Re: Clock monotonic a suggestion

[Joel Becker <Joel.Becker@oracle.com>]

On Fri, Mar 21, 2003 at 12:01:13AM -0800, george anzinger wrote:
> To carry this to the absurd, it also precludes most anything other 
> than a GPS or WWV based clock.  If we are to have any clock that is 

	No, cycle counters do just fine.
 
> >1    gettimeofday = 1000000000
> >2    driver delays 10s
> >3    gettimeofday = 1000000000
> >4    timer notices lag and adjusts
> 
> Uh, how is this done?  At this time there IS correction for delays up 
> to about a second built into the gettimeofday() code.  You seem to be 
> assuming that we can do better than this with clock monotonic.  Given 
> the right hardware, this may even be possible, but why not correct 
> gettimeofday in the same way?

	monotonic_clock as proposed uses a hardware clock.
Specifically, the TSC on vanilla intel, the Cyclone timer on x440, and
associated clocks on S/390 (to speak of platforms I've visited
recently).  Right now, the hangcheck-timer accesses the hardware counter
directly.  monotonic_clock is intended as a portable and consistent
accessor instead.
	The current gettimeofday() corrects for ~1s.  Even if we found a
way to correct for ~1000s or more, there would still be a race between
when the caller reads and when the correction happens.  A clock that
reads a hardware counter doesn't have this problem.
	If code doesn't need this accuracy, it can just use
gettimeofday().

Joel

-- 

"Glory is fleeting, but obscurity is forever."  
         - Napoleon Bonaparte

Joel Becker
Senior Member of Technical Staff
Oracle Corporation
E-mail: joel.becker@oracle.com
Phone: (650) 506-8127