High resolution replacement for Sleep

[David Roberts]

SleepX()

SleepX() is a high resolution replacement for Sleep.

The principle used has an analogy with HiFi were a class B amplifier is used to get us within a neighbourhood of a desired voltage and then uses a class A amplifier to fine tune. The cost of this approach is much less than using only a class A amplifier, which are expensive.

The class B amplifier analogy uses Sleep and the class A amplifier analogy uses the Performance Counter, which has a resolution of 100ns with Windows 10 and later.

The following code has the SleepX() code and a usage example.

This is a typical output.

Code:

Quarter of a millisecond: .2503 ms

2 seconds: 2.0000003 s

Silly: 1234.0003 ms

1  1.0001 ms
2  2.0001 ms
3  3.0002 ms
4  4.0001 ms
5  5.0002 ms
6  6.0002 ms
7  7.0002 ms
8  8.0001 ms
9  9.0001 ms
10  10 ms
11  11.0002 ms
12  12.0002 ms
13  13.0001 ms
14  14.0001 ms
15  15.0001 ms
16  16.0002 ms
17  17.0002 ms
18  18.0002 ms
19  19.0002 ms
20  20.0001 ms
21  21.0003 ms
22  22.0002 ms
23  23.0001 ms
24  24.0001 ms
25  25 ms
26  26.0002 ms
27  27.0002 ms
28  28.0002 ms
29  29.0002 ms
30  30.0002 ms

The first example looks at a delay of a quarter of a millisecond. I doubt that anyone will have a use for that. The second example looks at a delay of two seconds. The third example looks at a 'silly' delay of 1234ms, The following looks at delays from 1ms to 30ms in steps of 1ms.

All the results have a sub micro accuracy inline with the Performance Counter on Windows 10 and later.

SleepX() has two parts: The first part is used for delays <= to 3ms and only polls the Performance Counter. This is expensive but is a short-lived expense and should not impact on the system performance. The second part uses the construct 'Sleep ( n-3 )' to get us within a neighbourhood of the target delay, and then we poll the Performance Counter to fine tune. 'Sleep ( n-3 )' needs a resolution of 1ms and why we use SetHiRes.

'Sleep ( n-3 )' does not use any CPU load. The CPU load only kicks in when we enter the 'class A' mode, so is an absolute value and should not impact on the system performance.

Why was 'n <= 3'.  It is reasonable to expect a 1ms resolution to give a delay of between n ms and n+1 ms. In practice, we can exceed n+1 ms and about one third of delays do just that. It is very rare, but values approaching n+1.5 ms have been seen. To mitigate that issue, 'n <= 3' was chosen.

That is it: A very simple idea to give a high resolution replacement for Sleep with a negligible CPU load.

It is worth noting that

Code:

SetHiRes
<Code to time>
RevokeHiRes

is acceptable, but it should not be used within a loop; for example a graphics application with 60, or so, fps. That will upset the system clock. It is better to use SetHiRes at the beginning of an application and RevokeHiRes when the higher resolution is no longer needed. If you forget to use RevokeHiRes SetHiRes will be cancelled when the process terminates. Microsoft neglects to mention that.

Code:

#COMPILE EXE
#DIM ALL
#INCLUDE "win32api.inc"

' SOURCE CODE:

Macro QPC = QueryPerformanceCounter qTimeNow

Macro SetHiRes
  MacroTemp Time
  Dim Time As TIMECAPS
  TimeGetDevCaps( Time, SizeOf(Time) )
  TimeBeginPeriod(Time.wPeriodMin)
  'Sleep 16 ' Pre Windows 10 - high resolution does not 'byte' until next clock tick.
End Macro

Macro RevokeHiRes
  MacroTemp Time
  Dim Time As TIMECAPS
  TimeGetDevCaps( Time, SizeOf(Time) )
  TimeEndPeriod(Time.wPeriodMin)
End Macro

Global qFreq As Quad

Sub SleepX( ByVal n As Double )
  Local qTarget, qTimeNow As Quad
  QPC
  qTarget = qTimeNow + n*qFreq*0.001
  If n <= 3 then
    Do : QPC : Loop Until qTimeNow >= qTarget ' Class A amplifier analogy
  Else
    ' Class B amplifier analogy followed by Classs A amplifier analogy
    Sleep ( n-3 ) : Do : QPC : Loop Until qTimeNow >= qTarget
  End If
End Sub

' ====================

' EXAMPLE USAGE:

Function Pbmain () As Long
Local i as Long
Local qStart, qStop As Quad

  QueryPerformanceFrequency qFreq

  SetHiRes

  QueryPerformanceCounter qStart
    SleepX(0.25)
  QueryPerformanceCounter qStop
  Print "Quarter of a millisecond:";(qStop - qStart)*1000/qFreq;"ms"
  Print

  QueryPerformanceCounter qStart
    SleepX(2000)
  QueryPerformanceCounter qStop
  Print "2 seconds:";(qStop - qStart)/qFreq;"s"
  Print

  QueryPerformanceCounter qStart
    SleepX(1234)
  QueryPerformanceCounter qStop
  Print "Silly:";(qStop - qStart)*1000/qFreq;"ms"
  Print

  For i = 1 to 30
    QueryPerformanceCounter qStart
      SleepX(i)
    QueryPerformanceCounter qStop
    Print i;" ";(qStop - qStart)*1000/qFreq;"ms"
  Next

  RevokeHiRes

  WaitKey$

End Function