>> > The PIT crystal runs at 14.3181818 MHz (CGA dotclock, found on ISA, ...) >> > and is divided by 12 to get PIT tick rate >> > >> > 14.3181818 MHz / 12 = 1193182 Hz >> > >> > The reality is that the crystal is usually off by 50-100 ppm from the >> > standard value, depending on temperature. >> > >> > HZ ticks/jiffie 1 second error (ppm) >> > --------------------------------------------------- >> > 100 11932 1.000015238 15.2 >> > 200 5966 1.000015238 15.2 >> > 250 4773 1.000057143 57.1 >> > 300 3977 0.999931429 -68.6 >> > 333 3583 0.999964114 -35.9 >> > 500 2386 0.999847619 -152.4 >> > 1000 1193 0.999847619 -152.4 >> > >> > Some HZ values indeed fit the tick frequency better than others, up to >> > 333 the error is lost in the physical error of the crystal, for 500 and >> > 1000, it definitely is larger, and thus noticeable. >> > >> > Some (less round and nice) values of HZ would fit even better: >> > >> > HZ ticks/jiffie 1 second error (ppm) >> > --------------------------------------------------- >> > 82 14551 1.000000152 0.2 >> >> >> Most interesting... Would 838 Hz be a much better choice than 1000 then? >> (apart from the ugliness). > > No, 838 isn't significantly better. 864 and 627 would be better > candidates: > > HZ ticks/jiffie 1 second error (ppm) > --------------------------------------------------- > 627 1903 0.999999314 -0.7 > 838 1424 1.000109105 109.1 > 864 1381 1.000001829 1.8 > > A good HZ value would make ntpd significantly happier, if the crystal is > of reasonable quality. > > 152ppm (1000Hz) is 13 seconds a day, > 0.7 ppm (627Hz) is 22 seconds a year.
Does positive vs negative error make a difference to the timer subsystem? Nish was telling me they had to add 1 extra tick to timer inaccuracies because of the errors ... does changing the polarity of the error affect that (seems like it would ... but I got lost by now ;-))? M. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
