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

[JKSH]

Hi @stackprogramer ,

Can you please show us:

1. How do you call RunHardAlgorithm() in main()?
2. How do you run MyComputationThread?
3. How do you measure your code's speed?

[jsulm]

@stackprogramer "How i can assign multi core to a thread" - what do you mean? A thread is either running on one core or sleeping. It cannot run on several cores at the same time. That's why you need at least one thread for each core if you want to utilize all of them.

[stackprogramer]

@jsulm i have cpu corei5, i want to when main thread is busy, itshould be work(main thread is usded for receiving big data upd socket............),but som times it is not busy,

when data is not received, i only used one core for processing my Qthread, it is slow,i can not use in more optimization way of my cpu cores.......
i lost other core cpu in that time......................

@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..................

thanks

[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.