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#include "ntp_unixtime.h"
#include <stdio.h>
#define DEFAULT_SYS_PRECISION -99
int default_get_resolution();
int default_get_precision();
int
main(
int argc,
char *argv[]
)
{
printf("log2(resolution) = %d, log2(precision) = %d\n",
default_get_resolution(),
default_get_precision());
return 0;
}
/* Find the resolution of the system clock by watching how the current time
* changes as we read it repeatedly.
*
* struct timeval is only good to 1us, which may cause problems as machines
* get faster, but until then the logic goes:
*
* If a machine has resolution (i.e. accurate timing info) > 1us, then it will
* probably use the "unused" low order bits as a counter (to force time to be
* a strictly increaing variable), incrementing it each time any process
* requests the time [[ or maybe time will stand still ? ]].
*
* SO: the logic goes:
*
* IF the difference from the last time is "small" (< MINSTEP)
* THEN this machine is "counting" with the low order bits
* ELIF this is not the first time round the loop
* THEN this machine *WAS* counting, and has now stepped
* ELSE this machine has resolution < time to read clock
*
* SO: if it exits on the first loop, assume "full accuracy" (1us)
* otherwise, take the log2(observered difference, rounded UP)
*
* MINLOOPS > 1 ensures that even if there is a STEP between the initial call
* and the first loop, it doesn't stop too early.
* Making it even greater allows MINSTEP to be reduced, assuming that the
* chance of MINSTEP-1 other processes getting in and calling gettimeofday
* between this processes's calls.
* Reducing MINSTEP may be necessary as this sets an upper bound for the time
* to actually call gettimeofday.
*/
#define DUSECS 1000000
#define HUSECS (1024 * 1024)
#define MINSTEP 5 /* some systems increment uS on each call */
/* Don't use "1" as some *other* process may read too*/
/*We assume no system actually *ANSWERS* in this time*/
#define MAXSTEP 20000 /* maximum clock increment (us) */
#define MINLOOPS 5 /* minimum number of step samples */
#define MAXLOOPS HUSECS /* Assume precision < .1s ! */
int
default_get_resolution(void)
{
struct timeval tp;
struct timezone tzp;
long last;
int i;
long diff;
long val;
int minsteps = MINLOOPS; /* need at least this many steps */
gettimeofday(&tp, &tzp);
last = tp.tv_usec;
for (i = - --minsteps; i< MAXLOOPS; i++) {
gettimeofday(&tp, &tzp);
diff = tp.tv_usec - last;
if (diff < 0) diff += DUSECS;
if (diff > MINSTEP) if (minsteps-- <= 0) break;
last = tp.tv_usec;
}
printf("resolution = %ld usec after %d loop%s\n",
diff, i, (i==1) ? "" : "s");
diff = (diff *3)/2;
if (i >= MAXLOOPS) {
printf(
" (Boy this machine is fast ! %d loops without a step)\n",
MAXLOOPS);
diff = 1; /* No STEP, so FAST machine */
}
if (i == 0) {
printf(
" (The resolution is less than the time to read the clock -- Assume 1us)\n");
diff = 1; /* time to read clock >= resolution */
}
for (i=0, val=HUSECS; val>0; i--, val >>= 1) if (diff >= val) return i;
printf(" (Oh dear -- that wasn't expected ! I'll guess !)\n");
return DEFAULT_SYS_PRECISION /* Something's BUST, so lie ! */;
}
/* ===== Rest of this code lifted straight from xntpd/ntp_proto.c ! ===== */
/*
* This routine calculates the differences between successive calls to
* gettimeofday(). If a difference is less than zero, the us field
* has rolled over to the next second, so we add a second in us. If
* the difference is greater than zero and less than MINSTEP, the
* clock has been advanced by a small amount to avoid standing still.
* If the clock has advanced by a greater amount, then a timer interrupt
* has occurred and this amount represents the precision of the clock.
* In order to guard against spurious values, which could occur if we
* happen to hit a fat interrupt, we do this for MINLOOPS times and
* keep the minimum value obtained.
*/
int
default_get_precision(void)
{
struct timeval tp;
struct timezone tzp;
#ifdef HAVE_GETCLOCK
struct timespec ts;
#endif
long last;
int i;
long diff;
long val;
long usec;
usec = 0;
val = MAXSTEP;
#ifdef HAVE_GETCLOCK
(void) getclock(TIMEOFDAY, &ts);
tp.tv_sec = ts.tv_sec;
tp.tv_usec = ts.tv_nsec / 1000;
#else /* not HAVE_GETCLOCK */
GETTIMEOFDAY(&tp, &tzp);
#endif /* not HAVE_GETCLOCK */
last = tp.tv_usec;
for (i = 0; i < MINLOOPS && usec < HUSECS;) {
#ifdef HAVE_GETCLOCK
(void) getclock(TIMEOFDAY, &ts);
tp.tv_sec = ts.tv_sec;
tp.tv_usec = ts.tv_nsec / 1000;
#else /* not HAVE_GETCLOCK */
GETTIMEOFDAY(&tp, &tzp);
#endif /* not HAVE_GETCLOCK */
diff = tp.tv_usec - last;
last = tp.tv_usec;
if (diff < 0)
diff += DUSECS;
usec += diff;
if (diff > MINSTEP) {
i++;
if (diff < val)
val = diff;
}
}
printf("precision = %ld usec after %d loop%s\n",
val, i, (i == 1) ? "" : "s");
if (usec >= HUSECS) {
printf(" (Boy this machine is fast ! usec was %ld)\n",
usec);
val = MINSTEP; /* val <= MINSTEP; fast machine */
}
diff = HUSECS;
for (i = 0; diff > val; i--)
diff >>= 1;
return (i);
}