QThread correct usage

shav

Hi everyone!

I have a images gallery application. All images I'm loading in QThread. In QtCreator all works fine (Release and Debug mode). But if I try to load my app just click on MyApplication.app I can't get images list only at first time. When I try to refresh the list I'm receiving error:

2018-07-20 12:48:36.834 ShavSample[15543:506104] Persistent UI failed to open file file:///Users/andrewsh/Library/Saved%20Application%20State/com.consultica.ShavSample.savedState/window_1.data: Too many open files (24)
QThreadPipe: Unable to create pipe: Too many open files
QEventDispatcherUNIXPrivate(): Can not continue without a thread pipe
Abort trap: 6

I'm using Qt 5.11 on macOS 10.13.6.
Create a worker:

ShavGetImagesInfoWorker* worker = new ShavGetImagesInfoWorker(urls, callback);
if(!ShavTaskManager::isObjectCreated()) {
    ShavTaskManager::initManager();
}
ShavTaskManager::shared()->startWork(worker);

Start thread:

if(worker) {
    m_tasks.append(worker);
    QThread* newThread = new QThread();
    connect(newThread, &QThread::started, worker, &ShavThreadWorker::run);
    connect(worker, &ShavThreadWorker::finished, this, &ShavTaskManager::onFinished);
    connect(worker, &ShavThreadWorker::finished, newThread, &QThread::quit);
    connect(newThread, &QThread::finished, newThread, &QObject::deleteLater);
    worker->moveToThread(newThread);
    newThread->start(static_cast<QThread::Priority>(priority));
}

All worker is a subclass from class:

class ShavThreadWorker : public QObject
{
        Q_OBJECT
    private:
        ShavTaskResult* m_result;
        ShavTaskResult* m_progress;
        QMutex m_mutex;
    protected:
        virtual QObject* doTaskAction();
        void updateProgress(QVariant value, QVariant total, QVariant userData = QVariant());
    public:
        explicit ShavThreadWorker(QJSValue callback, QObject *parent = nullptr);
        explicit ShavThreadWorker(QJSValue callback, QJSValue progress, QObject *parent = nullptr);
        ~ShavThreadWorker();
    signals:
        void finished();
    private slots:
        inline void onFinishedResult() {
            emit finished();
        }
    public slots:
        void run();
        inline void stop() {
            this->thread()->requestInterruption();
        }
 };

and implementation of calls:

QObject *ShavThreadWorker::doTaskAction() {
    return nullptr;
}

void ShavThreadWorker::updateProgress(QVariant value, QVariant total, QVariant userData) {
    if(userData.isNull()) {
        QMetaObject::invokeMethod(m_progress, "updateProgress", Qt::QueuedConnection,
                                  Q_ARG(QVariant, value), Q_ARG(QVariant, total));  //Main thread
    } else {
        QMetaObject::invokeMethod(m_progress, "updateProgress", Qt::QueuedConnection,
                                  Q_ARG(QVariant, value), Q_ARG(QVariant, total), Q_ARG(QVariant, userData));  //Main thread
    }
}


ShavThreadWorker::ShavThreadWorker(QJSValue callback, QObject *parent) : QObject(parent) {
    static int index = 0;
    setObjectName(QString("%1_%2").arg(metaObject()->className()).arg(index));
    index += 1;

    m_result = new ShavTaskResult(callback);
    connect(m_result, SIGNAL(finished()), SLOT(onFinishedResult()));
    m_progress = nullptr;
}

ShavThreadWorker::ShavThreadWorker(QJSValue callback, QJSValue progress, QObject *parent) : QObject(parent) {
    static int index = 0;
    setObjectName(QString("%1_%2").arg(metaObject()->className()).arg(index));
    index += 1;

    m_result = new ShavTaskResult(callback);
    connect(m_result, SIGNAL(finished()), SLOT(onFinishedResult()));
    m_progress = new ShavTaskResult(progress);
}

ShavThreadWorker::~ShavThreadWorker() {
#ifdef QT_DEBUG
    qDebug()<<metaObject()->className()<<"was released!";
#endif
}

void ShavThreadWorker::run() {
    m_mutex.lock();
    QObject* res = doTaskAction();
    if(m_result != nullptr) {
        res->moveToThread(qApp->thread());
        QMetaObject::invokeMethod(m_result, "sendResult", Qt::QueuedConnection, Q_ARG(QObject*, res));  //Main thread
    } else {
        onFinishedResult();
    }
    m_mutex.unlock();
}

VRonin

Having too many threads makes performance worse than having just 1.

Rather than having a new thread having time you should distribute the work among QThread::IdealThreadCount threads

shav

Thanks for the reply! Could you share links to the example how to use QThread::IdealThreadCount? In Qt docs I can't find examples for this.

P.S. Also I try to use QThreadPool for call task But this also not working :(

VRonin

The closest i have to an example is https://github.com/VSRonin/ChatExample/blob/master/QtSimpleChatServerThreaded/chatserver.cpp

shav

Thanks, I will check!