Processes and threads

　 process

　　 We all know that the core of computers is CPU, It does all the computing , And the operating system is the manager of the computer , It is responsible for task scheduling , Allocation and management of resources ,

Command the whole computer hardware ; An application is a program that has some function , Programs run on the operating system .

　　 A process is a dynamic execution of a program with certain independent functions on a dataset , Is an independent unit of the operating system for resource allocation and scheduling , It's an app

The carrier of program running . Process is an abstract concept , There has never been a unified standard definition . The process is generally programmed by , Data collection and process control block are composed of three parts . The program is used to describe

Functions to be completed by the process , Is the instruction set that controls the execution of the process ; A data set is the data and workspace that a program needs to execute ; The program control block contains the description information and control information of the process

Is the only sign that a process exists

Characteristics of process ：

dynamic ： A process is an execution of a program , It's temporary , With a lifetime , It's dynamic , Dynamic extinction ;

concurrency ： Any process can execute concurrently with other processes ;

independence ： A process is an independent unit of the system for resource allocation and scheduling ;

Structural ： The process is programmed by , Data and process control block are composed of three parts

　　 Threads

　　 There was no thread concept in early operating systems , A process is the smallest unit that has resources and runs independently , It is also the smallest unit of program execution . Task scheduling adopts time slice

Round robin preemptive scheduling , Process is the smallest unit of task scheduling , Each process has a separate block of memory , Make the memory address of each process isolated from each other .

　　 later , With the development of computers , Yes CPU The demand is higher and higher , Switching between processes is expensive , Can't meet the requirements of more and more complex programs . So he invented

Thread , Thread is a single sequential control process in program execution , Is the smallest unit of program execution flow , Is the basic unit of processor scheduling and dispatching . A process can have one or

Multiple threads , Shared program memory space between threads ( That is, the memory space of the process ). A standard thread by thread ID, Current instruction pointer PC, Register and stack group

become . The process is composed of memory space ( Code , data , Process space , Open file ) And one or more threads .

　　 The difference between a process and a thread

1. A thread is the smallest unit of program execution , The process is the smallest unit of resources allocated by the operating system ;

2. A process consists of one or more threads , Threads are different execution paths of code in a process

3. Processes are independent of each other , But each thread in the same process shares the memory space of the program ( Including code snippets , Data sets , Pile etc. ) And some process level resources ( Such as opening documents and letters

Number, etc. ), Threads within a process are not visible to other processes ;

4. Scheduling and switching ： Thread context switching is much faster than process context switching

All in all , Threads and processes are abstract concepts , A thread is an abstraction smaller than a process , Threads and processes can be used for concurrency .

There was no thread concept in early operating systems , A process is the smallest unit that can have resources and run independently , It is also the smallest unit of program execution , It is equivalent to

There is only one thread in a process , Process itself is a thread . So threads are sometimes called lightweight processes

　　 later , With the development of computers , The efficiency of context switching between multiple tasks is required more and more , Just abstract out a smaller concept - Threads , In general, there will be more than one process

( It can also be a ) Threads .

Task scheduling

　　 The task scheduling of most operating systems adopts the preemptive scheduling mode of time slice rotation , In other words, after a task is executed for a short period of time, it is forced to pause to perform the next task , Every

Tasks are carried out in turn . A short period of time during which a task is performed is called a time slice , The status when the task is executing is called running status , After a task has been executed for a period of time, it is forced to pause for the next task , By

The suspended task is ready , Wait for the arrival of the next time slice that belongs to it . So that every task can be carried out , because CPU Very efficient implementation of , Time slice

Very short , Quickly switch between tasks , The feeling is that there are multiple tasks “ At the same time ”, This is what we call concurrency

 

Why not use multiple processes, but use multiple threads ？

　　　　 Thread cheap , The thread starts faster , Exit faster , The impact on system resources is also relatively small . And threads share most of the core objects with each other (File Handle) Ownership

　　　　 If you use multiple processes , But unexpected , And the test is difficult