Re: clock_getres(CLOCK_REALTIME, .) may return an outdated and too high resolution

[Christian Franke <Christian dot Franke at t-online dot de>]

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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