Threads and processes

First of all, let's make a figurative analogy :

CPU-> Equivalent to a project , Running all the time .

process -> Equivalent to the workshop of the factory , His representative CPU The single task that can be handled . Anytime ,CPU Always run a process , Other processes are not running .

There will be many workers in a workshop , They worked together on a task , Threads are equivalent to workers in the workshop , A process can contain multiple processes .

There can be more than one thread in a process , They share the resources of this process . Threads are also called mini processes , But it's easier to create than a process , It's also easier to undo .

Thread is the smallest unit of operation performed in a process , It is the basic unit of operating system execution processing mechanism . Each process has at least one thread , Threads can use the resources owned by the process to schedule and calculate .

But why do we often hear about multithreading instead of multiprocessing ?

Then let's talk about the progress :

The characteristics of a thread are similar to that of a process , He basically has all the processes , such as :

• The thread has a ready , Blocking , There are three basic states of operation , It also has the transition relationship between states .
• Threads can execute concurrently ,
• In a multiple CPU Under the environment of , Multithreading can also be assigned to different CPU Execute in parallel -> make the best of CPU

advantage : Multiple threads can exist simultaneously in the same process , These threads share the resources of the process , Process built communication must request operating system services ( because CPU To switch to kernel mode ), Costly , Between threads in the same process , No operating system is required to dare , It costs less .

however , Thread communication between different processes , You must also request operating system services

Concurrency between threads is less expensive than concurrency between processes , The system has been improved

Again , It should be noted that , Operate from threads of different processes , It will cause process switching ,, So it costs a lot .

These characteristics should lead to : Whether it's creating threads , Destruction of the thread , Scheduling threads are faster than processes

But the process also has advantages and disadvantages :

When a thread in a process crashes , It will cause all threads in the process to crash .

Since the introduction of processes and threads is so similar , So let's talk about putting threads and processes together

Before introducing threads , Process is the basic unit of resource allocation and independent scheduling . After introducing threads , Process is the basic unit of resource allocation , Threads are the basic unit of independent scheduling

Process and thread comparison

• Process is the basic unit of resource allocation in the operating system , All resources related to the process , Will be recorded in the process control block PCB in , All resources owned by this process have been indicated . All threads under the same process share all resources under the process
• Thread is the basic unit of allocating processor , It has nothing to do with resource allocation . in fact . The thread running on the processor , Not the process
• A process can only belong to one process , A process can belong to multiple threads , But at least one thread .
• Threads need to cooperate and synchronize when executing , Different processes should be synchronized by means of message communication .

The relationship between threads and processes

• Process will CPU Resource allocation to thread , That is true. CPU Running threads on
• The operating system is assigned to the process , All threads of the same process share all resources of the process

The difference between a process and a thread

• Threads in the same process share the same memory space , But processes are independent .
• The data of all threads in the same process is shared ( Process communication ), Data between processes is independent .
• For threads in the same process, other threads may be affected , But one process will not affect other processes
• Thread is a context execution instruction , A process is a cluster of resources related to operations .
• Threads of the same process can communicate directly , But the communication between processes needs to be realized with the help of intermediate agents .
• It's easy to create new threads , But creating a new process requires a copy of the parent process .
• One thread can operate other threads of the same process , But a process can only operate its child processes .
• Threads start fast , The process starts slowly （ But there is no comparability between the two ）.

Multi process

At the same time , If two or more processes are allowed to run in the same computer system , This is multitasking （ Multi process ）. Modern operating systems are almost all multi process operating systems , It can manage the running of multiple processes at the same time . The benefits of multiple processes are clear . However, multi processes have high requirements for system resources , The waste of resources is also serious . The most commonly used multi process scenarios are windows System , For example, open and run the software at the same time , Each software opening is equivalent to running a process .

Multithreading

In a complete piece of code , There is often a need for independent code modules , These independently running program segments are called “ Threads ”（Thread）, The concept of programming with multiple threads is called multithreading ( Multithreaded programming ), Multithreading is to accomplish multiple tasks synchronously , It's not to improve operational efficiency , But to improve the efficiency of resource utilization to improve the efficiency of the system . Multithreading is implemented when a program needs to complete multiple tasks at the same time . The purpose of multithreading is only to improve the efficiency of resource utilization . Each thread performs its own task , These threads can ” At the same time “. Simultaneously, not at the same time , But in a certain period of time , Complete all tasks , There is a sequence of tasks .