Thread Question - Understanding Use of Sleep

[David Roberts]

'Dialog DoEevnts 0' in MainThread doesn't help if MainThread is executed before Thread Create - NewThread doesn't get a look in until MainThread completes in this case either.

[David Roberts]

> ooh, ooh, one of my favorite topics.

I knew that and was hoping you'd come in.

However, the fact that the secondary thread does not start, even though it exists according to Task Manager, until the function MainThread has completed still remains unanswered.

[Michael Mattias]

>I'm playing with threads today,

ooh, ooh, one of my favorite topics.

>Control Set Text hDlg, 200, "Main" + Str$(iCount&)

"Assuming" CONTROL SET TEXT uses SetWindowText, a thread switch is forced when executed, becase SetWindowText (window owned by calling process) uses WM_SETTEXT message, and the window procedure always executes in the context of the window-owning thread.

So you are constantly switching threads.

In addition, the thread context in which the active window executes gets preferential treatment when it comes to getting CPU time.

The bottom line is, "empty loops" are no way to see what's going on in a multi-threaded program.


If you are looking at doing a "GUI with a background task in a separate thread of execution" you will want to look at and study this demo:
GUI + Worker Thread + Abort Demo 11-24-07

I have a bunch of other multi-threaded demos in the source code forum. However, most do not have "thread" in the title since I used additional threads because it seemed like the right thing to do, not because "THREAD CREATE" was particularly fascinating.

MCM

[David Roberts]

> the main thread executes almost immediately, then the second thread starts and runs really slowly (by comparison to the main thread)

In fact, the second thread does not start until the main thread has completed. Try 20000 instead of 2000.

After the main thread has completed left click and hold the title bar and watch the second thread run flat out. Release the button and the second thread slows down again.

If we put 'Dialog DoEevents 0' in MainThreads loop then MainThread still starts before NewThread but both functions now run flat out.

I did think I knew the reason for this behaviour but I have since changed my mind and am now scratching my head again.

[Gary Beene]

I'm playing with threads today, so I'm sure this is only the first of several questions I will ask.

I tried the simple code below, expecting both threads to run simultaneously. On my PC, the main thread executes almost immediately, then the second thread starts and runs really slowly (by comparison to the main thread). Regardless of the values I use in sleep (0, 1, 5, 50) , I get the same results - thread NewThread executes sequentially after MainThread.

From this Help comment on SLEEP, I expected NewThread to run in parallel with the main thread.

Pause the current thread of the application for a specified number of milliseconds (mSec), allowing other processes (or threads) to continue.

Can someone clarify for me what I'm misunderstanding?

'Compilable Example:
#Compile Exe
#Dim All
#Include "Win32API.inc"
Global hDlg As Dword, hThread As Dword
Function PBMain() As Long
   Dialog New Pixels, 0, "Test Code",300,300,200,200, %WS_OverlappedWindow To hDlg
   Control Add Button, hDlg, 100,"Start Thread", 50,10,100,20
   Control Add Label, hDlg, 200,"<main thread count>", 50,40,100,20
   Control Add Label, hDlg, 300,"<extra thread count>", 50,70,100,20
   Dialog Show Modal hDlg Call DlgProc
End Function

CallBack Function DlgProc() As Long
   If Cb.Msg = %WM_Command And Cb.Ctl = 100 And Cb.CtlMsg = %BN_Clicked Then
     Thread Create NewThread(0) To hThread       'start the new thread, argument not used
      Thread Close hThread To hThread    'suggested by PowerBASIC Inc. as good practice
      MainThread    'in the main thread, keep doing something
   End If
End Function

Function MainThread () As Long
     Local iCount&
     Do
        Incr iCount& : Control Set Text hDlg, 200, "Main" + Str$(iCount&)
        Sleep 0
     Loop Until iCount& > 2000
End Function

Thread Function NewThread (ByVal x As Long) As Long
     Local iCount&
     Do
        Incr iCount& : Control Set Text hDlg, 300, "Thread" + Str$(iCount&)
        Sleep 0
     Loop Until iCount& > 2000
End Function