Re: Proposal for kernel clock changes

[Warner Losh <imp%bsdimp.com@localhost>]

On Apr 1, 2014, at 1:50 PM, David Laight <david%l8s.co.uk@localhost> wrote:

> This may mean that you can (effectively) count the ticks on all your
> clocks since 'boot' and then scale the frequency of each to give the
> same 'time since boot' - even though that will slightly change the
> relationship between old timestamps taken on different clocks.
> Possibly you do need a small offset for each clock to avoid
> discrepencies in the 'current time' when you recalculate the clocks
> frequency.

If the underling clock moves in frequency, you need to have both a
scale on the frequency, and a time to count adjustment as well. Otherwise
on long-running systems you accumulate a fair amount of error. It doesn’t
take much more than 1ppm of error to accumulate a second of error in 10
days if you don’t have ‘on time’ marks that integrate all of time up to that
point. Then the error in phase will be related to the time since last phase
sync, rather than since time of boot.

Warner