Microsoft KB Archive/172338

= How To Use QueryPerformanceCounter to Time Code =

Article ID: 172338

Article Last Modified on 1/20/2007

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APPLIES TO


 * Microsoft Excel 2000 Standard Edition
 * Microsoft Visual Basic 5.0 Learning Edition
 * Microsoft Visual Basic 6.0 Learning Edition
 * Microsoft Visual Basic 5.0 Professional Edition
 * Microsoft Visual Basic 6.0 Professional Edition
 * Microsoft Visual Basic 5.0 Enterprise Edition
 * Microsoft Visual Basic 6.0 Enterprise Edition
 * Microsoft Visual Basic 5.0 Control Creation Edition
 * Microsoft Visual Basic 4.0 Standard Edition
 * Microsoft Visual Basic 4.0 Professional Edition
 * Microsoft Visual Basic 4.0 32-Bit Enterprise Edition
 * Microsoft Access 2002 Standard Edition
 * Microsoft Access 2000 Standard Edition
 * Microsoft Access 97 Standard Edition
 * Microsoft Access 95 Standard Edition
 * Microsoft Excel 2002 Standard Edition
 * Microsoft Excel 97 Standard Edition
 * Microsoft Excel 95 Standard Edition
 * Microsoft Word 2002 Standard Edition
 * Microsoft Word 2000 Standard Edition
 * Microsoft Word 97 Standard Edition

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This article was previously published under Q172338



SUMMARY
When timing code to identify performance bottlenecks, you want to use the highest resolution timer the system has to offer. This article describes how to use the QueryPerformanceCounter function to time application code.



MORE INFORMATION
Several timers of differing accuracy are offered by the operating system: Function                Units                      Resolution --- Now, Time, Timer        seconds                    1 second GetTickCount            milliseconds               approx. 10 ms TimeGetTime              milliseconds               approx. 10 ms QueryPerformanceCounter  QueryPerformanceFrequency  same If your system supports a high-resolution counter, you can use QueryPerformanceCounter and QueryPerformanceFrequency to do high-resolution timings.

The following sample code compares the various counters:

WARNING: ANY USE BY YOU OF THE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides this code "as is" without warranty of any kind, either express or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose.

Step-by-Step Procedures
  Enter the following code into a Module. If you enter it into a class, form, or report module, make the declarations Private. Option Explicit

Declare Function QueryPerformanceCounter Lib "Kernel32" _ (X As Currency) As Boolean Declare Function QueryPerformanceFrequency Lib "Kernel32" _ (X As Currency) As Boolean Declare Function GetTickCount Lib "Kernel32" As Long Declare Function timeGetTime Lib "winmm.dll" As Long

Sub Test_Timers Dim Ctr1 As Currency, Ctr2 As Currency, Freq As Currency Dim Count1 As Long, Count2 As Long, Loops As Long '     ' Time QueryPerformanceCounter '       If QueryPerformanceCounter(Ctr1) Then QueryPerformanceCounter Ctr2 Debug.Print "Start Value: "; Format$(Ctr1, "0.0000") Debug.Print "End Value: "; Format$(Ctr2, "0.0000") QueryPerformanceFrequency Freq Debug.Print "QueryPerformanceCounter minimum resolution: 1/" & _ Freq * 10000; " sec" Debug.Print "API Overhead: "; (Ctr2 - Ctr1) / Freq; "seconds" Else Debug.Print "High-resolution counter not supported." End If     ' ' Time GetTickCount '       Debug.Print Loops = 0 Count1 = GetTickCount Do         Count2 = GetTickCount Loops = Loops + 1 Loop Until Count1 <> Count2 Debug.Print "GetTickCount minimum resolution: "; _ (Count2 - Count1); "ms" Debug.Print "Took"; Loops; "loops" '     ' Time timeGetTime '       Debug.Print Loops = 0 Count1 = timeGetTime Do         Count2 = timeGetTime Loops = Loops + 1 Loop Until Count1 <> Count2 Debug.Print "timeGetTime minimum resolution: "; _ (Count2 - Count1); "ms" Debug.Print "Took"; Loops; "loops" End Sub  Run the function from the Debug/Immediate window. Your output should appear similar to the following:

Start Value: 3516284.3498

End Value: 3516284.3521

QueryPerformanceCounter minimum resolution: 1/1193182 sec

API Overhead: 1.92761875388667E-05 seconds

GetTickCount minimum resolution: 10 ms

Took 650 loops

timeGetTime minimum resolution: 10 ms

Took 1565 loops



Multiple statements execute before either GetTickCount or timeGetTime record a change. The actual number of loops will vary depending on the background tasks the operating system is executing.

On the other hand, QueryPerformanceCounter changes value between successive API calls, indicating its usefulness in high-resolution timing. The resolution in this case is on the order of a microsecond. Because the resolution is system-dependent, there are no standard units that it measures. You have to divide the difference by the QueryPerformanceFrequency to determine the number of seconds elapsed. In the case above, the overhead for just calling the API is about 19 microseconds. This would have to be subtracted when timing other code as follows: Private Sub Time_Addition Dim Ctr1 As Currency, Ctr2 As Currency, Freq As Currency Dim Overhead As Currency, A As Long, I As Long QueryPerformanceFrequency Freq QueryPerformanceCounter Ctr1 QueryPerformanceCounter Ctr2 Overhead = Ctr2 - Ctr1       ' determine API overhead QueryPerformanceCounter Ctr1 ' time loop For I = 1 To 100 A = A + I    Next I     QueryPerformanceCounter Ctr2 Debug.Print "("; Ctr1; "-"; Ctr2; "-"; Overhead; ") /"; Freq Debug.Print "100 additions took"; Debug.Print (Ctr2 - Ctr1 - Overhead) / Freq; "seconds" End Sub Sample output:

( 3630876.6256 - 3630876.6388 - 0.0013 ) / 119.3182

100 additions took 9.97333181358753E-05 seconds

NOTE: Because currency variables are used, the values returned are 10000 times smaller than the actual counters. Because the calculation of seconds involves a division operation, this factor is cancelled out.

