What is the best practices for qqueue shared among threads ?

RahibeMeryem

Hi,

I am still struggling to find a decent solution for my producer and workers system.

one of my thread producing cv::Mat s and ı want to consume this cv::Mat2s from 4 different threads.

I am really confused and lost ....

Is it possible to share a QQueue or a QVector among all threads and use mutex ? signal slots messaging is dropping the signals because workers not fast as producer.

any example could be very helpful

best..

EDIT 1: I looked zeromq and looks promising but seems complex. Decoupling workers and communicating with a fast messaging could be a solution also

jsulm

@RahibeMeryem said in What is the best practices for qqueue shared among threads ?:

Is it possible to share a QQueue or a QVector among all threads and use mutex ?

Yes it is.

"signal slots messaging is dropping the signals because workers not fast as producer" if you share QQueue/QVector you don't need signals/slots and use https://doc.qt.io/qt-5/qtcore-threads-semaphores-example.html to block the producer if the queue is too long already. See https://en.wikipedia.org/wiki/Producer–consumer_problem#Using_semaphores

RahibeMeryem

@jsulm

very stupid question :

my mainwindow.cpp starting threads with:

    detector_thread = new QThread;
    videofeeder = new VideoFeeder;
    videofeeder->moveToThread(detector_thread);

    bc_thread_01 = new QThread;
    bc_worker_1 = new Bc_worker_detector;
    bc_worker_1->moveToThread (bc_thread_01);
    bc_thread_01->start();

    bc_thread_02 = new QThread;
    bc_worker_2 = new Bc_worker_detector;
    bc_worker_2->moveToThread (bc_thread_02);
    bc_thread_02->start();

    bc_thread_03 = new QThread;
    bc_worker_3 = new Bc_worker_detector;
    bc_worker_3->moveToThread (bc_thread_03);
    bc_thread_03->start ();

there is a Bc_worker-detector.h and Bc_worker-detector.cpp

Also there is a SharedVar.h just include:

extern   QQueue <cv::Mat> shared_frames_que;

I declared it in videofeeder and include SharedVar.h

and messed up. not working

What is the proper way to share QQueue between seperate threads ? Really. I forget cpp these days due to python torture...

Best

jsulm

@RahibeMeryem said in What is the best practices for qqueue shared among threads ?:

not working

What is not working?
You need the definition for shared_frames_que as well in some cpp file.

RahibeMeryem

@jsulm it gives :

Undefined symbols for architecture x86_64:
  "_shared_frames_que", referenced from:
      Bc_worker_detector::frame_detect_mace(cv::Mat const&) in bc_worker_detector.o
ld: symbol(s) not found for architecture x86_64

jsulm

@RahibeMeryem As I said you need a definition also.
Or simply put it as member in your MainWindow and pass pointer to it to your workers.

RahibeMeryem

@jsulm

don't remember how to do that at he moment... really . :(

would you mind to show it to me ? my girlish power ZERO today. almost giving up......

thnx a lot

RahibeMeryem

@jsulm

I put

QQueue <cv::Mat> shared_frames_que;

in one one of the cpp but not worked also. (for definition)

same error

mrjj

@RahibeMeryem
Hi

In some .h file
extern QQueue <cv::Mat> shared_frames_que; (outside any functions/classes)
and then in some cpp file (outside any functions)
QQueue <cv::Mat> shared_frames_que;

should just work. ( make sure it knows cv:Mat also )
Try to make clean All + rebuild from build menu.

I just tested with
extern QQueue <int> shared_frames_que;
and 
QQueue <int> shared_frames_que;
and it just compiled.

RahibeMeryem

@mrjj

I will test soon let you know.

SGaist

Hi,

You should rather follow @jsulm´s suggestion which is simpler to put it place.

Declare you mutexed queue as member of your main window and pass it to the producer and consumer.