Multitask programming

3. Multitask programming

Multitasking refers to executing multiple tasks at the same time , for example : Now the operating systems installed on computers are multitasking operating systems , Can run multiple software at the same time

3.1 Multi task execution mode

3.1.1 Concurrent

• Alternate tasks over a period of time .

3.1.2 parallel

• For multi-core cpu Dealing with multitasking , The operating system will give cpu Each kernel of is scheduled to have an executing software
• Multiple cores are really executing software together . Here we need to pay attention to multi-core cpu It's multitasking in parallel , There are always multiple software executing together

3.2 process

A running program or software is a process , It is the basic unit of resource allocation in the operating system

At least one process after a program runs , A process has one thread by default , Multiple threads can be created in the process , Threads are attached to processes , No process, no thread

Multiple processes can accomplish multiple tasks

3.2.1 The use of multiprocessing

• Process package

  • multiprocessing

• Process Description of the process class Process([group [, target [, name [, args [, kwargs]]]]])

  • group： Specify process group , At present, you can only use None
  • target： Name of the target task to be executed
  • name： Process name
  • args： Pass parameters to the execution task in tuple mode
  • kwargs： Pass parameters to the execution task in a dictionary

• Process Common methods of creating instance objects

  • start()： Start subprocess instance （ Create child process ）
  • join()： Wait for subprocess execution to finish
  • terminate()： Whether the task is completed or not , Terminate child process now

3.2.2 Get the process number

• Get the current process number os.getpid()
• Get the current parent process number os.getppid()
• Get the process number to view the relationship between parent and child processes

3.2.3 Processes perform tasks with parameters

• args Parameters

  • Tuples

    • Tuple mode parameter transmission must be consistent with the order of parameters .

• kwargs Parameters

  • Dictionaries

    • Dictionary mode refers to... In the dictionary key Be sure to keep consistent with the parameter name .

3.2.4 Be careful

• Global variables are not shared between processes

  • Creating a child process copies the resources of the main process , That is, the child process is a copy of the main process , Like a pair of twins , The reason why global variables are not shared between processes , Because the global variables in the same process are not operated , It's just that the names of global variables in different processes are the same .

• The main process will wait for all child processes to finish executing

• How to make the main process exit and the child process destroy , Don't let the main process wait for the child process to execute

  • Guard the main process

    • Subprocess object .daemon = True

  • Destroy subprocesses

    • Subprocess object .terminate()

• Execution between processes is also out of order

  • Determined by the operating system scheduling

3.3 Threads

Thread is cpu The basic unit of dispatch , Each process has at least one thread , And this thread is what we usually call the main thread

Multithreading can also accomplish multitasking

3.3.1 Use of multithreading

• Thread module

  • threading

• Thread class Thread Parameter description

  • Thread([group [, target [, name [, args [, kwargs]]]]])

    • group: Thread group , At present, you can only use None
    • target: Name of the target task to be executed
    • args: Pass parameters to the execution task in the form of tuples
    • kwargs: Pass parameters to the execution task in a dictionary
    • name: The thread of , In general, there is no need to set

• sub_thread = threading.Thread(target= Task name )

• Start thread

  • start Method

3.3.2 Threads perform tasks with parameters

• args Parameters

  • Tuple mode parameter transmission must be consistent with the order of parameters .

• kwargs Parameters

  • Dictionary mode refers to... In the dictionary key Be sure to keep consistent with the parameter name .

3.3.3 Be careful

• Execution between threads is out of order

  • from cpu Scheduling decision

• The main thread will wait for all child threads to finish executing

  • How to make the main thread exit and the sub thread destroy , Don't let the main thread wait for the sub thread to execute

    • Guard the main thread

      • threading.Thread(target=show_info, daemon=True)
      • Process object .setDaemon(True)

• Sharing global variables between threads

• There is an error in sharing global variable data between processes

  • terms of settlement ： Thread synchronization

    • Thread waiting join()

      • th1.start()
        th1.join()
        th2.start()

    • The mutex

3.4 The mutex

Lock shared data , Ensure that only one thread can operate at the same time

3.4.1 Use

• Create a lock

  • mutex = threading.Lock()

• locked

  • mutex.acquire()

• The lock ( Liberation lock )

  • mutex.release()

3.4.2 Be careful

• acquire and release The code between methods can only have one thread to operate at the same time
• If you're calling acquire Method time Other threads already use this mutex , So at this time acquire Method will be blocked , The mutex cannot be locked again until it is released

Coupled with mutex, multitasking instantly becomes a single task , Performance will degrade , That is to say, only one thread can execute at the same time

3.5 Deadlock

The situation of waiting for the other party to release the lock is deadlock

result

• Will cause the application to stop responding , No more tasks

3.5.1 Avoid deadlock

• Release the lock in the right place

3.6 Process versus thread

3.6.1 Relationship comparison

• Threads are attached to processes , No process, no thread .
• A process provides a thread by default , Processes can create multiple threads .

3.6.2 Difference and contrast

• Global variables are not shared between processes
• Sharing global variables between threads , But we should pay attention to the problem of resource competition , terms of settlement : Mutex or thread synchronization
• The resource cost of creating a process is greater than that of creating a thread
• Process is the basic unit of operating system resource allocation , Thread is CPU The basic unit of dispatch
• Thread cannot execute independently , Must be in the process of existence
• Multi process development is more stable than single process and multi thread development

3.6.3 Comparison of advantages and disadvantages

• Process advantages and disadvantages :
  advantage ： You can use multi-core
  shortcoming ： Resources are expensive
• Advantages and disadvantages of threads :
  advantage ： The cost of resources is small
  shortcoming ： Multi core cannot be used