Need more help with C++ code syntax - QTConncurent and template

Anonymous_Banned275

Posting this has given me an idea to take a different approach

    What does QtConcurrent::run() do? What does it return?

I "runs" time consuming function " which scans (HCI inquiry) for nearby bluetooth devices and returns their address and name.

    What does QtConcurrent::map() do? What does it return?

It "runs" QProgressDialog and updates its "progress bar" in roughly 1 second intervals . The interval is not important.

Naturally - I have been coding these (two main ) tasks sequentially , with emphasis on the QtConncurent " functions" - mainly what they do. What I have missed is "block" approach which is little different that just " function returns..." when coding in event driven environment.

I will try to rebuild the code using this "block" philosophy , maybe it will make the code interaction easier to see.
Here is my goal in pseudo-code

{// QtConcurrent block START 
         {#  QtConcurrent::run() block 
         code 
        {#QtConcurrent::run()    block 
         {#   QtConcurrent::map() block 
                   {#   setup  spin vector array  block 
                code 
    ..           {#   setup  spin vector array  block  
              code 
        {# QtConcurrent::map()   block 
     code 

}// QtConcurrent block END 


Of course that still does not help to figure out how to process "the time consuming function results / return". 
What is puzzling - QtConcurennt  is multi threading  framework and by (my) definition one of the advantages of multi threading is NOT to have blocking processes. 
So why is "result" blocking  ? 
(I still cannot prove it, I have no "get actual thread ID" code , but it appears to   "run" in main process thread and if it blocks that thread - no good. ) 
Is it not part of the QtConcurrent framework ?

Pl45m4

@AnneRanch

result() is blocking the main thread, because you want to use the result there. If the result is not ready, it would not make any sense to continue with the code. Every use of result data would be invalid. So it waits until there is a result, then, for example assigns it to your local variable and continues.
Same as waitForFinished(), with the only difference that you don't care about the returned values, when using this.

The calculation of your result happens in multiple threads concurrently, but the calling thread (your main thread), where you want to use the result, has to wait.

Anonymous_Banned275

@Pl45m4 said in Need more help with C++ code syntax - QTConncurent and template:

@AnneRanch

result() is blocking the main thread, because you want to use the result there. If the result is not ready, it would not make any sense to continue with the code. Every use of result data would be invalid. So it waits until there is a result, then, for example assigns it to your local variable and continues.
Same as waitForFinished(), with the only difference that you don't care about the returned values, when using this.

The calculation of your result happens in multiple threads concurrently, but the calling thread (your main thread), where you want to use the result, has to wait.

I do understand that, however , where is the advantage of QtConcurrent then?

Either way , I still do not see "the connection / advantage " between plain function "return" , or processing the pointer passed to the function and this "result" .

Irregardless which way I access the data - it is not what I expect.
I need to "code it" differently.

I am already using the "map" letting "connect" do simple updating of progress bar, so why not utilize same method to keep "run" from screwing things up "waiting" for result ? I am using wrong method and in wrong place to boot.

The way I have it now - the "spin" is finished first and when it is finished - I should be able to see if "run" is also finished and then get "result" without any blocking.

JKSH

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

I am already using the "map" letting "connect" do simple updating of progress bar, so why not utilize same method to keep "run" from screwing things up "waiting" for result ?

QtConcurrent::run() runs your time-consuming function in another thread. When it finishes, your QFutureWatcher emits the finished() signal. Connect this signal to a slot; fetch the result in that slot.

    What does QtConcurrent::map() do? What does it return?
It "runs" QProgressDialog and updates its "progress bar" in roughly 1 second intervals . The interval is not important.

QtConcurrent::map() takes a function and a vector that contains N elements. It runs that function N times -- once per vector element. On a modern CPU, multiple copes of that function can run concurrently (in parallel).

Anonymous_Banned275

I am hoping this is my LAST post in this thread.
Even after I double check completion of the "time consuming function"
the "returnValue" takes over 1 second to complete.

First returnValue   QVector<QStringList> = futureWatcher_run.future().result() 
 elapsed time  1533 [mS]

And I really do not want to know why...

Cheers

EDIT / SOLVED

It takes 1.5 seconds to output the DEBUG message ! DUH !

       QElapsedTimer  timerHCI;
        timerHCI.start();

        QVector<QStringList> returnValue = futureWatcher_run.future().result();

       // should take no significant time
        qDebug()<< "time " <<  timerHCI.elapsed() ;  

       now the time = ZERO !

JonB

@AnneRanch
It will not take 1.5 seconds to execute qDebug()<< "time " << timerHCI.elapsed(). However, it may well take 1.5 seconds to get the return result back from futureWatcher_run.future().result(), if that's how long it takes to complete all of the threads.

SimonSchroeder

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

I do understand that, however , where is the advantage of QtConcurrent then?

There are two different use cases of QtConcurrent: 1) Similar to what you are doing you can launch multiple tasks in separate runs one after the other and only after all the runs call result(). This would mean that you can compute several tasks in parallel, but it also means that you have a synchronization point when you want to access the results. Furthermore, if you run a whole bunch of tasks using QtConcurrent it will use a thread pool in the background such that your CPU cores are not oversubscribed. This is the main advantage over QThread. 2) The other use case, as already meantioned, is to only call run in your function and connect the finished signals of the QFutureWatcher to a separate slot which will continue from where you want to access the results. Basically, this means that you need to split your function right there where you call result() (and you don't have to call result anymore because it will be the parameter of your function).

Furthermore, map and mapReduce automatically parallelize and call your function on each element. This is the really helpful part of QtConcurrent.

One quick fix to your problem (I haven't made up my mind if this is good or bad design) would be to run your existing function inside a separate thread. Then your separate thread would block instead of the main thread. However, this only works if you don't do any GUI stuff inside this thread. This would work something like this:

QThread::create([=]()
{
     // your whole function code here...
    ...
    futureWatcher_run.setFuture(QtConcurrent::run(HCI, &HCI_Scan_Dialog::HCI_Scan_Scan,stringList));
    ....
    QVector<QStringList> returnValue = futureWatcher_run.future().result(); // now this will block
})->start();

Anonymous_Banned275

Thanks for the post.

I seldom RTFM first, but this time I have made an exception.
I have chosen QtConcurrent because it is advertised as to "simplify multi threading". In principle that is what I am comfortable with, and it does the job.

To be clear - as an old fart - I believe there is "more than one way to skin a cat " , so I do not object to alternatives.

I actually got "impressed" by a sample using QProgressDialog and somehow lost my focus on the " time consuming function ".
My " time consuming function " provides intermediate data / result .
So I get intermediate SIGINAL(s) before I get the final "finished".

The MAIN issue was that QtConcurrent automatically utilizes all available CPU cores - so instead of updating the QProgressDialog in 1 seconds intervals it was four times faster....
That "bug" was not that easy to find...even after RTFM again.

Technically - I never bothered to check the actual "run:" threads.
I am happy as long as it is "non blocking".

I am sure QThread would do the job, but why fix it if it is not broken?

JKSH

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

The MAIN issue was that QtConcurrent automatically utilizes all available CPU cores - so instead of updating the QProgressDialog in 1 seconds intervals it was four times faster....
That "bug" was not that easy to find...even after RTFM again.

QtConcurrent's raison d'être is to complete tasks as quickly as possible by performing sub-tasks in parallel -- that's why it's called "concurrent". So, QtConcurrent::map() will finish faster when you have more cores and faster CPUs. It is intentionally designed this way.

If you want to update QProgressDialog at fixed intervals, QtConcurrent::map() and QThread are not suitable tools for the job. Instead, you want QTimer.

Anonymous_Banned275

@JKSH
Let me clear this - I have no objection to QConcurrent mode of operation.
I generally stick with a task , my - primary directive / objective - in this case multi threading / non blocking due to time consuming function.
It does not matter , to me , how many threads are being used.
Hence the primary task was to have non-blocking operation, secondary ( as I call "bells and whistles" ) some rough indication where is the "time consuming function" . Updating the status at 1 seconds intervals seems adequate.

Missing the "note" about "automatic CPU core selection" WAS MY MISTAKE.

I'll leave configuring QConcurrent to run only ONE CPU to the audience...

Cheers