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This article shows you the Linux operating system real-time learning example analysis, concise and easy to understand, absolutely can make you shine, through the detailed introduction of this article I hope you can gain something.

Basic concepts of real-time systems

POSIX defines a real-time system as one that provides the required level of service within a limited response time. Another definition proposed by Donald Gillies is: A real-time system means that the correctness of the calculation depends not only on the logical correctness of the program, but also on the time when the result is produced. Real-time systems have strict requirements on response time.

RTOS (Real Time Operation System) refers to an operating system that can respond correctly to internal or external abnormal events before a given deadline. In a real-time operating system, the correctness of the execution result of a process is not only related to the correctness of the logical operation or mathematical calculation result, but also related to the time when the process runs to the end of the result, that is, if the operation result of a process is correct, but because its completion time has exceeded the *** deadline given by the system, in a real-time system, this result is meaningless.

Real-time operating systems can be divided into two categories according to the response time constraints: hard real-time systems (HRT) and soft real-time systems (SRT).

Soft real-time operating system: The soft real-time operating system is not very strict with the time limit, and the response time can be relatively delayed, that is, within the range of not exceeding the *** specified time, the results are acceptable and will not have serious consequences.

Hard real-time operating system: For hard real-time operating system, not only the tasks must be able to run correctly, but also must be completed within a limited time, which is determined by the system itself, which is much stricter than the time constraints of soft real-time operating system. For example, in the embedded real-time operating system of aerospace field, if the desired calculation result is not obtained within the specified time, the consequence will be very serious.

Linux operating system real-time

1. Linux system framework

Macroscopically, the Linux framework is shown in Figure 1:

Figure 1 Linux framework diagram

To be more intuitive and easier to understand, Linux Framework Figure 1 can be abstracted into Figure 2 below.

Fig. 2

2. Factors restricting real-time performance of standard Linux kernel

There are plenty of unpreemptable regions in Linux

In Linux 2.6, kernel preemption is already possible, so real-time performance is enhanced."However, there are still a large number of non-preemptive regions in the kernel, such as critical regions protected by spinlocks.

Clock granularity coarse

Although Linux 2.6 kernel raises clock frequency to 1000 Hz and timing accuracy reaches 1ms, it is far from meeting the microsecond timing accuracy required by real-time system, such as 50us timing accuracy required by numerical control system.

shut down interrupt

In system calls and interrupt service routines,Linux turns off interrupts for long periods of time to protect critical section resources."Some system calls and interrupt service routines are also very long, which increases interrupt latency.

Lack of effective real-time task scheduling mechanism and scheduling algorithm

Linux system is designed according to the goal of time-sharing system, in order to achieve better average performance of the system, emphasizing the balance of response time between various processes to ensure fair CPU time consumption. Usually fixed time slice scheduling algorithm is adopted, kernel cannot preempt, and the behavior of real-time system depends more on complex unpredictable situation. These principles do not meet the requirements of short response times and deterministic execution behavior for real-time systems.

Priority reversal problem

Priority inversion occurs when a low-priority process occupies a resource, causing a higher-level process that also needs the resource to fail to run, and a ready process with a priority in between them gains control of the CPU, causing the higher-level task to wait for a lower-priority task. In Linux, since resources are non-preemptible and policies such as priority inheritance are not supported, priority inversion may occur, which affects the real-time performance of the system.

3. Improve Linux real-time performance

As you can see from the foregoing, the standard Linux kernel has many constraints on real-time performance, and it must be improved if Linux systems are to be used in real-time environments. At present, there are two ways to improve Linux real-time performance: direct kernel modification method and dual kernel method.

By analyzing the Linux principle and source code, modifying and optimizing the kernel process scheduling and interrupt service program, the real-time performance of the system is improved. In order to ensure the universality of the system, POSIX is required.

The relevant standards are subject to change. This approach yields good performance gains and good compatibility because it uses the same API as the standard Linux kernel. At present, there are many products that adopt kernel modification methods to improve Linux real-time performance, such as Kurt-Linux, TimesysLinux and Ingo's RT patch.

Dual kernel method, by adding a preemptive real-time kernel between Linux kernel and hardware interrupt, the standard Linux kernel is scheduled as a priority ** process of the real-time kernel, which can be intercepted by real-time processes.

Linux processes can still run on top of the Linux kernel, allowing you to use the services of the standard time-sharing operating system, Linux, while providing a low-latency, real-time environment. RT-Linux is a typical example of Linux real-time transformation with dual kernel method.

The above content is the Linux operating system real-time learning example analysis, have you learned knowledge or skills? If you want to learn more skills or enrich your knowledge reserves, please pay attention to the industry information channel.

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