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Date: Tue, 27 Mar 2012 20:01:33 +0200
From: Christian Franke <Christian DOT Franke AT t-online DOT de>
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To: cygwin AT cygwin DOT com
Subject: Re: clock_getres(CLOCK_REALTIME, .) may return an outdated and too high resolution
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Corinna Vinschen wrote:
> On Mar 26 19:00, Christian Franke wrote:
>> Corinna Vinschen wrote:
>>> I see your point, but what bugs me a bit is the fact that
>>> clock_getres(CLOCK_REALTIME) and clock_setres(CLOCK_REALTIME) will
>>> always return the same value coarsest, regardless what value has been set.
>> If clock_setres was called and succeeded, then clock_getres(.)
>> should return the value set before.
>>
>> If clock_setres was not called, the coarsest value is IMO the only
>> value that can be guaranteed.
>>
>> The actual value returned by NtQueryTimerResolution is simply
>> useless in this context: It is the minimum of all resolutions
>> currently set by all running processes. It may change at any time.
>> There is apparently no way the query the current setting of the
>> current process.
> Uh, right, I misunderstood.  I reverted the change to clock_setres.

Sorry, I probably forgot to mention that NtSetTimerResolution returns 
the same useless actual value than NtQueryTimerResolution.

I would suggest:

     status = NtSetTimerResolution (period, TRUE, &actual);
     if (!NT_SUCCESS (status))
       { ... return -1; }
  -  minperiod = actual;
  +  minperiod = period;


>>>> - Unlike on e.g. Linux, CLOCK_REALTIME does not provide a better
>>>> resolution than gettimeofday().
>>> We can only use what the OS provides.  Starting with Windows 8 there
>>> will be a new function call GetSystemTimePreciseAsFileTime:
>>> http://msdn.microsoft.com/en-us/library/windows/desktop/hh706895%28v=vs.85%29.aspx
>> This would provide an easy solution for>= Win8: clock_gettime
>> returns GetSystemTimePreciseAsFileTime, clock_getres returns
>> constant 1us.
> As far as I can tell from a quick debug session, the implementation
> of the underlying RtlGetSystemTimePrecise function is based on a spiffy
> combination of the standard clock tick with the performance counter.
> I'm not very good at assembler debugging, but the essence is access
> to some known and some unknown time values from SharedUserData, a
> call to RtlQueryPerformanceCounter, and a bit of arithmetic.
>
> Maybe we can implement something similar without waiting for W8?  Does
> anybody have code to combine a not so precise clock with a more precise
> counter to create a more precise clock?

The problem is that unlike the OS we don't have interrupts and probably 
don't want to start an extra thread which does timer calibrations.

It may be possible to implement a clock_gettime(CLOCK_REALTIME) which 
has the following properties (in most cases:-):
- Absolute time returned is in interval [GetSystemTime, 
GetSystemTime+coarsest)
- Differences between two returned times provide the resolution of the 
PerformanceCounter if the difference is small (tens of seconds).
I probably will have some time to check this next week.

Christian


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