Qt5 Tutorial QThreads - Priority - 2020

In this tutorial, we will learn how a thread behaves depending on its priority.

Most of the codes are the same as in the QThreads - Creating Threads except the portion that sets the thread priority.

The priority is set by the parameter passed into the start() method:

Its definition looks like this:

void QThread::start(Priority priority = InheritPriority)

The start() begins execution of the thread by calling run().

The operating system will schedule the thread according to the priority parameter. If the thread is already running, this function does nothing.

The effect of the priority parameter is dependent on the operating system's scheduling policy. In particular, the priority will be ignored on systems that do not support thread priorities.

enum QThread::Priority

Let's look at changes made in main.cpp:

#include <QCoreApplication>
#include "mythread.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    
    MyThread thread1("A"), thread2("B"), thread3("C");

    thread1.start(QThread::LowestPriority);
    thread2.start();
    thread3.start(QThread::HighestPriority);

    return a.exec();
}

The scheduler on my system seems respect the thread priority. The lowly "A" thread finished last!

...
"A"   77
"A"   78
"A"   79
"A"   80
"A"   81
"A"   82
"A"   83
"A"   84
"A"   85
"A"   86
"A"   87
"A"   88
"A"   89
"A"   90
"A"   91
"A"   92
"A"   93
"A"   94
"A"   95
"A"   96
"A"   97
"A"   98
"A"   99
"A"   100

There could be a priority inversion issue. Please visit for more on Priority Inversion.