Is Linux a real-time system or a time-sharing operating system?

The µC/OS, FreeRTOS, RT-Thread, and ThreadX we often share are all real-time operating systems (RTOS), so some readers may ask: What is a time-sharing operating system? Is Linux considered a real-time operating system?

Real-time Operating System (RTOS)

RTOS, the full name is Real Time Operating System, which is what we call real-time operating system.

1. Real-time operating system definition A real-time operating system (RTOS) is a system that can quickly accept and process external events or data when they are generated, and its processing results can be controlled within a specified time. The production process or processing system responds quickly and coordinates the operation of all real-time tasks.

Therefore, the main characteristics of a real-time operating system are to provide timely response and high reliability.

Real-time operating systems are divided into two types: hard real-time and soft real-time. Hard real-time systems require that operations must be completed within a specified time, which is guaranteed during the design of the operating system.

Soft real-time systems only need to complete the operation as quickly as possible according to the priority of the task. The operating system we usually use can become a real-time operating system after certain modifications.

A real-time operating system is an operating system that guarantees the completion of specific functions within a certain time limit. For example, an operating system could be designed to ensure that a robot on a production line can access an object. In a "hard" real-time operating system, if the calculations to make the object reachable cannot be completed within the allowed time, the operating system will terminate with an error.

In a "soft" real-time operating system, the production line can still continue to work, but the output of the product will be slowed down because the product cannot arrive within the allowed time, which causes the robot to have a short period of non-production. Some real-time operating systems are designed for specific applications, others are general-purpose.

Some general-purpose operating systems call themselves real-time operating systems. But to some extent, most general-purpose operating systems, such as Microsoft's Windows NT or IBM's OS/390, have real-time system characteristics. That is to say, Even if an operating system is not strictly a real-time system, they can solve some real-time application problems.

2. Characteristics of real-time operating system
1)Multi-tasking;
2) There is thread priority
3) Multiple interrupt levels

Small embedded operating systems often require real-time operating systems, and the kernel must meet the requirements of real-time operating systems.

3. Related concepts of real-time operating system
(1) Basic concepts
Critical section of code: refers to the indivisible part during processing code. Once this part of the code starts executing, no interrupt is allowed;

Resources:Any entity occupied by the task;

Shared resources: Resources that can be used by more than one task;

Task: Also called a thread, it is a simple program. Each task is given a certain priority, has its own set of CPU registers and its own stack space. Typically, each task is an infinite loop, and each task is in the following five states: sleeping state, ready state, running state, suspended state, and interrupted state;

Task switching: Save the current status of the running task (all contents in the CPU register) in the task's own stack area, and then change the current status of the next task to be run from the task's Reload the CPU registers into the stack and start running the next task;

Kernel: Responsible for managing various tasks, allocating CPU time to each task, and responsible for communication between tasks. Divided into inalienable cores and deprivable cores;

Scheduling: One of the main responsibilities of the kernel is to decide which task's turn to run. Generally based on priority scheduling method;

(2) Questions about priority
Task priority: It is divided into static priority whose priority cannot be changed and dynamic priority whose priority can be changed;

Priority Inversion: The priority inversion problem is the most common problem in real-time systems. The allocation of shared resources can cause low-priority tasks to run first and high-priority tasks to run later. The solution is to use a "priority inheritance" algorithm to temporarily change task priorities to curb priority inversion.

(3) Mutual exclusion
Although the shared data area simplifies the exchange of information between tasks, each task must be guaranteed to be exclusive in processing shared data. The general methods to satisfy mutual exclusion conditions include: turning off interrupts, using test and set instructions (TAS), prohibiting task switching, and using semaphores.

Because the significance of using a real-time operating system is to be able to handle various unexpected events in a timely manner, that is, to handle various interrupts, the most important and representative performance index parameters for measuring the embedded real-time operating system should undoubtedly be Interrupt response time. Interrupt response time is usually defined as:

Interrupt response time = interrupt delay time, time to save the CPU state, execution time of the ISR of the kernel entering the function.

Interrupt delay time = MAX (maximum time to turn off interrupts, maximum instruction time) The time to start executing the first instruction of the ISR.

Time Sharing Operating System (TSOS)

TSOS, the full English name is Time-sharing Operating System, that is, time-sharing operating system.

An operating system that enables a computer to serve several, dozens or even hundreds of users at the same time is called a time-sharing operating system. By connecting the computer to many end users, the time-sharing operating system switches the system processor time and memory space to the programs of each end user in turn at certain intervals.

Because the time interval is short, each user feels as if he has the computer exclusively. The characteristic of time-sharing operating system is that it can effectively increase resource utilization. For example, UNIX systems use deprived dynamic priority CPU scheduling to effectively support time-sharing operations.

The time-sharing system is a new type of OS formed to meet user needs. There is a completely different performance difference between it and the multi-channel batch processing system. User needs are specifically reflected in the following aspects: Human-computer interaction Shared hosting Convenient for users to get on the computer

1. The basic idea of ​​time-sharing system

Time slice: is to divide the computer's system resources (especially CPU time) into time. Each time period is called a time slice, and each user takes turns using the time slice.

Time sharing technology: Divide the running time of the processor into very short time slices, and allocate the processor to each online job in turn according to the time slice.

Time-sharing operating system: is an online multi-user interactive operating system. Generally, time slice rotation is used to enable one computer to serve multiple terminals. Ensure fast enough response time for each user and provide interactive session capabilities.

Design goals: Respond to user requests in a timely manner and maximize the utilization of system resources where possible.

Suitable for office automation, teaching and transaction processing and other occasions that require human-machine conversation.

2. Working method
A host is connected to several terminals; each terminal is used by a user; interactively makes command requests to the system; the system accepts commands from each user; uses time slice rotation to process service requests; and interacts with the terminal on The results are displayed to the user; the user issues the next command based on the results of the previous step

The key issue in the implementation of time-sharing system: timely reception. Deal with it promptly.

3. Features
Interactivity: Users engage in human-computer dialogue with the system.
Multiplexing: Multiple users use the same CPU on their respective terminals at the same time.
Independence: Users can operate independently of each other without interfering with or confusing each other.
Timeliness: Users can get timely answers from the system in a short period of time.
Factors that affect the response time: the number of terminals, the size of the time slice, the amount of information exchange, and the speed of information exchange.

the difference

RTOS and TSOS each have their own characteristics. RTOS is generally used for relatively low-speed MCUs, such as motion control, key input and other systems that require real-time processing, generally requiring ms level or even us level. response.

Time sharing: Now popular PCs and servers all adopt this operating mode, which divides the CPU operation into several time slices to process different computing requests.

Real-time: Generally used on microcontrollers, such as the up and down control of elevators, where real-time processing is required for actions such as button presses.

at last

Through the above analysis, it can be clear that linux is a time-sharing system, but it can be changed to real-time. For example: ucLinux is a real-time system modified from linux. As for their differences, you can quote from Baidu Similar answers:

The time-sharing system is a system that can serve two or more accounts at the same time!

A real-time system is an operating system that can respond to instructions immediately! Microsoft’s common systems can’t! And it’s still dead! The operating system in a fighter jet is a real-time system. Think about it, if the computer in the fighter jet responds to the pilot's last command or crashes when someone else is fighting, who would dare to fly the plane?

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