tbb concurrent_bounded_queue multiple threads access - c++

Question

I have the following code:
tbb::concurrent_bounded_queue<Image> camera_queue_;
camera_queue_.set_capacity(1);
struct Image
{
int hour_;
int minute_;
int second_;
int msec_;
QImage image_;
Image(){hour_ = -1; minute_ = -1; second_ = -1; msec_ = -1; image_ = QImage();}
Image& operator=(Image const& copy)
{
this->hour_ = copy.hour_;
this->minute_ = copy.minute_;
this->second_ = copy.second_;
this->msec_ = copy.msec_;
this->image_ = copy.image_;
return *this;
}
};
In a Qt Thread :
ThreadA:
tbb::concurrent_bounded_queue<Image> image_queue_;
image_queue_.set_capacity(1);
Image cur_image_;
void Worker::process() {
while(1)
{
if(quit_)
break;
{
camera_queue_.pop(cur_image_);
image_queue_.push(cur_image_);
}
emit imageReady();
}
emit finished();
}
Image Worker::getCurrentImage()
{
Image tmp_image;
image_queue_.pop(tmp_image);
return tmp_image;
}
In Another Thread:
ThreadB:
Producer::Producer(){
work_ = new Worker();
work_->moveToThread(workerThread_);
QObject::connect(workerThread_, &QThread::finished, work_, &QObject::deleteLater);
QObject::connect(this, &Producer::operate, work_, &Worker::process);
QObject::connect(work_, &Worker::imageReady, this, &Producer::displayImage);
QObject::connect(this, &Producer::stopDecode, work_, &Worker::stop);
workerThread_->start();
emit operate();
}
void Producer::process() {
while(1)
{
if(quit_)
break;
{
camera_queue_.push(GetImage());
}
}
}
void Producer::displayImage()
{
Image tmp = std::move(work_->getCurrentImage());
widget_->showImage(tmp.image_);
}
However, In main thread, I have a function that enables user to click a button to get current image:
bool Producer::SaveImage()
{
Image img = std::move(work_->getCurrentImage());
std::string fileName = std::to_string(img.hour_) + "-" + std::to_string(img.minute_) + "-" + std::to_string(img.second_) + "-" + std::to_string(img.msec_/1000) + ".jpg";
std::string outFileName = folder + "/" + fileName;
return img.image_.save(QString::fromStdString(outFileName));
}
The problem is:
When user does not click the button to invoke Producer::SaveImage(), the Image Decoding and Showing runs smoothly. But when user invoke Producer::SaveImage(), the whole program will get stuck (Caton phenomenon ?). The GUI response becomes not that smooth. The more user invokes SaveImage, the slower the GUI response becomes.
Can anyone help to explain why ? Is there a way to solve that ?

Answer 1

Why do you want to use concurrent queue? It looks like there is a syncronization mechanism in place and you rely mostly on it instead of using concurrent_queue for synchronisation and communication as it is supposed for.
The issue is that when you set capacity = 1, both operations of concurrent_bounded_queue will block until there is enough space of items in the queue. E.g. if the queue contains an item already, the push operation will block. And since you control your threads with another notification mechanism, you might catch a deadlock.
In particular, try to swap the operations like below:
camera_queue_.pop(cur_image_);
emit imageReady();
image_queue_.push(cur_image_);
This should prepare the thread which receives images (if I understand this correctly) and it will block on its image_queue_.pop() method, then this thread will put new image and unblocks the recipient. There might be other issues similar to this, so, please rethink all of your synchronization.