managing workerthreads

[darksaga]

hi all,

in my application I'd like to split the processing of some of my functions into several threads.

Several "workerthreads" should be processed simultaneously, increasing spead especially on multicore systems, and when accessing databases.

The "workerthreads" should be observerd & managed by a "observerthread".

in general it should look like:



worker1---
|
worker2-----<--observer---<---maineventLoop
|
worker3---

etc.


I've already implemented a minimum example, and would like to hear your opinions on how to improve my idea in terms of speed, threadsafety and adaptability.
i'm sure you'll find some glitches :rolleyes: , but its the first time i use the qt threading system in an expanded way...


simple main


#include "QueryThread.h"
#include <QApplication>

int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QueryThread ob(2);
ob.start();
return app.exec();
}


observer


#ifndef QUERYTHREAD_H
#define QUERYTHREAD_H

#include <QThread>
#include <QList>
#include <QWaitCondition>
#include <QMutex>
#include <WorkerThread.h>

class QueryThread : public QThread
{
Q_OBJECT

public:
QueryThread(int maxWorkers = 1, QObject *parent = 0) : QThread(parent){
_maxWorkers = maxWorkers;
_currentWorkers = 0;
}
~QueryThread (void){
for (int i = 0; i < _list.size(); i++){
_list.at(i)->terminate();
_list.at(i)->wait();
delete _list.at(i);
}
_list.clear();
}
void run(){
for (int i = 0; i < 10; i++){
_mutex.lock();
if(_currentWorkers >= _maxWorkers)
_wait.wait(&_mutex);
qDebug("Query %d", i);
createWorker();
_mutex.unlock();
}
}

public slots:
void workerThreadFinsihed(int id){
QMutex mutex;
mutex.lock();
_currentWorkers--;
freeList();
_wait.wakeOne();
mutex.unlock();
}

signals:
void queryFinished();

private:
void createWorker(void){
static int testId = 0;//testid
WorkerThread *work = new WorkerThread(testId++);
_currentWorkers++;
connect(work, SIGNAL(workFinished(int)), this, SLOT(workerThreadFinsihed(int)));
_list.append(work);
work->start();
}
void freeList(void){
for (int i = 0; i < _list.size(); i++){
if (_list.at(i)->isFinished()){
delete _list.at(i);
_list.removeAt(i);
}
}

}
QList<WorkerThread*> _list;
QWaitCondition _wait;
QMutex _mutex;
int _maxWorkers;
int _currentWorkers;
};
#endif


worker


#ifndef WORKERTHREAD_H
#define WORKERTHREAD_H

#include <QThread>

class WorkerThread : public QThread
{
Q_OBJECT

public:
WorkerThread(int id = 0, QObject *parent = 0) : QThread(parent){
_id = id;
}
~WorkerThread(void){qDebug("Worker %d delete", _id);}
void run(){
for (int i = 0; i < 5; i++){
qDebug("Worker %d, iteration %d", _id, i);
sleep(1);
}
emit workFinished(_id);
}
signals:
void workFinished(int id);

private:
int _id;
};
#endif


regards

[Michiel]

What you're creating here is commonly called a thread pool (http://en.wikipedia.org/wiki/Thread_pool). Maybe you can use that as a search term to find more information about them.

Actually, I'm surprised Qt doesn't have a thread pool implementation itself. :)

[wysota]

Actually, I'm surprised Qt doesn't have a thread pool implementation itself. :)

Why should it have an own thread pool implementation?

http://labs.trolltech.com/page/Projects/Threads/QtConcurrent

[Michiel]

I knew you'd come along to correct me eventually. ;)


Why should it have an own thread pool implementation?

http://labs.trolltech.com/page/Projects/Threads/QtConcurrent

Interesting. But how prone to change is that framework in this stage?

[wysota]

Very prone :) But you don't have to update it once it works ;) AFAIK it'll be part of Qt 4.4 (which will be released around January), so be patient while waiting for a stable release.