Messages in this thread | | | Date | Thu, 30 Oct 2003 13:33:00 -0800 | From | George Anzinger <> | Subject | Re: gettimeofday resolution seriously degraded in test9 |
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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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