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    SubjectRe: [PATCH/RFC] msleep() with hrtimers
    Hey, Roman,

    > One possible problem here is that setting up that timer can be
    > considerably more expensive, for a relative timer you have to read the
    > current time, which can be quite expensive (e.g. your machine now uses the
    > PIT timer, because TSC was deemed unstable).

    That's a possibility, I admit I haven't benchmarked it. I will say that
    I don't think it will be enough to matter - msleep() is not a hot-path
    sort of function. Once the system is up and running it almost never
    gets called at all - at least, on my setup.

    > One question here would be, is it really a problem to sleep a little more?

    "A little more" is a bit different than "twenty times as long as you
    asked for." That "little bit more" added up to a few seconds when
    programming a device which needs a brief delay after tweaking each of
    almost 200 registers.

    > BTW there is another thing to consider. If you already run with hrtimer/
    > dyntick, there is not much reason to keep HZ at 100, so you could just
    > increase HZ to get the same effect.

    Except that then, with the current implementation, you're paying for the
    higher HZ whenever the CPU is busy. I bet that doesn't take long to
    overwhelm any added overhead in the hrtimer msleep().

    In the end, I did this because I thought msleep() should do what it
    claims to do, because I thought that getting a known-to-expire timeout
    off the timer wheel made sense, and to make a tiny baby step in the
    direction of reducing the use of jiffies in the core code.

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      Last update: 2007-07-16 16:45    [W:0.037 / U:105.096 seconds]
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