mirror of
https://git.FreeBSD.org/src.git
synced 2024-12-20 11:11:24 +00:00
82 lines
2.7 KiB
C
82 lines
2.7 KiB
C
#include <sys/types.h>
|
|
#include <sys/time.h>
|
|
|
|
#define DEFAULT_SYS_PRECISION -99
|
|
|
|
int default_get_precision();
|
|
|
|
int
|
|
main() {
|
|
printf("log2(precision) = %d\n", default_get_precision());
|
|
return 0;
|
|
}
|
|
|
|
/* Find the precision 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 precision (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 precision < 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 MAXLOOPS HUSECS /* Assume precision < .1s ! */
|
|
|
|
int default_get_precision()
|
|
{
|
|
struct timeval tp;
|
|
struct timezone tzp;
|
|
long last;
|
|
int i;
|
|
long diff;
|
|
long val;
|
|
int minsteps = 2; /* 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("precision calculation given %dus after %d loop%s\n",
|
|
diff, i, (i==1) ? "" : "s");
|
|
|
|
diff = (diff *3)/2;
|
|
if (i >= MAXLOOPS) diff = 1; /* No STEP, so FAST machine */
|
|
if (i == 0) diff = 1; /* time to read clock >= precision */
|
|
for (i=0, val=HUSECS; val>0; i--, val >>= 1) if (diff >= val) return i;
|
|
return DEFAULT_SYS_PRECISION /* Something's BUST, so lie ! */;
|
|
}
|
|
|