Parallel computing Python / Raspberry Pi

Question

I am doing a project where I am required to measure vibration levels due to passing vehicles (using an accelerometer sampling at high rate) and capture video of the event "SIMULTANEOUSLY" when the vibrations exceed a certain threshold value. I want both the processes i.e. data acquisition and video recording to go simultaneously so that they are synchronized in time.

I am doing both the things on a raspberry pi. To achieve my goal I have 2 strategies in mind.

1) Write a function for

a) Data acquisition
b) Recording video

and run them in the same script.

2) Or while running the script for data acquisition, use a call to os.system, to run a script to capture video of the event.

Now I am curious in what order are functions executed in Python? So if I write

while 1:

   if v_threshold > some_value:

      Acquire_data(); Capture_Video();

Are they being executed simultaneously or does Capture_Video() only begin once Acquire_data() has finished execution? Or in general, how are functions executed in Python (or any other programming language).

What would be your advice to achieve the goal? Writing functions or running 2 parallel scripts? Or should I use the multiprocessing module?

Thanks

Nischal

They are executed sequentially, one after the other. Try making a dummy function for each that sleeps 5 seconds and you'll see it takes 10 seconds. You will need to use multiprocessing or multithreading to get both to run at once. — Mark Setchell
– Mark Setchell, Commented Dec 21, 2017 at 9:05

hansaplast · Accepted Answer · 2017-12-21 13:06:47Z

1

I would go for option 1 as you have more control over the whole process (e.g. you can easily wait until both subprocesses are done)

Your main setup would look like this:

from multiprocessing import Process
import time

def acquire_data(arg):
    for i in range(5):
        print('acquiring data: {}'.format(arg))
        time.sleep(1.1)

def capture_video():
    for i in range(5):
        print('capturing video')
        time.sleep(1)


if __name__ == '__main__':
    p_data = Process(target=acquire_data, args=('foo',))
    p_video = Process(target=capture_video)
    p_data.start()
    p_video.start()
    p_data.join() # wait until acquire_data is done
    p_video.join() # wait also until capture_video is done

Also: Which model is it? Because if both data-acquiring and video-capturing is taking 100% cpu then you will run into an issue with only a single core raspberry pi. Model 3 has 4 cores so this is fine.

answered Dec 21, 2017 at 13:06

hansaplast

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1 Comment

ArkanSaaS Over a year ago

Hi, Thanks for your advice. It is Pi3 model B

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