Re: [boost] Low-level precision timer proposal

From: Johan Nilsson (r.johan.nilsson_at_[hidden])
Date: 2006-05-16 08:20:10

Peter Dimov wrote:
> Johan Nilsson wrote:
>
>>> I'm not sure which clock do you mean when you say that a thread
>>> can't sleep for less than a clock tick. If you mean the CPU clock,
>>> then yes, a no-op Sleep function that returns immediately would
>>> probably consume more than one clock tick, so it is impossible to
>>> make a thread sleep for less.
>>
>> I referred to the timer tick period (~10-15 ms for the NT family).
>
> It used to be the case that you can't get better than 10ms precision
> on NT without calling timeBeginPeriod, but that was years ago, I
> think (NT 3.5,
> not sure about 4). I'm getting ms-precise timing (not hard realtime,
> obviously, but correct most of the time) on Windows XP.

I'm not so sure. Try out the following (VC compiler) _straight_ after a 
clean boot:

---
#include <windows.h>
#include <iostream>
#pragma comment(lib, "winmm")
int main(int argc, char* argv[])
{
    for (int i = 0; i != 1000; ++i)
    {
        std::cout << "MMTime: " << timeGetTime() << '\n';
        Sleep(1);
    }
    return 0;
}
---
The minimum interval between the output will be the default timer tick 
period (10-15ms). Calling timeBeginPeriod(1) before entering the loop will 
"fix" this. This will not affect the updating of the system time, though.
Note that the timer resolution is set "globally" - if one process requests a 
time period of 1 milliseconds it will also affect other processes' wake-up 
times.
// Johan