python threading.Timer如何确定经过的时间？

Question

I'm running some experiments in a virtual machine, which has its system time updated if its suspended. I want to know if I can suspend the virtual machine and not affect the timer. That is, does Timer use system time or wall time?

I've tried looking through the source code and got to _thread.lock.acquire before the code dips into C.

Below is my code. I delegate to a subprocess that outputs 'plans'. I keep collecting these plans until the optimal plan is found or the maximum allowed time has elapsed. The timer is used to terminate the process if time runs out (which is the expected state of affairs). I need to know that the Timer will not be effected by system time being updated as it will invalidate the experiment I'm running.

p = Popen(args, stdout=PIPE, cwd=self.working_directory)
timer = Timer(float(self.planning_time), p.terminate)
timer.start()

plan = None
while True:
    try:
        plan = list(decode_plan_from_optic(self.decode(p.stdout), 
                    report_incomplete_plan=True))
    except IncompletePlanException:
        break
timer.cancel()

Answer 1

Upon examination of the python source code for *unix it systems, I found that python eventually delegates to sem_timedwait from semaphore.h or pthread_cond_timedwait from pthread.h depending on support. Either way, both functions take a struct timespec from time.h as an absolute time to wait till -- timespec is the number of seconds and nanoseconds since the epoch.

So on the face of it seems that waiting in python is dependent on system time -- meaning my program would be effected by a change in system time. However, time.h specifies the constant CLOCK_MONOTONIC_RAW and the function clock_gettime , if a monotonically increasing clock is required. Showing there is an ability to wait independently of system time. However, sadly python uses gettimeofday (marked as obsolete since 2008) which is affected by changes to system time.

In short, waiting in python on *unix systems is effected by changes to system time.