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Date: Tue, 27 Mar 2012 23:28:29 +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 may return an outdated and too high resolution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 27 20:59, Christian Franke wrote:
>> Corinna Vinschen wrote:
>>> On Mar 27 20:01, Christian Franke wrote:
>>>> 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;
>>> But that's not right.  The "actual" value is not useless, but the value
>>> the resolution has actually been set to.
>> No, again this is the minimum of all resolutions currently set by
>> all processes.
>>
>>
>>>    The OS just doesn't support
>>> arbitrary values for the period.
>>>
>> Yes - but in 'actual' a smaller value than the value set for the
>> current process may be returned.
> Hmpf, ok.  Boy is that ugly.

Yes, aka broken by design :-)

There should be a function to query the actual setting for the current 
process only. But there is none.


>    Is there a chance that actual is bigger
> than period?  In that case we should perhaps set minperiod like this:
>
>    minperiod = MAX (actual, period);

According to some experiments this can only happen if period < finest. 
In this case NtSetTimerResolution also succeeds and returns actual == 
finest.

Christian


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