnumtasks: # Check for I/O events to handle if selfread_waiting or selfwrite_waiting: wait = 0 if selftask_queue else timeout r,w,e = selectselect(selfread_waiting, selfwrite_waiting, [], wait) for fileno in r: selfschedule(selfread_waitingpop(fileno)) for fileno in w: selfschedule(selfwrite_waitingpop(fileno)) # Run all of the tasks on the queue that are ready to run while selftask_queue: task = selftask_queuepopleft() try: result = taskrun() if isinstance(result,SystemCall): resulthandle(self,task) else: selfschedule(task) except StopIteration: selfnumtasks -= 1 # If no tasks can run, we decide if we wait or return else: if count > 0: count -= 1 if count == 0: return # Implementation of different system calls class ReadWait(SystemCall): def _ _init_ _(self,f): selff = f def handle(self,sched,task): fileno = selfffileno() schedreadwait(task,fileno) class WriteWait(SystemCall): def _ _init_ _(self,f): selff = f def handle(self,sched,task): fileno = selfffileno() schedwritewait(task,fileno) class NewTask(SystemCall): def _ _init_ _(self,target): selftarget = target def handle(self,sched,task): schednew(selftarget) schedschedule(task)
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The code in this example implements a very tiny operating system Here are some details concerning its operation: n All work is carried out by coroutine functions Recall that a coroutine uses the yield statement like a generator except that instead of iterating on it, you send it values using a send(value) method
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The Task class represents a running task and is just a thin layer on top of a coroutine A Task object task has only one operation, taskrun()This resumes the task and runs it until it hits the next yield statement, at which point the task suspendsWhen running a task, the tasksendval attribute contains the value that is to be sent into the task s corresponding yield expressionTasks run until they encounter the next yield statementThe value produced by this yield controls what happens next in the task: n If the value is another coroutine (typeGeneratorType), it means that the task wants to temporarily transfer control to that coroutineThe stack attribute of Task objects represents a call-stack of coroutines that is built up when this happensThe next time the task runs, control will be transferred into this new coroutine n If the value is a SystemCall instance, it means that the task wants the scheduler to do something on its behalf (such as launch a new task, wait for I/O, and so on)The purpose of this object is described shortly n If the value is any other value, one of two things happens If the currently executing coroutine was running as a subroutine, it is popped from the task call stack and the value saved so that it can be sent to the callerThe caller will receive this value the next time the task executes If the coroutine is the only executing coroutine, the return value is simply discarded n The handling of StopIteration is to deal with coroutines that have terminatedWhen this happens, control is returned to the previous coroutine (if there was one) or the exception is propagated to the scheduler so that it knows that the task terminated The SystemCall class represents a system call in the schedulerWhen a running task wants the scheduler to carry out an operation on its behalf, it yields a SystemCall instanceThis object is called a system call because it mimics the behavior of how programs request the services of a real multitasking operating system such as UNIX or Windows In particular, if a program wants the services of the operating system, it yields control and provides some information back to the system so that it knows what to do In this respect, yielding a SystemCall is similar to executing a kind of system trap The Scheduler class represents a collection of Task objects that are being managed At its core, the scheduler is built around a task queue (the task_queue attribute) that keeps track of tasks that are ready to runThere are four basic operations concerning the task queue new() takes a new coroutine, wraps it with a Task object, and places it on the work queue schedule() takes an existing Task and puts it back on the work queue mainloop() runs the scheduler in a loop, processing tasks one by one until there are no more tasksThe readwait() and writewait() methods put a Task object into temporary staging areas where it will wait for I/O events In this case, the Task isn t running, but it s not dead either it s just sitting around biding its time The mainloop() method is the heart of the schedulerThis method first checks to see if any tasks are waiting for I/O events If so, it arranges a call to select() in order to poll for I/O activity If there are any events of interest, the associated tasks are placed back onto the task queue so that they can run Next, the mainloop() method pops tasks off of the task queue and calls their run()
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