sgi revision 1.1.1.1.28.2
1 1.1.1.1.28.2 msaitoh adjtime, tick and tickadj: 2 1.1.1.1.28.2 msaitoh -------------------------- 3 1.1.1.1.28.2 msaitoh 4 1.1.1.1.28.2 msaitoh The SGI value for HZ is 100 under Irix 4, with the system clock running 5 1.1.1.1.28.2 msaitoh in nominal mode (ftimer off), so the value for tick is 10000 usec. 6 1.1.1.1.28.2 msaitoh Tickadj is a bit more tricky because of the behaviour of adjtime(), 7 1.1.1.1.28.2 msaitoh which seems to try to perform the correction over 100-200 seconds, with 8 1.1.1.1.28.2 msaitoh a rate limit of 0.04 secs/sec for large corrections. Corrections of 9 1.1.1.1.28.2 msaitoh less than 0.017 seconds generally complete in less than a second, 10 1.1.1.1.28.2 msaitoh however. 11 1.1.1.1.28.2 msaitoh 12 1.1.1.1.28.2 msaitoh Some measured rates are as follows: 13 1.1.1.1.28.2 msaitoh 14 1.1.1.1.28.2 msaitoh Delta Rate (sec/sec) 15 1.1.1.1.28.2 msaitoh 16 1.1.1.1.28.2 msaitoh > 1 0.04 17 1.1.1.1.28.2 msaitoh 0.75 0.04 18 1.1.1.1.28.2 msaitoh 0.6 0.004 19 1.1.1.1.28.2 msaitoh 0.5 0.004 20 1.1.1.1.28.2 msaitoh 0.4 0.0026 21 1.1.1.1.28.2 msaitoh 0.3 0.0026 22 1.1.1.1.28.2 msaitoh 0.2 0.0013 23 1.1.1.1.28.2 msaitoh 0.1 0.0015 24 1.1.1.1.28.2 msaitoh 0.05 0.0015 25 1.1.1.1.28.2 msaitoh 0.02 0.0003 26 1.1.1.1.28.2 msaitoh 0.01 0.015 27 1.1.1.1.28.2 msaitoh Strange. Anyway, since adjtime will complete adjustments of less than 28 1.1.1.1.28.2 msaitoh 17msec in less than a second, whether the fast clock is on or off, I 29 1.1.1.1.28.2 msaitoh have used a value of 150usec/tick for the tickadj value. 30 1.1.1.1.28.2 msaitoh 31 1.1.1.1.28.2 msaitoh Fast clock: 32 1.1.1.1.28.2 msaitoh ----------- 33 1.1.1.1.28.2 msaitoh 34 1.1.1.1.28.2 msaitoh I get smoother timekeeping if I turn on the fast clock, thereby making 35 1.1.1.1.28.2 msaitoh the clock tick at 1kHz rather than 100Hz. With the fast clock off, I 36 1.1.1.1.28.2 msaitoh see a sawtooth clock offset with an amplitude of 5msec. With it on, 37 1.1.1.1.28.2 msaitoh the amplitude drops to 0.5msec (surprise!). This may be a consequence 38 1.1.1.1.28.2 msaitoh of having a local reference clock which spits out the time at exactly 39 1.1.1.1.28.2 msaitoh one-second intervals - I am probably seeing sampling aliasing between 40 1.1.1.1.28.2 msaitoh that and the machine clock. This may all be irrelevant for machines 41 1.1.1.1.28.2 msaitoh without a local reference clock. Fiddling with the fast clock doesn't 42 1.1.1.1.28.2 msaitoh seem to compromise the above choices for tick and tickadj. 43 1.1.1.1.28.2 msaitoh 44 1.1.1.1.28.2 msaitoh I use the "ftimer" program to switch the fast clock on when the system 45 1.1.1.1.28.2 msaitoh goes into multiuser mode, but you can set the "fastclock" flag in 46 1.1.1.1.28.2 msaitoh /usr/sysgen/master.d/kernel to have it on by default. See ftimer(1). 47 1.1.1.1.28.2 msaitoh 48 1.1.1.1.28.2 msaitoh timetrim: 49 1.1.1.1.28.2 msaitoh --------- 50 1.1.1.1.28.2 msaitoh 51 1.1.1.1.28.2 msaitoh Irix has a kernel variable called timetrim which adjusts the system 52 1.1.1.1.28.2 msaitoh time increment, effectively trimming the clock frequency. Xntpd could 53 1.1.1.1.28.2 msaitoh use this rather than adjtime() to do it's frequency trimming, but I 54 1.1.1.1.28.2 msaitoh haven't the time to explore this. There is a utility program, 55 1.1.1.1.28.2 msaitoh "timetrim", in the util directory which allows manipulation of the 56 1.1.1.1.28.2 msaitoh timetrim value in both SGI and xntpd native units. You can fiddle with 57 1.1.1.1.28.2 msaitoh default timetrim value in /usr/sysgen/master.d/kernel, but I think 58 1.1.1.1.28.2 msaitoh that's ugly. I just use xntpd to figure out the right value for 59 1.1.1.1.28.2 msaitoh timetrim for a particular CPU and then set it using "timetrim" when 60 1.1.1.1.28.2 msaitoh going to multiuser mode. 61 1.1.1.1.28.2 msaitoh 62 1.1.1.1.28.2 msaitoh Serial I/O latency: 63 1.1.1.1.28.2 msaitoh ------------------- 64 1.1.1.1.28.2 msaitoh 65 1.1.1.1.28.2 msaitoh If you use a local clock on an RS-232 line, look into the kernel 66 1.1.1.1.28.2 msaitoh configuration stuff with regard to improving the input latency (check 67 1.1.1.1.28.2 msaitoh out /usr/sysgen/master.d/[sduart|cdsio]). I have a Kinemetrics OM-DC 68 1.1.1.1.28.2 msaitoh hooked onto /dev/ttyd2 (the second CPU board RS-232 port) on an SGI 69 1.1.1.1.28.2 msaitoh Crimson, and setting the duart_rsrv_duration flag to 0 improves things 70 1.1.1.1.28.2 msaitoh a bit. 71 1.1.1.1.28.2 msaitoh 72 1.1.1.1.28.2 msaitoh 73 1.1.1.1.28.2 msaitoh 12 Jan 93 74 1.1.1.1.28.2 msaitoh Steve Clift, CSIRO Marine Labs, Hobart, Australia (clift (a] ml.csiro.au) 75
Indexes created Mon Sep 07 00:24:45 UTC 2026