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.
Back to the top
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.
Back to the top
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:
( 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.
Back to the top
Microsoft Developer Network; topics: timeGetTime GetTickCount
QueryPerformanceCounter QueryPerformanceFrequency
Back to the top
Help and Support
