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    SubjectRe: [PATCH] i386: Selectable Frequency of the Timer Interrupt


    --Lee Revell <rlrevell@joe-job.com> wrote (on Tuesday, July 12, 2005 11:00:02 -0400):

    > On Tue, 2005-07-12 at 07:56 -0700, Martin J. Bligh wrote:
    >> --Lee Revell <rlrevell@joe-job.com> wrote (on Tuesday, July 12, 2005 10:24:59 -0400):
    >>
    >> > On Mon, 2005-07-11 at 21:30 -0700, Martin J. Bligh wrote:
    >> >> Exactly what problems
    >> >> *does* it cause (in visible effect, not "timers are less granular").
    >> >> Jittery audio/video? How much worse is it?
    >> >
    >> > Yes, exactly. Say you need to deliver a frame of audio or video every
    >> > 5ms.
    >>
    >> Ummm. that's a 200HZ refresh rate, is it not? That seems unreasonable
    >> (to a lay-person, as far as video goes).
    >>
    >
    > Seems unreasonable now but things like HDTV actually have much tighter
    > constraints than this. And frame times this small are quite often used
    > for audio, which admittedly is less affected by HZ because those devices
    > generate their own interrupts.
    >
    >> > You have a rendering thread and a display thread that communicate
    >> > via FIFOs. The main thread waits in select() for the next frame to
    >> > complete rendering or for the deadline to expire. That's next to
    >> > impossible with HZ=100, because the best you can do is the deadline
    >> > +-10ms. With HZ=1000 it's no problem.
    >>
    >> So if we have a 50HZ refresh rate, and a HZ rate of 250 or 300, it'll
    >> work fine then, right? I know that's actually some error in the timers,
    >> so it may be 2 or 3 ticks, not 1, but if we're running HZ at 5 or 6
    >> times the frequency of video, presumably that'd still work fine?
    >
    > Yes, for PAL/NTSC, but it's nice to be forward looking...

    Well, looking forward, you'll have sub-HZ timers, so none of this will
    matter. Actually, looking at the above, 150 seems perfectly reasonable
    to me, but 300 seems to be close enough. I'll run some numbers on both.

    >From your above email, I'm more convinced than ever that lowering HZ is
    the right thing to do ...

    M.


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