QThreadPool

2017/7/29 14:03:16

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QThreadPool

The QThreadPool class manages a collection of QThreads. QThreadPool manages and recycles QThread to help reduce thread creation and deconstruction costs in programs, especially in sever. As the following formula:

 #Mathjax not supported! So sad...
 cost = (create + destroy) / (create + destroy + run)

If we create the thread in a high concurrency server, the cost is unacceptable.

• QThreadPool
• QRunnable
• References

QThreadPool

To use one of the QThreadPool threads, subclass QRunnable and implement the run() virtual function. Then create an object of that class and pass it to QThreadPool::start().
QThreadPool deletes the QRunnable automatically by default. Use QRunnable::setAutoDelete() to change the auto-deletion flag.
Threads that are unused for a certain amount of time will expire. The default expiry timeout is 30000 milliseconds (30 seconds). This can be changed using setExpiryTimeout(). Setting a negative expiry timeout disables the expiry mechanism.

Key functions:

• QThreadPool *QThreadPool::globalInstance(): Each Qt application has a global thread pool.
• void QThreadPool::start(QRunnable *runnable, int priority = 0): Reserves a thread and uses it to run runnable, unless this thread will make the current thread count exceed maxThreadCount().

Example:

#include 
#include 
#include 
#include 
#include 

class Task : public QRunnable {
protected:
    virtual void run() {
        for ( int i = 0; i < 5; ++i ) {
            qDebug() << QThread::currentThreadId();
            QThread::msleep( 1000 );
        }
    }
};

int main( int argc, char **argv ) {
    QCoreApplication app( argc, argv );

    QThreadPool pool;
    pool.setMaxThreadCount( 3 );

    for ( int i = 0; i < 100; ++i ) {
        Task *task = new Task;
        pool.start( task );
    }

    return 0;
}

QRunnable

The QRunnable class is the base class for all runnable objects. The QRunnable class is an interface for representing a task or piece of code that needs to be executed, represented by your reimplementation of the run() function.

Key functions:

• void QRunnable::run(): Implement this pure virtual function in your subclass.

References

• QThreadPool class
• QRunnable class
• QRunnable and QThreadPool