Python Multi-process

1. Process VS Program

Code that has been written is called a program when it is not running.

Running code, it becomes a process

Process, in addition to containing code, there is a need to run the environment, etc., so there is a difference with the program

FORK

#Coding=utf-8ImportOSImportTimenum=0#Note that the fork function, which runs only on Unix/linux/mac, does not allow WindowsPID =Os.fork () #相当于创建了2个进程

ifPID = =0:num+=1Print('haha 1---num=%d'%num)Else: Time.sleep (1) Num+=1Print('haha 2---num=%d'%num)

Operation Result:

Summarize:

• In multiple processes, all data in each process (including global variables) has one copy, each of which does not affect
• Can only be executed in Linux/unix, no fork in Windows
• Fork produces a child process with a PID of 0 at execution time.

Results

Multiple fork Problems

If there are 2 fork function calls in a program, will there be 3 processes?

#Coding=utf-8ImportOSImport Time#Note that the fork function, which runs only on Unix/linux/mac, does not allow WindowsPID =os.fork ()ifPID = =0:Print('haha 1')Else:    Print('haha 2') PID=os.fork ()ifPID = =0:Print('haha 3')Else:    Print('haha 4') Time.sleep (1)

Operation Result:

So the if else above is divided into two processes, and two processes end up separating 2 processes in total four processes.

Order of execution of parent-child processes

The sequence of parent process and sub-process execution is not regular, it depends entirely on the operation system scheduling algorithm

# here is the fork bomb, do not believe you can.   while Ture:        os.fork

Multiprocessing

If you are going to write a multi-process service program, Unix/linux is undoubtedly the right choice. Because Windows does not have a fork call, isn't it possible to write multi-process programs in Python on Windows?

Because Python is cross-platform, nature should also provide a cross-platform, multi-process support. The multiprocessing module is a multi-process module with cross-platform versions.

The multiprocessing module provides a process class to represent a processing object, and the following example demonstrates starting a child process and waiting for it to end:

 fromMultiprocessingImportProcess#code to be executed by the child processdefTest (name):Print('child process running, name=%s, pid=%d...ppid=%d'%(name, Os.getpid (), Os.getppid ()))if __name__=='__main__':    Print('parent process%d.'%os.getpid ()) P= Process (Target=test, args= ('Test',)) Print('The child process is going to execute') P.start ()#let the process begin executing the code in the test functionP.join ()# wait for the child process to end    Print('child process has ended')

Operation Result:

Process

• Target: Represents the object called by this process instance
• Args: The parameter tuple that represents the calling object
• Name: Alias for the current process instance

Process Common methods:

1. Is_alive (): Determines whether the process instance is still executing;
2. Join ("timeout"): whether to wait for process instance execution to end, timeout: time-out
3. Start (): Start the process instance (Create child process)
4. Run (): When the Start () method is called on this object without a given target parameter, the Run method of the object is executed
5. Terminate (): Terminates immediately regardless of whether the task (process) has ended

Python Multi-process