82 lines
2.7 KiB
C
82 lines
2.7 KiB
C
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#include <sys/types.h>
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#include <sys/time.h>
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#define DEFAULT_SYS_PRECISION -99
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int default_get_precision();
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int
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main() {
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printf("log2(precision) = %d\n", default_get_precision());
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return 0;
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}
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/* Find the precision of the system clock by watching how the current time
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* changes as we read it repeatedly.
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*
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* struct timeval is only good to 1us, which may cause problems as machines
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* get faster, but until then the logic goes:
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*
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* If a machine has precision (i.e. accurate timing info) > 1us, then it will
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* probably use the "unused" low order bits as a counter (to force time to be
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* a strictly increaing variable), incrementing it each time any process
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* requests the time [[ or maybe time will stand still ? ]].
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*
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* SO: the logic goes:
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*
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* IF the difference from the last time is "small" (< MINSTEP)
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* THEN this machine is "counting" with the low order bits
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* ELIF this is not the first time round the loop
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* THEN this machine *WAS* counting, and has now stepped
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* ELSE this machine has precision < time to read clock
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*
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* SO: if it exits on the first loop, assume "full accuracy" (1us)
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* otherwise, take the log2(observered difference, rounded UP)
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*
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* MINLOOPS > 1 ensures that even if there is a STEP between the initial call
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* and the first loop, it doesn't stop too early.
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* Making it even greater allows MINSTEP to be reduced, assuming that the
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* chance of MINSTEP-1 other processes getting in and calling gettimeofday
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* between this processes's calls.
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* Reducing MINSTEP may be necessary as this sets an upper bound for the time
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* to actually call gettimeofday.
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*/
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#define DUSECS 1000000
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#define HUSECS (1024 * 1024)
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#define MINSTEP 5 /* some systems increment uS on each call */
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/* Don't use "1" as some *other* process may read too*/
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/*We assume no system actually *ANSWERS* in this time*/
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#define MAXLOOPS HUSECS /* Assume precision < .1s ! */
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int default_get_precision()
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{
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struct timeval tp;
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struct timezone tzp;
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long last;
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int i;
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long diff;
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long val;
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int minsteps = 2; /* need at least this many steps */
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gettimeofday(&tp, &tzp);
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last = tp.tv_usec;
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for (i = - --minsteps; i< MAXLOOPS; i++) {
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gettimeofday(&tp, &tzp);
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diff = tp.tv_usec - last;
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if (diff < 0) diff += DUSECS;
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if (diff > MINSTEP) if (minsteps-- <= 0) break;
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last = tp.tv_usec;
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}
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printf("precision calculation given %dus after %d loop%s\n",
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diff, i, (i==1) ? "" : "s");
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diff = (diff *3)/2;
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if (i >= MAXLOOPS) diff = 1; /* No STEP, so FAST machine */
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if (i == 0) diff = 1; /* time to read clock >= precision */
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for (i=0, val=HUSECS; val>0; i--, val >>= 1) if (diff >= val) return i;
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return DEFAULT_SYS_PRECISION /* Something's BUST, so lie ! */;
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}
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