SGI Hints

Last update: May 4, 2022 17:05 UTC (dbea9b7d4)

12 Jan 93

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adjtime, tick and tickadj

The SGI value for HZ is 100 under Irix 4, with the system clock running in nominal mode (ftimer off), so the value for tick is 10000 usec. tickadj is a bit more tricky because of the behaviour of adjtime(), which seems to try to perform the correction over 100-200 seconds, with a rate limit of 0.04 secs/sec for large corrections. Corrections of less than 0.017 seconds generally complete in less than a second, however.

Some measured rates are as follows:

	Delta       Rate (sec/sec)
	> 1		0.04
	0.75		0.04
	0.6		0.004
	0.5		0.004
	0.4		0.0026
	0.3		0.0026
	0.2		0.0013
	0.1		0.0015
	0.05		0.0015
	0.02		0.0003
	0.01		0.015

Strange. Anyway, since adjtime will complete adjustments of less than 17 msec in less than a second, whether the fast clock is on or off, I have used a value of 150 usec/tick for the tickadj value.

Fast Clock

I get smoother timekeeping if I turn on the fast clock, thereby making the clock tick at 1kHz rather than 100Hz. With the fast clock off, I see a sawtooth clock offset with an amplitude of 5 msec. With it on, the amplitude drops to 0.5 msec (surprise!). This may be a consequence of having a local reference clock which spits out the time at exactly one-second intervals - I am probably seeing sampling aliasing between that and the machine clock. This may all be irrelevant for machines without a local reference clock. Fiddling with the fast clock doesn’t seem to compromise the above choices for tick and tickadj.

I use the ftimer program to switch the fast clock on when the system goes into multiuser mode, but you can set the fastclock flag in /usr/sysgen/master.d/kernel to have it on by default. See ftimer(1).

timetrim

Irix has a kernel variable called timetrim which adjusts the system time increment, effectively trimming the clock frequency. xntpd could use this rather than adjtime() to do its frequency trimming, but I haven’t the time to explore this. There is a utility program, timetrim, in the util directory which allows manipulation of the timetrim value in both SGI and xntpd native units. You can fiddle with default timetrim value in /usr/sysgen/master.d/kernel, but I think that’s ugly. I just use xntpd to figure out the right value for timetrim for a particular CPU and then set it using timetrim when going to multiuser mode.

Serial I/O Latency

If you use a local clock on an RS-232 line, look into the kernel configuration stuff with regard to improving the input latency (check out /usr/sysgen/master.d/[sduart|cdsio]). I have a Kinemetrics OM-DC hooked onto /dev/ttyd2 (the second CPU board RS-232 port) on an SGI Crimson, and setting the duart_rsrv_duration flag to 0 improves things a bit.

Steve Clift, CSIRO Marine Labs, Hobart, Australia