QueryPerformanceCounter (VB6)

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Hi

I am using the QueryPerformanceCounter to time a piece of code. I need timing in microseconds.

My code compiles okay but the end result is 0 I think this is because my function is not accessing the kernel32.dll and so is not passing any value in the parameter. Do I have to do anything other than the Declare Function statements

This is what the code looks like at present

Option Explicit

Private Declare Function QueryPerformanceCounter Lib “KERNEL32” _
(lpPerformanceCount As Currency) As Long
Private Declare Function QueryPerformanceFrequency Lib “KERNEL32” _
(lpPerformanceCount As Currency) As Long

Sub BinarySearch()

Dim sec As Currency
Dim secOut As Currency

ProfileStart sec

Do the Binary Search

ProfileStop sec, secOut

End Sub

Sub ProfileStart(secStart As Currency)

Dim secFreq As Currency

If secFreq = 0 Then QueryPerformanceFrequency secFreq
QueryPerformanceCounter secStart

End Sub

Sub ProfileStop(secStart As Currency, secTiming As Currency)

Dim secFreq As Currency

QueryPerformanceCounter secTiming
If secFreq = 0 Then
secTiming = 0 ‘Handle old system with no high-resolution timer
Else
secTiming = (secTiming – secStart) / secFreq
End If

lblCheck.Visible = True
lblCheck.Caption = secTiming

End Sub

Now the code looks okay to me (famous last words) So what am I missing please?

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I don’t understand the role of secFreq in ProfileStart and ProfileStop. In neither procedure, a value is assigned to secFreq, so it is always 0. As a consequence, secTiming is always set to 0. What was secFreq intended to do?

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Does this do what you want?

Private Declare Function QueryPerformanceCounter Lib “KERNEL32” _
(lpPerformanceCount As Currency) As Long
Private Declare Function QueryPerformanceFrequency Lib “KERNEL32” _
(lpPerformanceCount As Currency) As Long

Sub BinarySearch()
Dim sec As Currency
ProfileStart sec
‘ do the binary search here
ProfileStop sec
End Sub

Sub ProfileStart(secStart As Currency)
QueryPerformanceCounter secStart
End Sub

Sub ProfileStop(secStart As Currency)
Dim secTiming As Currency
Dim secFreq As Currency

QueryPerformanceFrequency secFreq
QueryPerformanceCounter secTiming

If Not secFreq = 0 Then
secTiming = (secTiming – secStart) / secFreq
End If

lblCheck.Visible = True
lblCheck.Caption = secTiming
End Sub

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Almost!

There is an issue with

Sub ProfileStop(secStart As Currency)
Dim secTiming As Currency
Dim secFreq As Currency

QueryPerformanceFrequency secFreq
QueryPerformanceCounter secTiming

If Not secFreq = 0 Then
secTiming = (secTiming – secStart) / secFreq
End If

lblCheck.Visible = True
lblCheck.Caption = secTiming
End Sub

I have used lblCheck to find out that secFreq, secTiming and secStart all contain values at the point before the If Statement but the code still returns zero.

Further experimenting…

secTiming = (secTiming – secStart) / secFreq ‘used with or without IF statement
lblCheck.Caption = secTiming ‘returns zero

but
lblCheck.Caption = (secTiming – secStart) / secFreq ‘returns a result

Seems very odd.

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Sorry, no idea. It works for me, and I see no reason why

secTiming = (secTiming – secStart) / secFreq
lblCheck.Caption = secTiming

and

lblCheck.Caption = (secTiming – secStart) / secFreq

should produce different results.

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Hi

A bit more investigation. I now have amended the code and it is working.

Still don’t understanding what is happening. But the issue seems to be something to do with formatting. The binary search is completed very quickly – around 15 microseconds.

Code as previously written returns seconds. Amended code to convert to microseconds

If Not secFreq = 0 Then
secTiming = ((secTiming – secStart) / secFreq) * 1000000
End If

Which returns a result to to four decimal places

Whereas using lblCheck.Caption = ((secTiming – secStart) / secFreq) * 1000000
returns the same result only to around 15 decimal places

Without *1000000
lblCheck.Caption = (secTiming – secStart) / secFreq
returns the result to 15 decimal places to the power of minus 5

So as SecTiming is Currency format
does using

secTiming = (secTiming – secStart) / secFreq

return a number to secTiming which too small for it to display?

Anyway the code works. Once again thanks very much for all your help.

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Currency format stores numbers with 4 decimal places, so that explains your result. If you set the label directly, the result of the division is a double precision floating point number (with 15 decimals), it is not converted into currency (with 4 decimals).

(Timing a an action that only takes 15 microseconds doesn’t seem very useful. The overhead must be much more than 15 microseconds.)

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