[ntp:questions] Is 24PPM an Excessive Real-Time Clock Correction?

Per Hedeland per at hedeland.org
Mon Jul 9 21:17:37 UTC 2007


In article <4692484C.4050705 at comcast.net> "Richard B. Gilbert"
<rgilbert88 at comcast.net> writes:
>Spoon wrote:
>> Richard B. Gilbert wrote:
>> 
>>> David T. Ashley wrote:
>>>
>>>> I have ntpd running on a RHEL Linux Dell 1U rack server in another city.
>>>>
>>>> I have no experience with other servers (to know how accurate the 
>>>> clocks are or are not).
>>>>
>>>> The value of "drift" that gets settled on after a few days is 24 PPM 
>>>> (about 14 seconds/week).
>>>>
>>>> This strikes me as a little high, because even my $20 Timex watch 
>>>> does better than this.
>>>>
>>>> Any insight into whether 24 PPM is excessive for my server?
>>>
>>>
>>> Consider that a computer manufacturer typically spends something like 
>>> $2 US on the components for the clock!
>> 
>> 
>> You say "the clock" like as if there were only one.
>> 
><snip>
>I meant the time of day clock; it's the only one that ntpd concerns 
>itself with.  I thought it would be obvious from the context!

The "time of day clock" is normally used to refer to the CMOS or
equivalent one that ticks even when power is off - that would be "RTC"
in the previous post I believe - it's the one clock that ntpd absolutely
does *not* concern itself with.

The one it *does* concern itself with is the "system" clock, a software
clock that is advanced by the kernel based on interrupts from one of the
other sources listed - most commonly the PIT or APIC timer AFAIK. But of
course the interesting part isn't the timers (i.e. counters), but the
crystal(s) etc that drive them.

--Per Hedeland
per at hedeland.org




More information about the questions mailing list