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From: mogul@wrl.dec.com (Jeffrey Mogul)
Newsgroups: comp.protocols.time.ntp
Subject: Re: clock resolutions etc..
Message-ID: <1991Jan15.020110.25941@pa.dec.com>
Date: 15 Jan 91 02:01:10 GMT
References: <9101112100.AA22695@snook.ecs.umass.edu>
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In article <9101112100.AA22695@snook.ecs.umass.edu> rayaprol@umvlsi.ecs.umass.edu writes:
>I was going throught the man pages for adjtime(2) on DECStation 5000
>and found that the resolution on MIPS is 3.906ms and that on VAX is
>10ms. The man page also goes on saying that the ticks of 9ms or 11ms
>are used depending on the adjustment to be made. This only makes sense
>for a VAX machine. Could anyone tell me what the rate of adjustment is
>for a MIPS machine? 

I think this part of the manual page is confused.  Basically, once per
clock interrupt the time variable is updated (ignoring roundoff error)

	if (TotalDelta != 0) {
		time += (1/HZ) + delta;
		TotalDelta -= delta;
	}
	else
		time += 1/HZ;

The adjtime(2) call sets TotalDelta to the amount of correction
that you ask for, and sets delta like this

	if (TotalDelta > bigadj)
		delta = 10 * tickadj;
	else
		delta = tickadj;

and then rounds TotalDelta to an exact multiple of delta.

So, the interesting parameters are the kernel globals bigadj
(defaults to 1 second) and tickadj to 4000/hz == 15 (for RISC)
or 1 (for Vax).  I really don't know why the RISC and Vax versions
are different; the consensus of the NTP community is that "1"
is the right value, but 15 seems to work pretty well.  (On older
Vax/Ultrix systems, tickadj seems to be 4000/hz == 40, as well.)

To answer your question, then:

System		Default increment	Fast increment  	Slow increment
RISC		 3906 uSec		 3921 uSec		 3891 uSec
VAX		10000 uSec		10001 uSec		 9999 uSec

assuming that the total adjustment requested is under 1 second (which
is always the case with ntpd).

-Jeff