[ntp:questions] ntp server specs

Richard B. Gilbert rgilbert88 at comcast.net
Thu Apr 21 14:05:14 UTC 2011

On 4/20/2011 10:48 PM, unruh wrote:
> On 2011-04-21, Richard B. Gilbert<rgilbert88 at comcast.net>  wrote:
>> On 4/20/2011 8:51 PM, unruh wrote:
>>> On 2011-04-21, Richard B. Gilbert<rgilbert88 at comcast.net>   wrote:
>>>> On 4/20/2011 5:49 PM, unruh wrote:
>>>>> On 2011-04-20, C BlacK<rblak at non.net>    wrote:
>>>>>> If I am setting up an ntp server for a lot machines, I have some questions.
>>>>>> If the timing on client machines needs to be tight, should the ntp server be an x86 machine with a less than accurate clock
>>>>>> or a ntp server with a very accurate clock.
>>>>> ? Clearly the server should have as good a clock as possible. And it
>>>>> should not CPU speed type power management. And it should not be on a GB
>>>>> network.
>>>>> Note that if you run the server with a GPS PPS you can get it down to
>>>>> 5usec accuracy, at which point the type of machine is largely
>>>>> irrelevant.
>>>>>> I want the clients to track closely to server.
>>>>> What does "track closely" mean? 1 hour? 1 sec? 1ms? 1us, 1ns?
>>>> <snip>
>>>> It's reasonable to expect NTPD to track the correct time within 10
>>>> milliseconds or better using a server that is close to you in *Net
>>> More like 10-100us.
>> Ten to one hundred microseconds over the net is a rare bird indeed.  At
>> least it is here in the Philadelphia area.
> I was getting it on an adsl line at home from the U.
>> Performance is MUCH better between 2:00AM and 5:00AM local time.  Delays
>> are fewer and there is less "phase noise".
>> <snip>
>> I use a GPS Timing Receiver that gets me within 50 ns. of the correct
>> time.  Of course that gets a little muddled while it wends its way into
>> my computer.
> I do not know what this means. The time is accurate to withing better
> than 10ns on the GPs sattelite.-- things just get a little muddled while
> it wends its way onto your computer.
> We are talking about ntp, and getting it into the computer is what it is
> all about. What time can you discipline your computer time to.

The typical computer is not designed for precision timekeeping!!  The 
latency from the "tick" to the computer's clock is not necessarily 
fixed, nor is it easy to measure.

When I look at my computer's "clock" it tells me the time to the nearest 
second.  NTPD is more precise but not usefully so for most things that I 
do.  I live my life with minutes, hours, days, etc.

NTPD will tell me the time to the nearest 50 or 100 nanoseconds but I'm 
generally satisfied with the correct minute.

There are people to whom nanoseconds are important; I'm just not one of 

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