Re: gettimeofday resolution seriously degraded in test9

[George Anzinger <george@mvista.com>]

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