Transcript OS Timing Services

OS Timing Services
Dr. Hugh Melvin, Dept. of IT, NUI,G
Typical PC Clock System
• RTC (Real Time Clock)
• Low freq oscillator (Eg. 32 kHz)
• Minimises Battery Consumption
• Second-level accuracy
• Software Clock (System Clock)
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Counter
Interrupt driven: ticks are added to counter value
Counter value translated into time standard eg. UTC
At system startup, counter value taken from RTC
Interrupts generated by separate oscillator
• Eg. Intel 8254 1.193182 MHz Clock is commonly used
Interrupt rate determines granularity or resolution
Dr. Hugh Melvin, Dept. of IT, NUI,G
System Clock
• Interrupt rate  tick size  system clock resolution
• Note: Little point in implementing NTP for msec
accuracy if timing services have poor resolution
• Unix/Linux platforms
– Interrupt rate upgraded to 1000 Hz (1msec) from 100Hz
(10 msec) in recent releases
• Windows Platforms
– 10 15 msec ticksize still common
– Eg. GetSystemTimeAsFileTime()
Dr. Hugh Melvin, Dept. of IT, NUI,G
Granularity/Resolution
• System Clock Granularity
– Significant restriction for precision timing requirements of many
RTS applications
• Process timeslice also typically set by timer interrupts
– Further restriction on Real Time applications
– Limits responsiveness
• Windows
– Not yet implemented
– Manual workarounds available
• See notes
– NTP workaround based on interpolation eg. using 8254 clock
counter
Dr. Hugh Melvin, Dept. of IT, NUI,G
Granularity/Resolution
• Unix/Linux
– gettimeofday() circumvents clock granularity
(100/1000Hz) by interpolating between ticks using
main CPU processor counter
– gettimeofday()
• Stores time in timeval structure (sec + usec)
• struct timeval{
time_t tv_sec;
time_t tv_usec;
}
– time_t is long integer
Dr. Hugh Melvin, Dept. of IT, NUI,G
Windows Resolution
//File handler
ofstream outFile( argv[1],ios::out);
SYSTEMTIME st;
int i=0;
while (i<5000)
{
::GetSystemTime(&st);
outFile<< st.wHour << ':'<< st.wMinute << ':'<<
st.wSecond << '.' << st.wMilliseconds << '\n';
i++;
}
• Indicates granularity
See http://msdn.microsoft.com/msdnmag/issues/04/03/HighResolutionTimer/
Dr. Hugh Melvin, Dept. of IT, NUI,G
Windows Resolution
• See Code Extract
– Store 5000 intervals
– Examine distribution
• Not all equal to fundamental resolution!
• OS issue process preempted  time has advanced by > 1 tick
5000 samples. Smallest delta: 15.625ms; Largest
delta: 46.875ms; Sum of Delta: 78234.4ms;
Average: 15.6469ms
Distribution:
156250: 4993
312500: 4
468750: 2
Total Runtime was : 78.2656 seconds
Exercise: Load up OS .. Examine distribution
Dr. Hugh Melvin, Dept. of IT, NUI,G
Windows Resolution: Workaround
• Improve resolution via separate High Performance/
Resolution Counter (HPRC)
– ::QueryPerformanceCounter(&li);
– Can access its frequency
– ::QueryPerformanceFrequency(&frequency);
• 3.57 MHz Clock
• Interpolate between ticks of system clock
• Need to establish baseline ‘synchronise’ with system
clock
– Baseline or reference has
• System clock at instant of change
• Corresponding HPRC value
– Use baseline for interpolation
Dr. Hugh Melvin, Dept. of IT, NUI,G
void simplistic_synchronize(reference_point& ref_point)
{
FILETIME ft0 = { 0, 0 }, ft1 = { 0, 0 };
LARGE_INTEGER li;
//
// Spin waiting for a change in system time. Get the matching
// performance counter value for that time.
//
::GetSystemTimeAsFileTime(&ft0);
do {
::GetSystemTimeAsFileTime(&ft1);
::QueryPerformanceCounter(&li); }
while ((ft0.dwHighDateTime == ft1.dwHighDateTime) &&
(ft0.dwLowDateTime == ft1.dwLowDateTime));
ref_point.file_time = ft1;
ref_point.counter = li;
}
Dr. Hugh Melvin, Dept. of IT, NUI,G
Skew
System Clock : Ref Sys Time + Δ HPRC
Actual System Clock
Reference Point (Ref Sys Time, Ref HPRC)
Dr. Hugh Melvin, Dept. of IT, NUI,G
Unix/Linux Timing
• gettimeofday()
– Microsec granularity
• Process timeslice can however impact on
timing services
– Eg. usleep( ) .. microsec level argument
– In practice, limited by timeslice
Dr. Hugh Melvin, Dept. of IT, NUI,G