[ntp:questions] Re: time reset, syncronisation lost and Large PPM values

Tom Smith smith at cag.zko.hp.com
Fri Oct 21 19:16:00 UTC 2005

Sorry, I should have also mentioned the hardware possibilities.

If re-initializing ntpd leaves you right back where you started,
you may have hardware-related issues.

If your system is losing time against the servers, you may be
losing interrupts. Heavy IDE drive activity can cause this,
as can certain other device drivers that hog interrupts.

If your system is gaining time relative to the servers, APIC
may be at fault, especially on SMP systems. Disabling APIC in
the kernel seems to be where many people with this problem have
ended up.

ACPI (the power management stuff) presents a different class of
problem. If the power management (or anything else) modifies the
hardware clock frequency (slowing it down to conserve power),
NTP isn't really equipped to deal with this. NTP assumes that the
system has a single characteristic frequency and computes a
compensating correction factor (drift rate) to allow the system
clock to move forward at the rate of "real" (UTC) time. If the
hardware is speeding up and slowing down, this completely screws
up NTP's calculation of what frequency correction needs to be made.
Laptops often have this problem. If the power management
can be configured to leave the clock speed alone, that
should be sufficient to avoid this problem. Otherwise, acpi
also has to be disabled in the kernel.


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