使用多线程的程序在使用多处理时挂起

Question

我正在练习多线程和多处理，我尝试使用这两种方法实现相同的程序。
这个想法是有两个并行的threads/processes：一个从网络摄像头读取帧并将它们发送到队列，另一个检索它们并显示它们。

import json
import queue, threading
from queue import Queue
import trigger_events
import multiprocessing as mp
import cv2


class system_manager():
    def __init__(self, source):

        ## camera reader
        self.camera_queue = queue.Queue(maxsize=1)
        # self.cam_read = threading.Thread(target=Camera_Reader, args=(source, self.camera_queue))
        self.cam_read = mp.Process(target=Camera_Reader, args=(source, self.camera_queue))
        self.cam_read.daemon = True
        self.cam_read.start()

        ## module for managing triggers (user/mug position)
        # self.cam_display = threading.Thread(target=Camera_Display, args=(self.camera_queue,))
        self.cam_display = mp.Process(target=Camera_Display, args=(self.camera_queue,))
        self.cam_display.daemon = True
        self.cam_display.start()



def Camera_Reader(source, camera_queue):
    print("Cam Loading...")
    cap=cv2.VideoCapture(source)
    print("Cam Loaded...")
    while(True):
        # print(f"frame: {counter}")
        ret, frame = cap.read()
        # counter += 1
        camera_queue.put(frame)


def Camera_Display(camera_queue):
    counter_frame = 0
    while(True):
        try:
            print(f"\nFrame: {counter_frame}")
            frame = camera_queue.get()
            key = cv2.waitKey(1)
            if (key == ord('q')):
                break
            print("Here")
            counter_frame += 1
            cv2.imshow("Frame", frame)
        except camera_queue.Empty:
            print("Nothing in queue!!!")
    cv2.destroyAllWindows()



if __name__ == "__main__":
    SM = system_manager(source=0)

奇怪的是，如果我对两个单独的任务使用线程，一切都会正常进行。另一方面，如果我将它们分配给不同的进程，应该显示帧 (Camera_Display) 的进程挂起...我得到输出：

Frame: 0
Cam Loaded...

所以，print("Here") 行没有得到执行，并且进程在 while 循环的第一次迭代期间挂起。我假设这两种方法都应该适用于这个问题，我不明白当我使用多处理时出了什么问题。

Answer 1

用进程替换线程是不够的，你还必须使用multiprocessing.Queue而不是queue.Queue。后者默认用于线程。

https://docs.python.org/3/library/multiprocessing.html#exchanging-objects-between-processes

使用多线程的程序在使用多处理时挂起

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