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