multithreading in Python: extremely high CPU consumption

Question

Below is the code of a simple timer that prints out a new number in the command line every 0.1 sec, and -- simultaneously -- awaits for a keypress (Enter). As soon as Enter is pressed, the timer is switched into pause, stops printing and waits for another Enter keypress that would resume counting / printing.

The problem I face is that while in the 'pause' state, CPU consumption grows to almost 100% (and seems to be below 1% while running / printing out).

Why is it? What am I doing wrong?

Here goes the code:

import time, thread, sys

time_passed = 0
l = []
trig = True

def input(l):
    """ Waits for a keypress """
    raw_input()
    l.append(None)

def timer():
    """ Prints out a number every millisecond """
    global time_passed
    time.sleep(.1)
    time_passed += 1
    sys.stdout.write(format(time_passed)+"\r")
    sys.stdout.flush()

def running():
    """ Toggles play/pause state if Enter is pressed """
    global trig, l
    while trig:
        timer()
        if l:
            l = []
            trig = False
            return trig, l

def stopped():
    """ Toggles pause/play state if Enter is pressed """
    global trig, l
    while not trig:
        if l:
            l = []
            trig = True
            return trig, l

def main():
    """ Waits for a keypress while either running or stopping the timer """
    global l, trig
    while True:
        thread.start_new_thread(input, (l,))
        if trig:    # The timer is running
            running()
        else:       # The timer is stopped
            stopped()

main()

Thank you.

luk32 · Accepted Answer · 2014-04-22 09:48:24Z

1

Well, if its in a "stopped" state you run this:

def stopped():
    """ Toggles pause/play state if Enter is pressed """
    global trig, l
    while not trig:
        if l:
            l = []
            trig = True
            return trig, l

this loop has no pause/wait. So it runs at full speed without getting back to system, while within the running you do call time.sleep. Putting time.sleep(0.1) before the if will make it work as you wish.

Also a friendly personal note, you could refactor it a little bit. Two functions with the same description working on globals make it a little bit hard to understand =).

I'd make something like:

import time, thread, sys, threading

time_passed = 0
l = []

def input(l):
    """ Waits for a keypress """
    raw_input()
    l.append(None)

def timer():
    """ Prints out a number every millisecond """
    global time_passed
    time_passed += 1
    sys.stdout.write(format(time_passed)+"\r")
    sys.stdout.flush()

def running(state):
    """ Toggles play/pause state if Enter is pressed """
    global l
    while True:
        time.sleep(.1)
        if state:
            timer()
        if l:
            l = []
            state = not state
            return state

def main():
    """ Waits for a keypress while either running or stopping the timer """
    global l
    state = True
    while True:
        thread.start_new_thread(input, (l,))
        state = running(state)

main()

and I would think how to signal the toggle better. I somehow don't like the idea of returning from the function, you don't need really to return from running, just flip the state. But it is just personal preference I guess.

edited Apr 22, 2014 at 9:48

answered Apr 22, 2014 at 9:20

luk32

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2 Comments

A S Over a year ago

Thanks! As for refactoring, those functions, running() and stopped() do not have exactly the same description :) (but I do agree, it may be quite misleading). I used to have 'em integrated in main(), do you think it'd be better?

luk32 Over a year ago

I edited the answer and added refactored version of running, that bases its behaviour on a state passed as an argument. I still think that two while True loops are too much, but I think the code is easier to understand this way. You can pass it through Code Review SE. They are better at this.

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