Re: Clock monotonic a suggestion

[george anzinger <george@mvista.com>]

Martin Waitz wrote:
> On Thu, Mar 20, 2003 at 04:16:13PM -0800, george anzinger wrote:
> 
>>Define CLOCK_MONOTONIC to be the same as
>>(gettimeofday() + wall_to_monotonic).
> 
> 
> why don't you simply use asm("rdtsc") ?
> (ok, you should make sure that you always ask the same processor and
> stuff, but using the built in TSC seems to do everything you want...)
> 
> 
I don't really understand how :(

I want a tick on CLOCK_MONOTONIC to be the same size as a tick on 
gettimeofday() over the life of the system.  I.e. lock step.  The only 
difference is that CLOCK_MONOTONIC can not be set, so, if we use the 
above, wall_to_monotonic must be adjusted when the gettimeofday() 
clock is set (but not when it is "adjusted" by NTP).

asm("rdtsc") is, first of all, only useful on x86 platforms, 
CLOCK_MONOTONIC is in the POSIX clocks and timers code and in all 
platforms.  Second, each platform has an equivalent, best guess, way 
of filling in the time information below the 1/HZ level (and yes, some 
x86 platforms use TSC) already in the gettimeofday() code.  Except 
that the system settime code is platform dependent, this solution is 
platform independent.

-- 
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