Messages in this thread | | | Subject | Re: Is the clockevent resolution fine-grained enough? | From | Thomas Gleixner <> | Date | Sat, 03 Mar 2007 09:54:30 +0100 |
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On Fri, 2007-03-02 at 09:24 -0800, Marko Rauhamaa wrote: > > That's an academic exercise, or are you talking about some real world > > hardware which runs Linux ? > > Real hardware running linux.
quantum computer ?
> > > In our application, we need periodic clock interrupts at about 100 > > > kHz. > > > > With a stock kernel ? > > Well, with a "clockevent" patch of our own. We'd like to use a stock > kernel, though.
100khz on a stock kernel with real world hardware:
ROTFL, You made my day.
> > > If the (programmable) frequency must be rounded to the nearest > > > nanosecond, we have a cumulative error of > > > > > > 100,000 * 0.5 ns/s = 50 µs/s > > > > clockevents is based on the monotonic system clock and depends on the > > accuracy of that and the device which deliveres the interrupts. > > [...] > > There is nothing to nugde. The clockevent subsystem operates on > > absolute time, so there is no cummulative error, except you setup your > > timers relative per event. > > I'm afraid you didn't quite understand what I was getting at. Say the > user programs the frequency to be 109,000 Hz. That means a nominal clock > interval of ~9174.3119 ns. Now the clockevent interface forces me to > round it down to 9174 ns. That means the clock interrupts fall behind > with respect to the other parts in the system that implement 109,000 Hz > much more to the letter. The error grows by 34 µs every second so that > after 8 hours, we are lagging by a whole second.
Sorry man. Did you actually read what I wrote ?
> The clockevent subsystem operates on absolute time, so there is no > cummulative error
Again: clockevents operate on absolute time, so it is simply the fault of the user, when he decides to do something stupid like:
femto_seconds_t interval;
timer_function() { do_whatever_you_need_to_do(); set_next_event(interval, RELATIVE); }
instead of
yokto_seconds_t interval; yokto_seconds_t next_event;
timer_function() { do_whatever_you_need_to_do();
next_event += interval; set_next_event(yokto_seconds_to_nsec(next_event), ABSOLUTE); }
Please read _AND_ understand the clockevents code. Your uber_clockevents patch is solving PEBKAC.
tglx
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