Why when I use QThread in Qt is slower from a simple Qt main program?

[jsulm]

@stackprogramer Sorry I don't understand what you're writing. Sounds like you're doing multi-threading in a wrong way.
You should read https://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/

[stackprogramer]

@jslum sorry i am in hurry
i saw this links
How To Really, Truly Use QThreads; The Full Explanation

http://blog.debao.me/2013/08/how-to-use-qthread-in-the-right-way-part-1/

http://www.bogotobogo.com/Qt/Qt5_QThreads_Creating_Threads.php

http://blog.debao.me/2013/08/how-to-use-qthread-in-the-right-way-part-1/

https://github.com/fabienpn/simple-qt-thread-example

https://john.nachtimwald.com/2015/05/02/effective-threading-using-qt/

but my problem has so complication algorithm, i need a effective way. any way thanks very much

[J.Hilk]

@jsulm

I think the issue here is not with QThread

@stackprogramer wants his program to use more than 1 CPU core for each thread to make the most out of the available hardware

Mulitthread != Multicore

[jsulm]

@J.Hilk said in Why when I use QThread in Qt is slower from a simple Qt main program?:

use more than 1 CPU core for each thread

This is not possible. A thread can only run on one CPU at given time. If you want to utilize 4 cores you need at least 4 threads.

[stackprogramer]

@J-Hilk @jsulm thanks for attention
i think i should switch to multicorre like cuda gpu programming or a same tools.
thread is good but it is not Excellent for my problem.
any way thanks ,best regards stackprogramer

[JKSH]

@stackprogramer said in Why when I use QThread in Qt is slower from a simple Qt main program?:

@JKSH

1. i run RunHardAlgorithm() in a signal udp data received, after that data is buffered in class upd i run RunHardAlgorithm().....................

2)MyComputationThread ,......
i run in a timer ........
3)i measure seconds..................

Sorry, your descriptions are unclear... please post your code.

@stackprogramer said in Why when I use QThread in Qt is slower from a simple Qt main program?:

i think i should switch to multicorre like cuda gpu programming or a same tools.
thread is good but it is not Excellent for my problem.

Do you mean you want to do parallel computing? What is the name of your algorithm?

[J.Hilk]

@stackprogramer

You can always break it up "manually"

QList<int> lInt;

int cnt = QThread::idealThreadCount();

if(cnt > 1){
  int size = lInt.size()/cnt;
   for(int i = 0; i < cnt; i++){
      QList<int> newlInt = lInt.mid(size*i,size);
      QtConcurrent::run(this, &MyClass::RunHardAlgorithm, newlInt);
   }
}else{
   QtConcurrent::run(this, &MyClass::RunHardAlgorithm, lInt);
}

[kshegunov]

From what I was able to grasp from your explanation it seems to me you need to parallelize your algorithm rather. However this depends on what your calculation does, and making a sequential computation a parallel one is a big universe; there's no one approach to help with that, and there's no easy fix by just using a class or w/e. As @JKSH said, you need to post your code, so we can examine it and provide a constructive suggestion.

@J.Hilk said in Why when I use QThread in Qt is slower from a simple Qt main program?:

Mulitthread != Multicore

For most cases that's not true. The only distinction between the two can be made if one considers the memory - if it's shared or distributed, and even then the argument is rather weak. @jsulm makes a good point, that a thread can be executed only on a single core and a core can execute a single thread at any one time.

[J.Hilk]

@kshegunov said in Why when I use QThread in Qt is slower from a simple Qt main program?:

@J.Hilk said in Why when I use QThread in Qt is slower from a simple Qt main program?:

Mulitthread != Multicore

For most cases that's not true. The only distinction between the two can be made if one considers the memory - if it's shared or distributed, and even then the argument is rather weak.

What I tried to say was, that one does not a necessarily implicate the other.

You can have multiple threads that are handled from a single core.
Also, creating a new thread, its also not automaticaly assured that its handled from a different core as well, I believe that is handled by the OS and the current CPU-load?

[kshegunov]

@J.Hilk said in Why when I use QThread in Qt is slower from a simple Qt main program?:

What I tried to say was, that one does not a necessarily implicate the other.

I'm arguing they're one and the same. :)

You can have multiple threads that are handled from a single core

Yes, by means of context switching.

Also, creating a new thread, its also not automaticaly assured that its handled from a different core as well, I believe that is handled by the OS and the current CPU-load?

What core executes what thread at a given time slice is handled by the OS, yes. The actual implementation is in the OS' scheduler.