为什么即使使用多线程也不快？

Question

我使用下面的代码多线程下载图像，但我发现不是那么快，我不知道为什么，有人为我expalin

正常下载：

for imageSecond in imageSeconds:
    urlServer = imageSecond.get("src")
    pathLocal = formatPath(downloadLocationPath, ntpath.basename(urlServer))
    downloadImage(browser, urlServer, pathLocal)    

def downloadImage(browser, urlServer, pathLocal):

线程下载：

ts = []
for imageSecond in imageSeconds:
    urlServer = imageSecond.get("src")
    pathLocal = formatPath(downloadLocationPath, ntpath.basename(urlServer))
    ts.append(createNewDownloadThread(browser, urlServer, pathLocal))

for t in ts:
    t.join()

def createNewDownloadThread(browser, urlServer, pathLocal):
    download_thread = threading.Thread(target=downloadImage, args=(browser, urlServer, pathLocal))
    download_thread.start()
    return download_thread#.join()

def downloadImage(browser, urlServer, pathLocal):

Answer 1

从Python docs：

join(timeout=None) Wait until the thread terminates. This blocks the calling thread until the thread whose join() method is called terminates – either normally or through an unhandled exception – or until the optional timeout occurs.

因此，您正在创建一个线程，等待它完成下载，然后为后续下载创建另一个线程，该线程与原始解决方案基本相同

更新您可以使用^{}，它提供了简单的界面：

import concurrent.futures as cf

def square(x):
    return x*x

with cf.ThreadPoolExecutor(10) as executor:
    futures = [executor.submit(square, i) for i in range(5)]
    cf.wait(futures)

    for f in futures:
        print(f.result())

输出：

0
1
4
9
16

对于您的代码，它看起来像这样：

NUM_THREADS = 10
with cf.ThreadPoolExecutor(NUM_THREADS) as executor:
    futures = []
    for imageSecond in imageSeconds:
        urlServer = imageSecond.get("src")
        pathLocal = formatPath(downloadLocationPath, ntpath.basename(urlServer))
        futures.append(executor.submit(downloadImage, browser, urlServer, pathLocal))

    cf.wait(futures)

Answer 2

使用如下代码

ts = []

for:
    t = createThread()
    t.start()
    ts.append(t)

for t in ts:
    t.join()

代替代码

for:
    t = createThread()
    t.start()
    t.join()