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    SubjectRe: [2.4.17/18pre] VM and swap - it's really unusable
    Alan Cox wrote:
    >
    > Andrew's patches give you 1mS worst case latency for normal situations, that
    > is below human perception, and below scheduling granularity.

    The full ll patch is pretty gruesome though.

    The high-end audio synth guys claim that two milliseconds is getting
    to be too much. They are generating real-time audio and they do
    have more than one round-trip through the software. It adds up.

    Linux is being used in so many different applications now. You are,
    I think, one of the stronger recognisers of the fact that we do not
    only use Linux to squirt out html and to provide shell prompts to snotty
    students. Good scheduling responsiveness is a valuable feature.

    I haven't seen any figures for embedded XP, but it is said that
    if you bend over backwards you can get 10 milliseconds out of NT4,
    and 4-5 out of the fabled BeOS. This is one area where we can
    fairly easily be very much the best. It's low-hanging fruit.

    Internal preemptability is, in my opinion, the best way to deliver
    this.

    I accept your point about it making debugging harder - I would
    suggest that the preempt code be altered so that it can be disabled
    at runtime, rather than via a rebuild. I suspect this can be
    done at zero cost by setting init_task's preempt count to 1000000
    via a kernel boot option. And at almost-zero cost via a sysctl.

    I would further suggest that support be added to the kernel to
    allow general users to both detect and find the source of latency
    problems. That's actually pretty easy - realfeel running at
    2 kHz only consumes 2-3% of the CPU. It can just be left ticking
    over in the background.

    With preemptability merged in 2.5 we can then work to fix the
    long-held locks. Most of them are simple. Some of them are
    very much not. I'll gladly help with that.

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