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    SubjectRe: gettimeofday resolution seriously degraded in test9
    Peter Chubb wrote:
    >>>>>>"Stephen" == Stephen Hemminger <shemminger@osdl.org> writes:
    >
    >
    > Stephen> On Wed, 29 Oct 2003 11:07:45 +0100 Gabriel Paubert
    > Stephen> <paubert@iram.es> wrote:
    >
    >>>for example.
    >
    >
    > Stephen> The suggestion of using time interpolation (like ia64) would
    > Stephen> make the discontinuities smaller, but still relying on fine
    > Stephen> grain gettimeofday for controlling servo loops with NTP
    > Stephen> running seems risky. Perhaps what you want to use is the
    > Stephen> monotonic_clock which gives better resolution (nanoseconds)
    > Stephen> and doesn't get hit by NTP.
    >
    > monotonic_clock:
    > -- isn't implemented for most architectures
    > -- even for X86 only works for some timing sources
    > -- and for the most common case is variable rate because of
    > power management functions changing the TSC clock rate.
    >
    > As far as I know, there isn't a constant-rate monotonic clock
    > available at present for all architectures in the linux kernel. The
    > nearest thing is scheduler_clock().

    What you want is the POSIX clocks and timers CLOCK_MONOTONIC which is available
    on all archs (as of 2.6). The call is:

    cc [ flag ... ] file -lrt [ library ... ]

    #include <time.h>

    int clock_gettime(clockid_t which_clock, struct timespec *setting);

    where you want "which_clock" to be CLOCK_MONOTONIC.


    --
    George Anzinger george@mvista.com
    High-res-timers: http://sourceforge.net/projects/high-res-timers/
    Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml

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