These process knowledge you must know

What is the process

A program in a computer is about a running activity on a data set , Is the system resource allocation and scheduling of the basic unit , Is the foundation of the operating system architecture . In modern thread oriented computer architecture , A process is a container for threads . A program is an instruction 、 Description of data and its organization , A process is an entity of a program . Official explanation , You can understand it without speaking in vernacular , It's rare , In the language of programmers , Program is a class , Processes are objects .

The origin of the process

The existence of anything has its inevitability , Then why is there a process ？

• In order to improve the efficiency of computer system ． Enhance the parallel operation ability of various hardware in the computer system ． The operating system requires that the program structure must meet the needs of concurrent processing ． To this end, the concept of process is introduced
• Process is the core of the operating system , All operating systems based on multiprogramming are based on the concept of process
• Process is a powerful tool for designing and analyzing operating system
• Clearly depict the internal laws of dynamic systems , Effective management and scheduling of programs running into the main memory of a computer system

Characteristics of the process

• dynamic ： The essence of a process is an execution of a program in a multiprogramming system , The process is dynamic , Dynamic extinction
• concurrency ： Any process can be executed concurrently with other processes
• independence ： A process is a basic unit that can run independently , At the same time, it is also an independent unit of system resource allocation and scheduling
• asynchrony ： Due to the inter process constraints , Make the process execution intermittent , That is, processes are independent of each other 、 Advance at an unpredictable speed

Structure of process

• Program
• data
• Process control block

A program is the runnable code written by a programmer , Data is generated during program operation , The process control block is used to indicate the status of the process , Its function is to make a program that cannot run independently in a multiprogramming environment （ Contain data ）, Become a basic unit that can run independently or a process that can execute concurrently with other processes . In addition, multiple different processes can contain the same program , A program in different data sets constitutes different processes , Can get different results , But in the process of execution , The program can't change .

What has the process gone through

When the user sends instructions to the operating system that I want to start the program , First establish a process instance by the operating system , Each process has its own virtual address 、 Process control block 、 Page table and other data structures , Program entry returned from loader （ Virtual address ） Start , Operating system commands CPU Execute instructions from this virtual address , Because the code is not added to memory ,CPU Find the mapped physical address from the page table , Read the code in the hard disk into the memory through the page missing exception processing , then CPU Modify page table status , Indicates that the code has been loaded into memory , At this time, from the program entry （ Virtual address ） Start execution , With execution , The code is slowly added to memory , Keep running , There will be a blockage in the middle 、 Pending status, etc , Finally it's over , Release all data in memory .

Address assignment of the process

Each process will allocate the same virtual address space , And the same virtual address space , In fact, the corresponding physical addresses are different , But it can also be the same , So this provides favorable conditions for interprocess communication , The specific corresponding physical address actually consists of CPU Of MMU Module management . The kernel space holds the code and data of the kernel .

Process management

Why need management ？ We all know ,CPU Only one thing can be handled at a time , Only one process can be processed , And the process is definitely better than CPU More than one , So there are multiple processes using one CPU The situation of , So the significance of process management lies in the rational allocation of computer resources , The resources here include CPU、 The Internet 、 Keyboard, etc , How reasonable ？ You need to create and release at the right time , Reasonable pit occupation is the basic morality of a process . How many states does a process usually have ？ as follows

Process life cycle

• Running state
• Sleep state
• Wait state
• New state
• A dead state

The conversion process of the above states is as follows ：

The significance of process state is that it tells us , What can it do now , Especially for the operating system dad , It will load or release resources according to different states of the process , To avoid waste , Then we need a mechanism to accomplish such things , That is, the following process scheduling

Process scheduling

Scheduling means being dominated , To put it bluntly, it is dominated by the system with some algorithm , Of course, the success or failure of the algorithm , It is directly related to the throughput of the process 、CPU utilization 、 Fairness of resource acquisition , Scheduling algorithm is extremely complex , But there is a simple algorithm rule you must have heard , That is the priority algorithm , Always select the process with the highest priority , As the next running process , But when the highest priority process is not found , What to do ？ Who should run at this time ？ In fact, the system creates an idle process for us , If there is no idling process, can it ？ answer ： Can not be , Because the basic principle of process scheduling is to find the current process , Find the process to run , And then switch , If the current process is NULL, The next process to run is also NULL, How else can the scheduler execute ？ It will cause the whole system to stop , Unable to continue . Speaking of process switching , Let's have a deep understanding ：

Process switching

The most essential question is what has been switched ？ Understand this , You will understand , Let me draw a picture , Help you understand

Each process corresponds to one PCB data structure , Used to store process control information , With this information, you can continue to run the next time you come back .PCB by Process Control Block Process control block , The data structure is as follows ：

• Process status , No explanation
• pid, process ID, Uniqueness
• register , Generally, there are general registers 、 Instruction register , General access by user space , The instruction register stores the next instruction , In this way, you will know what to do when you resume operation
• priority , Priority of the process , The essence is right CPU Control priority size
• Communication information , Interprocess communication relationship , Communication channel provided by the operating system
• resources , Show the demand for resources , Like open files
• wait

There is no complete list , Because for different operating systems , There may be different data structures , So I won't list them one by one , I hope you can understand , The essence of process switching is to replace the current process object , It's just an object with the same class structure as him .

expand ：

• The source power of process switching is the trap mechanism , Belonging to a kind of interruption , Interrupts are divided into hardware interrupts and software interrupts , For short, hard medium 、 Soft medium , Hard medium 45、 Soft medium 65, Um. … Can't afford , Soft is the trap mechanism , Traps themselves also need to be driven , Then its drive is ？ If the program is abnormal , Illegal operation 、 The address is out of bounds 、 The breakpoint 、 Divide 0 wait .
• When it comes to interruptions , Have to mention , Interrupt is the only way from user mode to kernel mode , It's the only one .

author ：i The headmaster
link ：https://juejin.cn/post/7119117491330088967