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    SubjectRe: [PATCH/RFC] msleep() with hrtimers
    On 7/16/07, Ray Lee <ray-lk@madrabbit.org> wrote:
    > On 7/16/07, Roman Zippel <zippel@linux-m68k.org> wrote:
    > > On Mon, 16 Jul 2007, Jonathan Corbet wrote:
    > > > > 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.
    > >
    > > That's a bit my problem - we have to consider other setups as well.
    > > Is it worth converting all msleep users behind their back or should we
    > > just a provide a separate function for those who care?
    >
    > As a driver author (2.4 timeframe, embedded platform, see gitinc.com
    > for the hardware description), I would rather msleep did what it says
    > it's going to do. If the current one can wait 20 times longer than you
    > ask for, then that's just broken.

    Well, before these changes, the only guarantee msleep() could make,
    just like the only guarantee schedule_timeout() could make, was that
    it would not return early. The 1-jiffy sleep was always tough to deal
    with, because of rounding and such. And it's simply exacerbated with
    HZ=100. It's not technically 20 times longer in all cases, it's 2
    jiffies longer, which depends on HZ, so varies from 2 msecs longer to
    20 msecs longer.

    Thanks,
    Nish
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