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

Richard B. Gilbert rgilbert88 at comcast.net
Tue Jul 10 13:23:02 UTC 2007


Spoon wrote:
> Richard B. Gilbert wrote:
> 
>> Per Hedeland wrote:
>>
>>> Richard B. Gilbert wrote:
>>>
>>>> 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" 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.
>>
>>
>> Sorry about that.  Too many clocks!  In any event, the crystal 
>> oscillator gets very little attention from the manufacturer; the 
>> components are just about the cheapest available and no attempt is 
>> made to tune the crystal oscillator to bring it exactly on frequency.  
>> It's generally within 100 PPM of the desired value and that's plenty 
>> good enough for most people.  If the customers cared, manufacturers 
>> would include a higher quality crystal in a temperature compensated 
>> oscillator circuit and adjust it at the factory to the correct frequency.
>>
>> Most people buy computers by the G Hz of the processor, not the 
>> accuracy of the clock.  Most don't really need the accuracy we can get 
>> with NTP.  And a lot of people use NTP only to synchronize clocks to 
>> each other; they couldn't care less what time it is as long as all the 
>> computers agree on a time.
> 
> 
> I think you missed my point.
> 
> The HPET is supposed to replace all the other clocks I mentioned.
> 

What is "HPET"??




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