lkml.org 
[lkml]   [2007]   [Mar]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: Is the clockevent resolution fine-grained enough?
    From
    Date
    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


    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2007-03-03 09:51    [W:0.024 / U:66.500 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site