[ntp:questions] NTP + kernel frequency

Spoon root at localhost.invalid
Fri Nov 16 10:28:31 UTC 2007


David L. Mills wrote:

 > Hal Murray wrote:
 >
 >> There is a lot of work going on in the Linux kernel to avoid
 >> unnecessary timer interrupts in order to save power on laptops
 >> and other systems running off batteries.
 >
> Any work on the Linux kernel to avoid timer interrupts is incompatible 
> with NTP. This is not a bad or good judgement, just and engineering 
> constraint. If timer interrupts are disabled, disable NTP.
> 
> On the issue about timer interrupts at frequencies other than 100 Hz, 
> this is easy to fix. The nanokernel code that left here uses a constant 
> SHIFT_PLL that must be scaled inversely as the timer interrupt 
> frequency. It does not need to be exact, but close. I don't know how or 
> even whether this code is mangled in the Linux kernel, but there you 
> have it. If provisions to fix this are not in the Linux kernel and the 
> timer frequency is other than 100 Hz, then the Linux build and install 
> process should not include ntpd; alternatively, the kernel discipline 
> must be disabled.

One can browse the Linux kernel NTP implementation here:
http://lxr.linux.no/source/kernel/time/ntp.c

The entire time directory might be of interest:
http://lxr.linux.no/source/kernel/time/

The NTP_INTERVAL_FREQ macro is defined as:

#ifdef CONFIG_NO_HZ
#define NTP_INTERVAL_FREQ  (2)
#else
#define NTP_INTERVAL_FREQ  (HZ)
#endif




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