Messages in this thread | | | Subject | Re: [RFC] New timeofday proposal (v.A1) | From | john stultz <> | Date | Thu, 09 Dec 2004 00:29:52 -0800 |
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On Thu, 2004-12-09 at 08:57 +0100, Ulrich Windl wrote: > On 8 Dec 2004 at 15:36, john stultz wrote: > > [...] > > Take a look at the adjtimex man page as well as the ntp.c file from the > > timeofday core patch. There are number of different types of adjustments > > that are made, possibly at the same time. Briefly, they are (to my > > understanding - I'm going off my notes from awhile ago): > > o tick adjustments > > how much time should pass in a _user_ tick > > tick adjustments are considered obsolete, because if a lcok implementation (or > hardware) is severly broken, users should reject using that stuff. Meaning: tick > adjustments are ment to be set once in the life of a computer system. No > continuous tuning. > > > o frequency adjustments > > long term adjustment to correct for constant drift), > > Actually, you are compensating for a "tick problem" on a smaller scale (constant > part), and variations caused by temperature, voltage, and others (variable part). > > > o offset adjustments > > additional ppm adjustment to correct for current offset from the ntp > > server > > o single shot offset adjustments > > old style adjtime functionality > > > > Tick, frequency and offset adjustments can be precalculated and summed > > to a single ppm adjustment. This is similar to the style of adjustment > > you propose directly onto the time source frequency values. > > > > However, there is also this short term single shot adjustments. These > > adjustments are made by applying the MAX_SINGLESHOT_ADJ (500ppm) scaling > > for an amount of time (offset_len) which would compensate for the > > offset. This style is difficult because we cannot precompute it and > > apply it to an entire tick. Instead it needs to be applied for just a > > specific amount of time which may be only a fraction of a tick. When we > > Yes, that's the "precise" variant of implementing it. Poor implementations are > just accurate to one tick.
Thanks for your knowledgeable clarifications. Its good to know someone out there deeply understands this stuff more then at a "this is what the code is doing, and I have my own interpretation as to why" level. :)
thanks again, -john
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