[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

Bill Unruh unruh at physics.ubc.ca
Thu Jan 31 17:07:45 UTC 2008

On Wed, 30 Jan 2008, Danny Mayer wrote:

> Unruh wrote:
>>  "David L. Mills" <mills at udel.edu> writes:
>> >  David,
>> >  We can argue about the Hurst parameter, which can't be truly random-walk 
>> >  as I have assumed, but the approximation is valid up to lag times of at 
>> >  least a week. However, as I have been cautioned, these plots are really 
>> >  sensitive to spectral lines due to nonuniform sampling. I was very 
>> >  careful to avoid such things.
>>  But the "lines" I am refering to are not artifacts, they are there because
>>  of the way the computer is used. -- the temp fluctuations caused by people
>>  running the machine daily, except on weekends. These are not part of any
>>  "random walk" process. They are real jumps in the drift rate of the
>>  machine, large jumps, and definitely not random.
> Well of course. You are running Linux and losing interrupts. FreeBSD and 
> friends don't suffer from that problem. I seem to remember setting HZ=100 
> mostly eliminates that problem, at the price of rebuilding the kernel.
> Danny

No they are not lost interrupts. They are NOT jumps in the offset, they are
jumps in the frequency, which will last for a few hours and then jump back.
Lost interrupts do not act like that-- they would jump the offset by 10ms
(or 4ms) which is definitely not happening. Andit is hard to gain


William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
UBC, Vancouver,BC  |   Program in Cosmology |     unruh at physics.ubc.ca
Canada V6T 1Z1     |      and Gravity       |  www.theory.physics.ubc.ca/

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