/contrib/ntp/html/build/hints/sgi

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