Re: after ids (Helmut Giese)

On 7/5/08, Helmut Giese <hgiese@...> wrote:
>
> >Donal K. Fellows wrote
> >While I could always see how to do it with single sets of timers, I
> >could never work out how to do it reliably with multiple event
> >sequences on different scales (e.g. a 13ms interval and a 1s interval)
> >where the event processing takes an appreciable time w.r.t. the
> >shorter intervals. Which happens for sure when using, say, Tk; redraws
> >are quite expensive. With each pending timer event assigned an
> >absolute time at which it becomes eligible for execution, it's pretty
> >simple to handle. But without it...
>
> Could well be, that I am way off of what is discussed here, but what I
> understood seems rather easy to realize (at least on PC hardware).
> Here's the idea:
> Any reasonably recent PC hardware has a free running 64 bit timer
> (sometimes called the Multi Media Timer) - since Pentium 1 IIRC.
> There are 2 system calls:
> - Query this timer's frequency (QueryPerformanceFrequency) and
> - get its current count (QueryPerformanceCounter).
>
> On startup Tcl would ask for the frequency and would once calculate the
> number of timer ticks to a millisecond.
> Now someone comes along and wants to be notified after 10 msec. Ok,
> Tcl gets the current timer value, adds the appropriate value for 10 ms and
> puts the timeout value into a list.
> 1 msec later someone else comes around and asks for a delay of 3 msec.
> Tcl does the same and (this would be a micro-optimisation) orders the list, so
> that the new event is placed _before_ the one from step 1.
>
> At any moment when Tcl wants to check if any timer event has expired, it walks
> the list and notifies any caller whose time has expired (stopping at the
> first time
> which is still in the future, if the list is ordered).
>
> There would be no dependency on wall clock time - and one could probably
> even expect a finer grained control than 1 msec - non-GUI apps, of course - on
> my (somewhat dated PC) this timer runs at 1.8 GHz.
>
> Ok, I apologise if I am off topic - but it sounded strangely familiar with
> something
> I just did with an embedded controller.
> Best regards
> Helmut Giese

Thanks for the idea, but isn't QueryPerfCounter a Windows thing ?
In that case, there's no problem since GetTickCount() is OK for the job.
Am I missing something ?

-Alex

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