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Posted by Jerry Stuckle on 06/26/07 22:41
Moe Trin wrote:
> On Sun, 24 Jun 2007, in the Usenet newsgroup comp.os.linux, in article
> <YdWdncWaU_Y1E-PbnZ2dnUVZ_rfinZ2d@comcast.com>, Jerry Stuckle wrote:
>
>> David T. Ashley wrote:
>
>>> "Jerry Stuckle" <jstucklex@attglobal.net> wrote
>
>>>> So they have to be adjusted. And even though NTP tries to speed up and
>>>> slow the clock - it's not perfect. And at times the clock WILL jump
>>>> forwards or backwards at least one second.
>
> The only time the clock will be jumping is a boot time, before any
> applications are running, as kernel time jumps from "zero" to what ever
> the BIOS is claiming. There is a mechanism in NTP where you can also
> SET the time, but this should only be done at boot time.
>
The only time when the system sets it automatically, you mean.
>>> What kind of events are you speaking of that might cause discontinuities?
>> It depends on a lot of things. Many systems do not have the necessary
>> hardware adjust clock speed.
>
> When was the last time you saw a hardware adjustment for clock speed?
> Not to many of us are using IBM Type 1 motherboards from 1985. The
> Pentium class motherboards often don't even have a separate oscillator
> module. with the interrupt source being the oscillator that is part of
> the RTC.
>
True.
>> So just like any clock, if it's running fast, it must be set back
>> sooner or later.
>
> This kernel is compiled to run the IRQ0 interrupt at 100 "ticks" per
> second (others use 1000 "ticks"), and that means the the divider is
> normally running at 18432:1. To _drift_ the clock into correspondence
> with "real" time, that division ratio is altered. As the crap crystal
> oscillator used as a timing source is accurate to +/-100 ppm, the
> division ration is periodically altered over the range of 18430 to
> 18434 to make it read something approaching the desired time. No big
> magic deal - and no "major" steps. It's just that the 1/100 of a
> second may vary by up to 54 microseconds. If your kernel is compiled
> for a 1000 Hertz interrupt, the kernel is already doing this shuffle
> step routine, as the divider is only programed to integers, and not the
> 1843.2:1 r quired with a perfectly set oscillator.
>
> Old guy
First of all, uncompensated clocks may or may not be 100ppm. I've got
one server, for instance, which is off more than 500 ppm. Several
others run in the 200-400ppm range.
You've come up with a lot of arguments. So you are saying you can
guarantee - WITHOUT QUALIFICATION - that the clock will NEVER be set
back while things are running?
I didn't think so.
--
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
==================
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