Re: after ids

On 05/07/2008, at 19:58, Donal K. Fellows wrote:

> On Darwin[*], times() is documented as returning the number
> of clock ticks since the Unix epoch, and so is inherently coupled to
> absolute time.

the Darwin implementation of times() comes from FreeBSD and is  
completely trivial, the return value is just gettimeofday() in ticks  
(c.f. Darwin source file Libc/gen/times-fbsd.c reproduced below).  
Might be worthwhile checking if current *BSDs still use this technique  
as well...

However, Darwin does have a platform-specific function to get a  
monotonic time reference: mach_absolute_time(), which we are using in  
Tcl already for high-resolution clicks, c.f. TclpGetWideClicks() and  
TclpWideClicksToNanoseconds() in tclUnixTime.c.

mach_absolute_time() is based on the CPU TSC (adjusted for CPU nap/ 
sleep) and is guaranteed to be monotonically increasing, see e.g. the  
following Darwin kernel sources for details:
     xnu/osfmk/i386/rtclock.c
     xnu/osfmk/i386/machine_routines_asm.s
     xnu/osfmk/i386/ommpage/commpage_mach_absolute_time.s
     xnu/osfmk/i386/commpage/commpage.c (commpage_set_nanotime)

All the implementations of relative timers in the OS (nanosleep(),  
pthread_cond_timedwait_relative_np() etc) are based on this reference.

Finding a function that returns an monotonic time reference on a  
system does not solve the problem of implementing relative timers in  
Tcl however, you also need a way for the notifier to wait for a  
relative time interval, e.g. the current threaded unix notifier  
waiting is based on pthread_cond_timedwait(), which uses absolute time  
(c.f. Tcl_ConditionWait()).
Darwin has pthread_cond_timedwait_relative_np() for exactly this  
purpose, I don't know if other systems have something equivalent.

There is also the problem of what to do in the notifier in the  
presence of both absolute and relative timers, i.e. use an absolute or  
a relative timeout? (converting next absolute timer fire to a relative  
time interval may result in waiting for too long if there are changes  
to the gettod base).

Cheers,

Daniel

-- 
** Daniel A. Steffen                   **
** <mailto:das@...>  **


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#if defined(LIBC_SCCS) && !defined(lint)
static char sccsid[] = "@(#)times.c	8.1 (Berkeley) 6/4/93";
#endif /* LIBC_SCCS and not lint */
#include <sys/cdefs.h>
__FBSDID("$FreeBSD: src/lib/libc/gen/times.c,v 1.2 2002/02/01 01:08:48  
obrien Exp $");

#include <sys/param.h>
#include <sys/time.h>
#include <sys/times.h>
#include <sys/resource.h>

/*
  * Convert usec to clock ticks; could do (usec * CLK_TCK) / 1000000,
  * but this would overflow if we switch to nanosec.
  */
#define	CONVTCK(r)	(r.tv_sec * CLK_TCK + r.tv_usec / (1000000 /  
CLK_TCK))

clock_t
times(tp)
	struct tms *tp;
{
	struct rusage ru;
	struct timeval t;

	if (getrusage(RUSAGE_SELF, &ru) < 0)
		return ((clock_t)-1);
	tp->tms_utime = CONVTCK(ru.ru_utime);
	tp->tms_stime = CONVTCK(ru.ru_stime);
	if (getrusage(RUSAGE_CHILDREN, &ru) < 0)
		return ((clock_t)-1);
	tp->tms_cutime = CONVTCK(ru.ru_utime);
	tp->tms_cstime = CONVTCK(ru.ru_stime);
	if (gettimeofday(&t, (struct timezone *)0))
		return ((clock_t)-1);
	return ((clock_t)(CONVTCK(t)));
}

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