1999/0225/port/tod.c (diff list | history)

1999/0219/sys/src/9/port/tod.c:5,26 – 1999/0225/sys/src/9/port/tod.c:5,103 (short | long | prev | next)
1999/0219		#include "fns.h" #include "../port/error.h"
1999/0225		// compute nanosecond epoch time from the fastest ticking clock // on the system. converting the time to nanoseconds requires // the following formula // // t = (((1000000000<<s1)/f)*(ticks>>s2))>>(s1-s2) // // where // // 'f' is the clock frequency // 'ticks' are clock ticks // 's1' and 's2' are shift ammounts to avoid 64 bit // overflows in the calculations // // to avoid too much calculation in gettod(), we calculate // // mult = (1000000000<<s1)/f // // each time f is set. f is normally set by a user level // program writing to /dev/fastclock. // // To calculate s1 and s2, we have to avoid overflowing our // signed 64 bit calculations. Also we wish to accomodate // 15 minutes of ticks. This gives us the following // constraints: // // 1) log2(1000000000<<s1) <= 63 // or s1 <= 33 // 2) accomodate 15 minutes of ticks without overflow // or log2(((1000000000<<s1)/f)*((15*60*f)>>s2)) <= 63 // or log2(mult) + 12 + log2(f) - s2 <= 63 // or log2(mult) + log2(f) - 51 <= s2 // // by definition // // 3) log2(mult) = log2(1000000000) + s1 - log2(f) // or log2(mult) = 30 + s1 - log2(f) // // To balance the accuracy of the multiplier and the sampled // ticks we set // // 4) log2(mult) = log2(f>>s2) // or log2(mult) = log2(f) - s2 // // Combining 2) and 4) we get // // 5) log2(f) - s2 + log2(f) - 51 <= s2 // or 2*log2(f) - 51 <= 2*s2 // or log2(f) - 25 <= s2 // // Combining 3) and 4) // // 6) 30 + s1 - log2(f) = log2(f) - s2 // or s1 = 2*log2(f) - s2 - 30 // // Since shifting ticks left doesn't increase accuracy, and // shifting 1000000000 right loses accuracy // // 7) s2 >= 0 // 8) s1 >= 0 // // As an example, that gives us the following // // for f = 100, log2(f) = 7 // // s2 = 0 // s1 = 0 // // for f = 267000000, log2(f) = 28 // // s2 = 3 // s1 = 23 // // for f = 2000000000, log2(f) = 31 // // s2 = 6 // s1 = 26 // // for f = 8000000000, log2(f) = 33 // // s2 = 8 // s1 = 28
1999/0219		// frequency of the tod clock #define TODFREQ 1000000000LL enum { Log2mult= 22, // multiplier should have 22 bits Log2todfreq= 30, // number of significant bits of TODFREQ };                  static vlong logtab[40]; struct { Lock; int shift; // time = (ticks*multiplier)>>shift
1999/0225		int s1; // time = ((ticks>>s2)*multiplier)>>(s1-s2)
1999/0219		vlong multiplier; // ...
1999/0225		int s2; // ... vlong maxdiff; // max diff between ticks and last to avoid overflow
1999/0219		vlong hz; // frequency of fast clock vlong last; // last reading of fast clock vlong off; // offset from epoch to last
1999/0219/sys/src/9/port/tod.c:65,76 – 1999/0225/sys/src/9/port/tod.c:142,165
1999/0219		void todsetfreq(vlong f) {
1999/0225		int lf;
1999/0219		// this ensures that the multiplier has 22 bits ilock(&tod); tod.hz = f; tod.shift = Log2mult - Log2todfreq + log2(f); tod.multiplier = (TODFREQ<<tod.shift)/f;
1999/0225		lf = log2(f); tod.s2 = lf - 25; if(tod.s2 < 0) tod.s2 = 0; tod.s1 = 2*lf - tod.s2 - 30; if(tod.s1 < 0) tod.s1 = 0; if(tod.s1 > 33) tod.s1 = 33; tod.multiplier = (TODFREQ<<tod.s1)/f; tod.maxdiff = 1LL<<(10 + lf);
1999/0219		iunlock(&tod);
1999/0225		//print("hz %lld mult %lld s1 %d s2 %d maxdiff %lld\n", tod.hz, tod.multiplier, tod.s1, tod.s2, tod.maxdiff);
1999/0219		} //
1999/0219/sys/src/9/port/tod.c:109,119 – 1999/0225/sys/src/9/port/tod.c:198,208
1999/0219		diff = ticks - tod.last; // convert to epoch x = (diff*tod.multiplier)>>tod.shift;
1999/0225		x = ((diff>>tod.s2)*tod.multiplier)>>(tod.s1-tod.s2);
1999/0219		x += tod.off; // protect against overflows if(diff > (1LL<<(63-Log2mult))){
1999/0225		if(diff > tod.maxdiff){
1999/0219		tod.last = ticks; tod.off = x; }
1999/0219/sys/src/9/port/tod.c:135,144 – 1999/0225/sys/src/9/port/tod.c:224,239
1999/0219		{ if((MACHP(0)->ticks % HZ) != 0) return;
1999/0225		// once a second apply correction
1999/0219		ilock(&tod); if(tod.n > tod.i++) tod.off += tod.delta; iunlock(&tod);
1999/0225		// once a minute, make sure we don't overflow if((MACHP(0)->ticks % (60*HZ)) == 0) todget();
1999/0219		} long