Question: I am trying to pause an encode of FFmpeg while it is in a non-shell subprocess (This is important to how it plays into a larger program). This can be done by presssing the "Pause / Break" key on the keyboard by itself, and I am trying to send that to Popen. The command itself must be cross platform compatible, so I cannot wrap it in any way, but I can send signals or run functions that are platform specific as needed. I looked at how to send a "Ctrl+Break" to a subprocess via pid or handler and it suggested to send a signal, but that raised a "ValueError: Unsupported signal: 21" from subprocess import Popen, PIPE import signal if __name__ == '__main__': command = "ffmpeg.exe -y -i example_video.mkv -map 0:v -c:v libx265 -preset slow -crf 18 output.mkv" proc = Popen(command, stdin=PIPE, shell=False) try: proc.send_signal(signal.SIGBREAK) finally: proc.wait()  Then attempted to use GenerateConsoleCtrlEvent to create a Ctrl+Break event as described here https://learn.microsoft.com/en-us/windows/console/generateconsolectrlevent from subprocess import Popen, PIPE import ctypes if __name__ == '__main__': command = "ffmpeg.exe -y -i example_video.mkv -map 0:v -c:v libx265 -preset slow -crf 18 output.mkv" proc = Popen(command, stdin=PIPE, shell=False) try: ctypes.windll.kernel32.GenerateConsoleCtrlEvent(1, proc.pid) finally: proc.wait()  I have tried psutil pause feature, but it keeps the CPU load really high even when "paused". Even though it wouldn't work with the program overall, I have at least tried setting creationflags=CREATE_NEW_PROCESS_GROUP which makes the SIGBREAK not error, but also not pause it. For the Ctrl-Break event will entirely stop the encode instead of pausing it.

Answer: Linux/Unix solution: import subprocess, os, signal # ... # Start the task: proc = subprocess.Popen(..., start_new_session=True) # ... def send_signal_to_task(pid, signal): #gpid = os.getpgid(pid) # WARNING This does not work gpid = pid # But this does! print(f"Sending {signal} to process group {gpid}...") os.killpg(gpid, signal) # ... # Pause and resume: send_signal_to_task(proc.pid, signal.SIGSTOP) send_signal_to_task(proc.pid, signal.SIGCONT)  Notice start_new_session=True in Popen call and using of os.killpg instead of os.kill in send_signal_to_task function. The reason for this is the same as as reason for why you have large CPU usage even in paused state as you reported. ffmpeg spawns a number of child processes. start_new_session=True will create a new process group and os.killpg sends signal to all processes in group. Cross-platform solution is supposed to be provided by psutil module but you probably should research whether it supports process groups as the variant above: >>> import psutil >>> psutil.pids() [1, 2, 3, 4, 5, 6, 7, 46, 48, 50, 51, 178, 182, 222, 223, 224, 268, 1215, 1216, 1220, 1221, 1243, 1244, 1301, 1601, 2237, 2355, 2637, 2774, 3932, 4176, 4177, 4185, 4187, 4189, 4225, 4243, 4245, 4263, 4282, 4306, 4311, 4312, 4313, 4314, 4337, 4339, 4357, 4358, 4363, 4383, 4395, 4408, 4433, 4443, 4445, 4446, 5167, 5234, 5235, 5252, 5318, 5424, 5644, 6987, 7054, 7055, 7071] >>> p = psutil.Process(7055) >>> p.suspend() >>> p.resume()  Reference: https://pypi.org/project/psutil/ Some pointers on emulating process groups in Windows: Popen waiting for child process even when the immediate child has terminated