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