python - 在新进程中执行 python 代码比在主进程中慢得多

Question

我开始在 python 中学习 multiprocessing 并且我注意到相同的代码在主进程上的执行速度比在使用 multiprocessing模块。

这是我的代码的简化示例，其中我首先在 main process 上执行代码，并打印前 10 次计算的时间和总计算的时间。然后在 new process 上执行相同的代码(这是一个长时间运行的进程，我可以随时发送 new_pattern)。

import multiprocessing
import random
import time


old_patterns = [[random.uniform(-1, 1) for _ in range(0, 10)] for _ in range(0, 2000)]
new_patterns = [[random.uniform(-1, 1) for _ in range(0, 10)] for _ in range(0, 100)]


new_pattern_for_processing = multiprocessing.Array('d', 10)
there_is_new_pattern = multiprocessing.Value('i', 0)
queue = multiprocessing.Queue()


def iterate_and_add(old_patterns, new_pattern):
    for each_pattern in old_patterns:
        sum = 0
        for count in range(0, 10):
            sum += each_pattern[count] + new_pattern[count]


print_count_main_process = 0
def patt_recognition_main_process(new_pattern):
    global print_count_main_process
    # START of same code on main process
    start_main_process_one_patt = time.time()
    iterate_and_add(old_patterns, new_pattern)
    if print_count_main_process < 10:
        print_count_main_process += 1
        print("Time on main process one pattern:", time.time() - start_main_process_one_patt)
    # END of same code on main process


def patt_recognition_new_process(old_patterns, new_pattern_on_new_proc, there_is_new_pattern, queue):
    print_count = 0
    while True:
        if there_is_new_pattern.value:
            #START of same code on new process
            start_new_process_one_patt = time.time()
            iterate_and_add(old_patterns, new_pattern_on_new_proc)
            if print_count < 10:
                print_count += 1
                print("Time on new process one pattern:", time.time() - start_new_process_one_patt)
            #END of same code on new process
            queue.put("DONE")
            there_is_new_pattern.value = 0


if __name__ == "__main__":
    start_main_process = time.time()
    for new_pattern in new_patterns:
        patt_recognition_main_process(new_pattern)
    print(".\n.\n.")
    print("Total Time on main process:", time.time() - start_main_process)

    print("\n###########################################################################\n")

    start_new_process = time.time()
    p1 = multiprocessing.Process(target=patt_recognition_new_process, args=(old_patterns, new_pattern_for_processing, there_is_new_pattern, queue))
    p1.start()
    for new_pattern in new_patterns:
        for idx, n in enumerate(new_pattern):
            new_pattern_for_processing[idx] = n
        there_is_new_pattern.value = 1
        while True:
            msg = queue.get()
            if msg == "DONE":
                break
    print(".\n.\n.")
    print("Total Time on new process:", time.time()-start_new_process)

这是我的结果:

Time on main process one pattern: 0.0025289058685302734
Time on main process one pattern: 0.0020127296447753906
Time on main process one pattern: 0.002008199691772461
Time on main process one pattern: 0.002511262893676758
Time on main process one pattern: 0.0020067691802978516
Time on main process one pattern: 0.0020036697387695312
Time on main process one pattern: 0.0020072460174560547
Time on main process one pattern: 0.0019974708557128906
Time on main process one pattern: 0.001997232437133789
Time on main process one pattern: 0.0030074119567871094
.
.
.
Total Time on main process: 0.22810864448547363

###########################################################################

Time on new process one pattern: 0.03462791442871094
Time on new process one pattern: 0.03308463096618652
Time on new process one pattern: 0.034590721130371094
Time on new process one pattern: 0.033623456954956055
Time on new process one pattern: 0.03407788276672363
Time on new process one pattern: 0.03308820724487305
Time on new process one pattern: 0.03408670425415039
Time on new process one pattern: 0.0345921516418457
Time on new process one pattern: 0.03710794448852539
Time on new process one pattern: 0.03358912467956543
.
.
.
Total Time on new process: 4.0528037548065186

为什么执行时间相差这么大？

Answer 1

有点微妙，但问题在于

new_pattern_for_processing = multiprocessing.Array('d', 10)

它不保存 python float 对象，它保存原始字节，在本例中足以保存 10 个 8 字节机器级 double。当您读取或写入此数组时，python 必须将 float 转换为 double 或相反。如果您只读取或写入一次，这没什么大不了的，但您的代码会在循环中执行多次，并且这些转换占主导地位。

为了确认，我将机器级数组复制到 python float 列表一次，并让流程对其进行处理。现在它的速度和父级一样。我的更改仅在一个函数中

def patt_recognition_new_process(old_patterns, new_pattern_on_new_proc, there_is_new_pattern, queue):
    print_count = 0
    while True:
        if there_is_new_pattern.value:
            local_pattern = new_pattern_on_new_proc[:]
            #START of same code on new process
            start_new_process_one_patt = time.time()
            #iterate_and_add(old_patterns, new_pattern_on_new_proc)
            iterate_and_add(old_patterns, local_pattern)
            if print_count < 10:
                print_count += 1
                print("Time on new process one pattern:", time.time() - start_new_process_one_patt)
            #END of same code on new process
            there_is_new_pattern.value = 0
            queue.put("DONE")