How To Use QueryPerformanceCounter to Time Code (172338)

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

1. 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
   						

2. 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: 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.