ntpd struggling to keep up - how to fix?

[David Malone]

On Fri, Feb 12, 2010 at 11:46:04AM -0800, Jeremy Chadwick wrote:
> Technical footnote: I wish I understood 1) the difference between
> ACPI-safe and ACPI-fast, and 2) how the system or OS "ranks" the
> timecounters (the higher the value in parenthesis, supposedly the more
> accurate/preferred it is).  Xin, do you happen to know how this works?

1) When you read the ACPI timing register, you should get a sensible
answer. However on some (most?) hardware, you can read the register
and get it half way through an update. When the kernel finds the
ACPI timer, it tries reading it a few times in a row, and checks
the results look good - if they do, you get ACPI-fast. If it catches
a half-updated register, then you get ACPI-slow, which reads the
register multiple times in an effort to avoid the problem.

2) The ranking of timers is essentially hard wired, though for some
times it is adjusted in some way. For example, the ranking of the
TSC may be reduced if it looks like an SMP system. I believe the
ranking was originally intended to be a measure of how fast the
counter could be read, but things have turned out to be complicated
by difficult hardware.

	David.