[ntp:questions] drifting on crystal

Brad Knowles brad at stop.mail-abuse.org
Thu Jan 13 22:18:33 UTC 2005

At 11:01 PM +0100 2005-01-13, Enrique Perez-Terron wrote:

>  The TSC is said to have its own problems because of variations of the
>  cpu frequencies on multi-cpu computers, and because of such things as
>  power saving laptops switching frequencies.

	Both of these are absolute death for NTP.

>  By the way, does ntp have support for such factors?  Do we need to
>  maintain separate drift information for each of the frequencies the
>  CPU(s) can switch to?

	With regards to SMP systems, see the article by Poul-Henning Kamp 

	If your CPU changes speeds, that will absolutely kill NTP.  NTP 
assumes that the CPU clock speed is always stable (albeit perhaps 
slightly fast or slow), and that there are lots of other variables 
that it needs to measure, which it does against the CPU clock speed, 
and then it will apply slight corrections to the CPU clock speed 
based on the calculations.

	If the CPU speed itself is variable, well, there's not much worse 
that you could do to NTP.

Brad Knowles, <brad at stop.mail-abuse.org>

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."

     -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
     Assembly to the Governor, November 11, 1755

   SAGE member since 1995.  See <http://www.sage.org/> for more info.

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