[Qt Threads] In-depth exploration of QThread thread priority: principles, applications and best practices

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Detailed explanation of QThread thread priority

Chapter 1: Basic concepts of QThread thread priority

1.1 What is QThread?

QThread is a class in the Qt library used for multi-threaded programming. It provides a cross-platform way to manage threads. QThread is typically used to perform time-consuming or blocking tasks to improve application responsiveness.

Code example:

class MyThread : public QThread
{
     
     
protected:
    void run() override {
     
     
        // 线程执行的任务
    }
};

In this example, we define a MyThreadclass called , which inherits from QThread. We overridden run()the method and it will be executed in a new thread.

1.2 What is thread priority?

Thread Priority is a criterion used by the operating system to decide which thread should get CPU time. In a multi-threaded environment, threads with higher priority usually get CPU time earlier than threads with lower priority.

As Bjarne Stroustrup said in "The C++ Programming Language":
"Concurrency is about structure, parallelism is about execution."

1.3 How to set QThread thread priority

In QThread, you can setPriority()set the priority of the thread through methods. This method accepts an QThread::Priorityenum value as parameter.

Code example:

MyThread thread;
thread.setPriority(QThread::HighPriority);
thread.start();

In this example, we create an MyThreadobject and setPriority()set its priority via methods HighPriority.

Table: QThread priority enumeration values

QThread::Priority enumeration value	describe
QThread::IdlePriority	lowest priority
QThread::LowestPriority	lower priority
QThread::LowPriority	low priority
QThread::NormalPriority	normal priority
QThread::HighPriority	high priority
QThread::HighestPriority	higher priority
QThread::TimeCriticalPriority	time critical priority
QThread::InheritPriority	Inherit the priority of the creation thread

In this part, we learned the basic concepts of QThread thread priority, including what QThread is, what thread priority is, and how to set thread priority in QThread. In the following sections, we’ll dive into the inner workings and application scenarios of these concepts.

Note:
In the Qt source code, setPriority()the method is implemented in qthread_unix.cpp(Unix system) and (Windows system). qthread_win.cppThese files handle thread priority settings on Unix and Windows platforms respectively.

Chapter 2: Internal mechanism and working principle of QThread thread priority

2.1 Internal Mechanism: How to Implement Thread Priority

QThread thread priority is implemented through the thread scheduling mechanism of the underlying operating system. When you set the priority of a QThread object, the Qt library calls the corresponding system call to change the thread's priority.

Note:
In Linux systems, this function is pthread_setschedparamimplemented through a function, which is defined in pthread.hthe header file.

2.2 How it works: How priority affects thread scheduling

The way thread priority affects thread scheduling depends on the thread scheduling algorithm used. Generally speaking, threads with higher priority will get CPU time earlier than threads with lower priority. However, in some cases, in order to prevent "starvation", the operating system may temporarily promote a lower priority thread.

As Bjarne Stroustrup said in "The C++ Programming Language":
"Resource acquisition is initialization."

2.3 Application scenarios: When should thread priorities be adjusted?

1. Real-time processing : When a thread needs to respond quickly to external events, a higher priority should be set.
2. Background tasks : For tasks that do not require immediate response, such as logging or data backup, you can set a lower priority.

Table: Application scenarios and recommended priority settings

Application scenarios	Recommended priority settings
real time processing	HighPriority
User interaction	NormalPriority
Background tasks	LowPriority

Code example:

// 实时处理任务
MyThread realtimeThread;
realtimeThread.setPriority(QThread::HighPriority);
realtimeThread.start();

// 后台任务
MyThread backgroundThread;
backgroundThread.setPriority(QThread::LowPriority);
backgroundThread.start();

In this example, we set up two threads with different priorities, one for real-time processing and the other for background tasks.

Note:
In the source code of Qt, setPriority()the specific implementation of the method can be found in qthread_unix.cpp(Unix system) and qthread_win.cpp(Windows system). These files describe in detail how to set thread priorities on various platforms.

Chapter 3: Practical applications, problems and solutions

3.1 Practical application: How to use QThread thread priority in projects

In actual projects, setting thread priorities reasonably can significantly improve the performance and response speed of the program. For example, in an image processing application, you may need a high-priority thread to display the image in real time, while using a low-priority thread to save the image.

Code example:

// 高优先级线程用于图像显示
MyThread displayThread;
displayThread.setPriority(QThread::HighPriority);
displayThread.start();

// 低优先级线程用于图像保存
MyThread saveThread;
saveThread.setPriority(QThread::LowPriority);
saveThread.start();

In this example, displayThreadit is used to display images in real time, so a high priority is set. And saveThreadfor image saving, low priority is set.

3.2 Frequently Asked Questions and Solutions

1. 问题：线程优先级设置不生效
   **解决方案：**确保操作系统和编译器支持线程优先级设置，并检查是否有足够的系统权限。

2. 问题：线程“饥饿”
   **解决方案：**使用动态优先级调整或者时间片轮转等调度算法。

3. 问题：跨平台兼容性问题
   **解决方案：**使用Qt提供的跨平台API，并避免使用平台特定的线程优先级设置。

表格：常见问题与解决方案

常见问题	解决方案
线程优先级设置不生效	确保操作系统和编译器支持，检查系统权限
线程“饥饿”	使用动态优先级调整或时间片轮转
跨平台兼容性问题	使用Qt的跨平台API

3.3 最佳实践与建议

1. 避免频繁改变线程优先级：这会增加CPU调度的复杂性。
2. 权衡优先级与资源使用：设置过高的优先级可能会导致其他线程得不到足够的CPU时间。
3. 测试与调优：在实际环境中测试线程优先级的效果，并根据需要进行调优。

正如Bjarne Stroustrup在《The C++ Programming Language》中所说：
“Make interfaces precisely and strongly typed.”

这一部分中，我们探讨了如何在实际项目中应用QThread线程优先级，以及可能遇到的问题和解决方案。通过合理地设置和管理线程优先级，我们不仅可以提高程序的性能，还可以更好地满足用户需求。

注意：
在Qt的源码中，setPriority()方法的具体实现可以在qthread_unix.cpp（Unix系统）和qthread_win.cpp（Windows系统）中找到。这些文件详细描述了如何在各个平台上设置线程优先级。

结语

在我们的编程学习之旅中，理解是我们迈向更高层次的重要一步。然而，掌握新技能、新理念，始终需要时间和坚持。从心理学的角度看，学习往往伴随着不断的试错和调整，这就像是我们的大脑在逐渐优化其解决问题的“算法”。

这就是为什么当我们遇到错误，我们应该将其视为学习和进步的机会，而不仅仅是困扰。通过理解和解决这些问题，我们不仅可以修复当前的代码，更可以提升我们的编程能力，防止在未来的项目中犯相同的错误。

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