初步解析Python下的多进程编程
要让Python程序实现多进程（multiprocessing），我们先了解操作系统的相关知识。
Unix/Linux操作系统提供了一个fork()系统调用，它非常特殊。普通的函数调用，调用一次，返回一次，但是fork()调用一次，返回两次，因为操作系统自动把当前进程（称为父进程）复制了一份（称为子进程），然后，分别在父进程和子进程内返回。
子进程永远返回0，而父进程返回子进程的ID。这样做的理由是，一个父进程可以fork出很多子进程，所以，父进程要记下每个子进程的ID，而子进程只需要调用getppid()就可以拿到父进程的ID。
Python的os模块封装了常见的系统调用，其中就包括fork，可以在Python程序中轻松创建子进程：
# multiprocessing.py import os print 'Process (%s) start...' % os.getpid() pid = os.fork() if pid==0: print 'I am child process (%s) and my parent is %s.' % (os.getpid(), os.getppid()) else: print 'I (%s) just created a child process (%s).' % (os.getpid(), pid)
运行结果如下：
Process (876) start... I (876) just created a child process (877). I am child process (877) and my parent is 876.
由于Windows没有fork调用，上面的代码在Windows上无法运行。由于Mac系统是基于BSD（Unix的一种）内核，所以，在Mac下运行是没有问题的，推荐大家用Mac学Python！
有了fork调用，一个进程在接到新任务时就可以复制出一个子进程来处理新任务，常见的Apache服务器就是由父进程监听端口，每当有新的http请求时，就fork出子进程来处理新的http请求。
multiprocessing
如果你打算编写多进程的服务程序，Unix/Linux无疑是正确的选择。由于Windows没有fork调用，难道在Windows上无法用Python编写多进程的程序？
由于Python是跨平台的，自然也应该提供一个跨平台的多进程支持。multiprocessing模块就是跨平台版本的多进程模块。
multiprocessing模块提供了一个Process类来代表一个进程对象，下面的例子演示了启动一个子进程并等待其结束：
from multiprocessing import Process
import os
# 子进程要执行的代码
def run_proc(name):
print 'Run child process %s (%s)...' % (name, os.getpid())
if __name__=='__main__':
print 'Parent process %s.' % os.getpid()
p = Process(target=run_proc, args=('test',))
print 'Process will start.'
p.start()
p.join()
print 'Process end.'
执行结果如下：
Parent process 928. Process will start. Run child process test (929)... Process end.
创建子进程时，只需要传入一个执行函数和函数的参数，创建一个Process实例，用start()方法启动，这样创建进程比fork()还要简单。
join()方法可以等待子进程结束后再继续往下运行，通常用于进程间的同步。
Pool
如果要启动大量的子进程，可以用进程池的方式批量创建子进程：
from multiprocessing import Pool
import os, time, random
def long_time_task(name):
print 'Run task %s (%s)...' % (name, os.getpid())
start = time.time()
time.sleep(random.random() * 3)
end = time.time()
print 'Task %s runs %0.2f seconds.' % (name, (end - start))
if __name__=='__main__':
print 'Parent process %s.' % os.getpid()
p = Pool()
for i in range(5):
p.apply_async(long_time_task, args=(i,))
print 'Waiting for all subprocesses done...'
p.close()
p.join()
print 'All subprocesses done.'
执行结果如下：
Parent process 669. Waiting for all subprocesses done... Run task 0 (671)... Run task 1 (672)... Run task 2 (673)... Run task 3 (674)... Task 2 runs 0.14 seconds. Run task 4 (673)... Task 1 runs 0.27 seconds. Task 3 runs 0.86 seconds. Task 0 runs 1.41 seconds. Task 4 runs 1.91 seconds. All subprocesses done.
代码解读：
对Pool对象调用join()方法会等待所有子进程执行完毕，调用join()之前必须先调用close()，调用close()之后就不能继续添加新的Process了。
请注意输出的结果，task 0，1，2，3是立刻执行的，而task 4要等待前面某个task完成后才执行，这是因为Pool的默认大小在我的电脑上是4，因此，最多同时执行4个进程。这是Pool有意设计的限制，并不是操作系统的限制。如果改成：
p = Pool(5)
就可以同时跑5个进程。
由于Pool的默认大小是CPU的核数，如果你不幸拥有8核CPU，你要提交至少9个子进程才能看到上面的等待效果。
进程间通信
Process之间肯定是需要通信的，操作系统提供了很多机制来实现进程间的通信。Python的multiprocessing模块包装了底层的机制，提供了Queue、Pipes等多种方式来交换数据。
我们以Queue为例，在父进程中创建两个子进程，一个往Queue里写数据，一个从Queue里读数据：
from multiprocessing import Process, Queue
import os, time, random
# 写数据进程执行的代码:
def write(q):
for value in ['A', 'B', 'C']:
print 'Put %s to queue...' % value
q.put(value)
time.sleep(random.random())
# 读数据进程执行的代码:
def read(q):
while True:
value = q.get(True)
print 'Get %s from queue.' % value
if __name__=='__main__':
# 父进程创建Queue，并传给各个子进程：
q = Queue()
pw = Process(target=write, args=(q,))
pr = Process(target=read, args=(q,))
# 启动子进程pw，写入:
pw.start()
# 启动子进程pr，读取:
pr.start()
# 等待pw结束:
pw.join()
# pr进程里是死循环，无法等待其结束，只能强行终止:
pr.terminate()
运行结果如下：
Put A to queue... Get A from queue. Put B to queue... Get B from queue. Put C to queue... Get C from queue.
在Unix/Linux下，multiprocessing模块封装了fork()调用，使我们不需要关注fork()的细节。由于Windows没有fork调用，因此，multiprocessing需要“模拟”出fork的效果，父进程所有Python对象都必须通过pickle序列化再传到子进程去，所有，如果multiprocessing在Windows下调用失败了，要先考虑是不是pickle失败了。
小结
在Unix/Linux下，可以使用fork()调用实现多进程。
要实现跨平台的多进程，可以使用multiprocessing模块。
进程间通信是通过Queue、Pipes等实现的。
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
AI Hentai Generator
Generate AI Hentai for free.
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
1377
52
Do mysql need to pay
Apr 08, 2025 pm 05:36 PM
MySQL has a free community version and a paid enterprise version. The community version can be used and modified for free, but the support is limited and is suitable for applications with low stability requirements and strong technical capabilities. The Enterprise Edition provides comprehensive commercial support for applications that require a stable, reliable, high-performance database and willing to pay for support. Factors considered when choosing a version include application criticality, budgeting, and technical skills. There is no perfect option, only the most suitable option, and you need to choose carefully according to the specific situation.
How to use mysql after installation
Apr 08, 2025 am 11:48 AM
The article introduces the operation of MySQL database. First, you need to install a MySQL client, such as MySQLWorkbench or command line client. 1. Use the mysql-uroot-p command to connect to the server and log in with the root account password; 2. Use CREATEDATABASE to create a database, and USE select a database; 3. Use CREATETABLE to create a table, define fields and data types; 4. Use INSERTINTO to insert data, query data, update data by UPDATE, and delete data by DELETE. Only by mastering these steps, learning to deal with common problems and optimizing database performance can you use MySQL efficiently.
MySQL can't be installed after downloading
Apr 08, 2025 am 11:24 AM
The main reasons for MySQL installation failure are: 1. Permission issues, you need to run as an administrator or use the sudo command; 2. Dependencies are missing, and you need to install relevant development packages; 3. Port conflicts, you need to close the program that occupies port 3306 or modify the configuration file; 4. The installation package is corrupt, you need to download and verify the integrity; 5. The environment variable is incorrectly configured, and the environment variables must be correctly configured according to the operating system. Solve these problems and carefully check each step to successfully install MySQL.
MySQL download file is damaged and cannot be installed. Repair solution
Apr 08, 2025 am 11:21 AM
MySQL download file is corrupt, what should I do? Alas, if you download MySQL, you can encounter file corruption. It’s really not easy these days! This article will talk about how to solve this problem so that everyone can avoid detours. After reading it, you can not only repair the damaged MySQL installation package, but also have a deeper understanding of the download and installation process to avoid getting stuck in the future. Let’s first talk about why downloading files is damaged. There are many reasons for this. Network problems are the culprit. Interruption in the download process and instability in the network may lead to file corruption. There is also the problem with the download source itself. The server file itself is broken, and of course it is also broken when you download it. In addition, excessive "passionate" scanning of some antivirus software may also cause file corruption. Diagnostic problem: Determine if the file is really corrupt
How to optimize MySQL performance for high-load applications?
Apr 08, 2025 pm 06:03 PM
MySQL database performance optimization guide In resource-intensive applications, MySQL database plays a crucial role and is responsible for managing massive transactions. However, as the scale of application expands, database performance bottlenecks often become a constraint. This article will explore a series of effective MySQL performance optimization strategies to ensure that your application remains efficient and responsive under high loads. We will combine actual cases to explain in-depth key technologies such as indexing, query optimization, database design and caching. 1. Database architecture design and optimized database architecture is the cornerstone of MySQL performance optimization. Here are some core principles: Selecting the right data type and selecting the smallest data type that meets the needs can not only save storage space, but also improve data processing speed.
How to optimize database performance after mysql installation
Apr 08, 2025 am 11:36 AM
MySQL performance optimization needs to start from three aspects: installation configuration, indexing and query optimization, monitoring and tuning. 1. After installation, you need to adjust the my.cnf file according to the server configuration, such as the innodb_buffer_pool_size parameter, and close query_cache_size; 2. Create a suitable index to avoid excessive indexes, and optimize query statements, such as using the EXPLAIN command to analyze the execution plan; 3. Use MySQL's own monitoring tool (SHOWPROCESSLIST, SHOWSTATUS) to monitor the database health, and regularly back up and organize the database. Only by continuously optimizing these steps can the performance of MySQL database be improved.
Does mysql need the internet
Apr 08, 2025 pm 02:18 PM
MySQL can run without network connections for basic data storage and management. However, network connection is required for interaction with other systems, remote access, or using advanced features such as replication and clustering. Additionally, security measures (such as firewalls), performance optimization (choose the right network connection), and data backup are critical to connecting to the Internet.
Solutions to the service that cannot be started after MySQL installation
Apr 08, 2025 am 11:18 AM
MySQL refused to start? Don’t panic, let’s check it out! Many friends found that the service could not be started after installing MySQL, and they were so anxious! Don’t worry, this article will take you to deal with it calmly and find out the mastermind behind it! After reading it, you can not only solve this problem, but also improve your understanding of MySQL services and your ideas for troubleshooting problems, and become a more powerful database administrator! The MySQL service failed to start, and there are many reasons, ranging from simple configuration errors to complex system problems. Let’s start with the most common aspects. Basic knowledge: A brief description of the service startup process MySQL service startup. Simply put, the operating system loads MySQL-related files and then starts the MySQL daemon. This involves configuration
