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5.6.2. Asynchronous sleep function and benchmarks

An asynchronous sleep function is highly desirable to let the GUI loop (at the moment, the Qt event loop) run while pyrpl is waiting for curves from the instruments.

The benchmark can be found in timers.ipynb. It was executed on python 3.5 on a windows 10 anaconda system. Methods compatible with python 2:

We first compare 4 different implementations of the sleep function that are all fully compatible between python 2 and python 3. The normal time.sleep function (which is not asynchronous)

Calling time.sleep(delays) with delays ranging continuously from 0 to 5 ms gives the following distribution of measured delay vs requested delay:

As stated in the doc, sleep never returns before the requested delay, however, it will try its best not to return more than 1 ms too late. Moreover, we clearly have a problem because no qt events will be processed since the main thread is blocked by the current execution of time.sleep: for instance a timer’s timeout will only be triggered once the sleep function has returned, this is what’s causing freezing of the GUI when executing code in the jupyter console. Constantly calling APP.processEvents()

The first work around, is to manually call processEvents() regularly to make sure events are processed while our process is sleeping.

from timeit import default_timer

def sleep_pe(delay):
    time0 = default_timer()

first comment: we need to use timit.default_timer because time.time has also a precision limited to the closest millisecond.

We get, as expected, an almost perfect correlation between requested delays and obtained delays. Some outliers probably result from the OS interrupting the current process execution, or even other events from the GUI loop being executed just before the requested time.

We also see that the CPU load is quite high, even though we don’t do anything but waiting for events. This is due to the loop constantly checking for the current time and comparing it to the requested delay. Running the QEventLoop locally

A better solution, as advertised here, is to run a new version of the QEventLoop locally:

def sleep_loop(delay):
   loop = QtCore.QEventLoop()
   timer = QtCore.QTimer()
   loop.exec() # la loop prend en charge elle-même l'évenement du timer qui va la faire mourir après delay.

The subtlety here is that the loop.exec() function is blocking, and usully would never return. To force it to return after some time delay, we simply instanciate a QTimer and connect its timeout signal to the quit function of the loop. The timer’s event is actually handled by the loop itself. We then get a much smaller CPU load, however, we go back to the situation where the intervals are only precise at the nearest millisecond. The hybrid approach

A compromise is to use a QTimer that will stop 1 ms earlier, and then manually call processEvents for the remaining time. We get at the same time a low CPU load (as long as delay >> 1 ms, which is not completely verified here), and a precise timing.

def my_sleep(delay):
    tic = default_timer()
    if delay>1e-3:
        sleep_loop(delay - 1e-3)
        APP.processEvents() Benchmark in the presence of other events

To simulate the fact that in real life, other events have to be treated while the loop is running (for instance, user interactions with the GUI, or another instrument running an asynchronous measurement loop), we run in parallel the following timer:

from PyQt4 import QtCore, QtGui
n_calc = [0]
def calculate():

timer = QtCore.QTimer()

By looking at how fast n_calc[0] gets incremented, we can measure how blocking our sleep-function is for other events. We get the following outcomes (last number “calc/s” in the figure title): time.sleep

As expected, time.sleep prevents any event from being processed calling processEvents

40 000 events/seconds. running the eventLoop locally

That’s approximately identical our custom function

Still more or less identical (but remember that the big advantage compared to the previous version is that in the absence of external events, the CPU load is close to 0). Async programming in python3(.5):

A description of async programming in python 3.5 is given in “Requirements for an asynchronous interface compatible with python 3 asyncio”. To summarize, it is possible to use the Qt event loop as a backend for the beautiful syntax of coroutines in python 3 using quamash. Of course, because the quamash library is just a wrapper translating the new python asynchronous syntax into QTimers, there is no magic on the precision/efficiency side: for instance, the basic coroutine asyncio.sleep gives a result similar to “Running a local QEventLoop”:

async def sleep_coroutine(delay):
   await asyncio.sleep(delay)

But, obviously, we can play the same trick as before to make a precise enough coroutine:

async def sleep_coroutine(delay):
   tic = default_timer()
   if delay>0.001:
       await asyncio.sleep(delay - 0.001)
   while default_timer() < tic + delay: