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    SubjectRe: [patch 0/9] x86_64: reliable TSC-based gettimeofday
    On Thu, Feb 01, 2007 at 12:20:59PM +0100, Andi Kleen wrote:
    > On Thursday 01 February 2007 10:59, wrote:
    > >
    > > Inter-CPU monotonicity can not, however, be guaranteed in a vsyscall, so
    > > vsyscall is not used by default.
    > Only for unsynchronized machines I hope

    yes, sorry, only on unsynchronized machines

    > The big strategic problem is how to marry your patchkit to John Stultz's
    > clocksources work which is also competing for merge. Any thoughts on that?

    I'll look into that next week. Sorry, I wanted to do that a long time
    ago, but I spent weeks (over a month) fighting a nasty livelock
    in the code. (Morale: think twice before using a spinlock inside
    a {do .. while (read_seqretry(..))} loop)

    > >When strict inter-CPU monotonicity is not needed, the vsyscall version of
    > >gettimeofday may be forced using the "nomonotonic" command line parameter.
    > >gettimeofday()'s monotonicity is guaranteed on a single CPU even with the very
    > >fast vsyscall version. Across CPUs, the vsyscall version of gettimeofday is
    > >not guaranteed to be monotonic, but it should be pretty close. Currently, we
    > >get errors of tens/hundreds of microseconds.
    > I think a better way to do this would be to define a new CLOCK_THREAD_MONOTONOUS
    > (or better name) timer for clock_gettime().

    I absolutely agree. Will do that. This should give userspace a
    decently accurate and very fast time source.

    Jiri Bohac <>
    SUSE Labs, SUSE CZ

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