From: <[EMAIL PROTECTED]>
On 07/05/07, Chuck Swiger <[EMAIL PROTECTED]> wrote:
On May 4, 2007, at 9:10 AM, RW wrote:
> On Thu, 03 May 2007 11:07:34 -0400
> Chuck Swiger <[EMAIL PROTECTED]> wrote:
>> Sun SPARC machines have good HW clocks, and also some of the newer
>> Macs also seem to have consistently low values in ntp.drift and
>> handle timekeeping well.
> Does that matter?
A good question-- the answer seems to be that it depends.
A low value in ntp.drift is inconsequential compared
to a constant or near constant value, which many
motherboards do not "support".
> The RTC time is almost immediately overridden by ntpdate. The
> drift is a systematic error that ntpd allows for. I would
> have thought that the only significant issue, is whether the system
> loses timer interrupts under load.
There are limits to how rapidly ntpd will slew the clock via adjtime
(); the smaller the intrinsic drift of the HW clock, the sooner any
adjustment (beyond the initial stepping at system boot via ntpdate)
will complete. This only matters to stratum-2 and higher systems--
anything with a primary reference clock (GPS/WWV/ACTS/etc) is going
to sync to that and ignore the local HW clock entirely.
If you really need that ultimate precision, by all means
ntpd -> ntpd on the LAN is probably the Right Thing,
in conjunction with close temperature control. For most
uses (keeping two or more given machines within 10ms
or so on the same LAN) timed with one machine synced
to the outside world via ntpd is simpler at the very least.
If you have a 10ms tolerance you fall out of range rather quickly with
rather small errors. 1.1ppm over 2.4 hours is about 10ms. And that is
the range of variance that you can expect with temperature changes. That
is why NTP has a locked loop. It can sense the temperature changes over
the polling interval and compensate.
Note that typical motherboard oscillators are specified as plus or minus
100 ppm. AT cut crystals pretty much used to be 50 ppm devices until the
mass market PC crystals appeared. A reasonably good AT cut crystal should
show a plus to minus 25 ppm variance over -20 C to +70 C or there abouts.
And if the manufacturer felt good the day yours was made it will show a
turnover temperature about that of the inside of your case. But with the
really cheap crystals floating around - don't bet on it.
Machine rooms have more constant temperatures as a rule. That translates
to better stability. And the machines are on 24/7. That translates to
MUCH better stability. (Crystals "age" VERY rapidly for the first few
hours after turn on. This has to do with particulates and even air
molecules settling on the quartz surface when they are not oscillating.)
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