12.7 创建一个线程池

## 问题 You want to create a pool of worker threads for serving clients or performing other kindsof work. ## 解决方案 The concurrent.futures library has a ThreadPoolExecutor class that can be used forthis purpose. Here is an example of a simple TCP server that uses a thread-pool to serveclients: from socket import AF_INET, SOCK_STREAM, socketfrom concurrent.futures import ThreadPoolExecutor def echo_client(sock, client_addr): ‘''Handle a client connection‘''print(‘Got connection from', client_addr)while True: > > msg = sock.recv(65536)if not msg: > > break > sock.sendall(msg) print(‘Client closed connection')sock.close() def echo_server(addr): pool = ThreadPoolExecutor(128)sock = socket(AF_INET, SOCK_STREAM)sock.bind(addr)sock.listen(5)while True: > client_sock, client_addr = sock.accept()pool.submit(echo_client, client_sock, client_addr) echo_server((‘',15000)) If you want to manually create your own thread pool, it’s usually easy enough to do itusing a Queue. Here is a slightly different, but manual implementation of the same code: from socket import socket, AF_INET, SOCK_STREAMfrom threading import Threadfrom queue import Queue def echo_client(q): ‘''Handle a client connection‘''sock, client_addr = q.get()print(‘Got connection from', client_addr)while True: > > msg = sock.recv(65536)if not msg: > > break > sock.sendall(msg) print(‘Client closed connection') sock.close() def echo_server(addr, nworkers): # Launch the client workersq = Queue()for n in range(nworkers): > t = Thread(target=echo_client, args=(q,))t.daemon = Truet.start() # Run the serversock = socket(AF_INET, SOCK_STREAM)sock.bind(addr)sock.listen(5)while True: > client_sock, client_addr = sock.accept()q.put((client_sock, client_addr)) echo_server((‘',15000), 128) One advantage of using ThreadPoolExecutor over a manual implementation is that itmakes it easier for the submitter to receive results from the called function. For example,you could write code like this: from concurrent.futures import ThreadPoolExecutorimport urllib.request def fetch_url(url):u = urllib.request.urlopen(url)data = u.read()return data pool = ThreadPoolExecutor(10)# Submit work to the poola = pool.submit(fetch_url, ‘[http://www.python.org](http://www.python.org)‘)b = pool.submit(fetch_url, ‘[http://www.pypy.org](http://www.pypy.org)‘) # Get the results backx = a.result()y = b.result() The result objects in the example handle all of the blocking and coordination neededto get data back from the worker thread. Specifically, the operation a.result() blocksuntil the corresponding function has been executed by the pool and returned a value. ## 讨论 Generally, you should avoid writing programs that allow unlimited growth in the num‐ber of threads. For example, take a look at the following server: from threading import Threadfrom socket import socket, AF_INET, SOCK_STREAM def echo_client(sock, client_addr): ‘''Handle a client connection‘''print(‘Got connection from', client_addr)while True: > > msg = sock.recv(65536)if not msg: > > break > sock.sendall(msg) print(‘Client closed connection')sock.close() def echo_server(addr, nworkers): # Run the serversock = socket(AF_INET, SOCK_STREAM)sock.bind(addr)sock.listen(5)while True: > client_sock, client_addr = sock.accept()t = Thread(target=echo_client, args=(client_sock, client_addr))t.daemon = Truet.start() echo_server((‘',15000)) Although this works, it doesn’t prevent some asynchronous hipster from launching anattack on the server that makes it create so many threads that your program runs outof resources and crashes (thus further demonstrating the “evils” of using threads). Byusing a pre-initialized thread pool, you can carefully put an upper limit on the amountof supported concurrency.You might be concerned with the effect of creating a large number of threads. However,modern systems should have no trouble creating pools of a few thousand threads.Moreover, having a thousand threads just sitting around waiting for work isn’t going tohave much, if any, impact on the performance of other code (a sleeping thread does justthat—nothing at all). Of course, if all of those threads wake up at the same time andstart hammering on the CPU, that’s a different story—especially in light of the GlobalInterpreter Lock (GIL). Generally, you only want to use thread pools for I/O-boundprocessing.One possible concern with creating large thread pools might be memory use. For ex‐ample, if you create 2,000 threads on OS X, the system shows the Python process usingup more than 9 GB of virtual memory. However, this is actually somewhat misleading.When creating a thread, the operating system reserves a region of virtual memory tohold the thread’s execution stack (often as large as 8 MB). Only a small fragment of thismemory is actually mapped to real memory, though. Thus, if you look a bit closer, youmight find the Python process is using far less real memory (e.g., for 2,000 threads, only 70 MB of real memory is used, not 9 GB). If the size of the virtual memory is a concern,you can dial it down using the threading.stack_size() function. For example: import threadingthreading.stack_size(65536) If you add this call and repeat the experiment of creating 2,000 threads, you’ll find thatthe Python process is now only using about 210 MB of virtual memory, although theamount of real memory in use remains about the same. Note that the thread stack sizemust be at least 32,768 bytes, and is usually restricted to be a multiple of the systemmemory page size (4096, 8192, etc.).