Messages in this thread | | | Date | Wed, 8 Dec 2004 15:53:58 -0800 (PST) | From | Christoph Lameter <> | Subject | Re: [RFC] New timeofday proposal (v.A1) |
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On Wed, 8 Dec 2004, 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 > o frequency adjustments > long term adjustment to correct for constant drift), > 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 > start talking about systems with irregular tick frequency (via > virtualization, or tickless systems) it becomes even more problematic.
We would need to schedule a special tick like event at a certain time but otherwise I do not see a problem. Is there a requirement that these "specific amounts of time" are less than 1 ms? The timer hardware (such as the RTC clock) can generate an event in <200ns that could be used to change the scaling. For a tickless system we would need to have such scheduled events anyways.
> If this can be fudged then it becomes less of an issue. Or at worse, we > have to do two mult/shift operations on two "parts" of the time interval > using different adjustments.
That looks troublesome. Better avoid that.
> Its starting to look doable, but its not necessarily the simplest thing > (for me at least). I'll put it on my list, but patches would be more > then welcome.
I am still suffering from my limited NTP knowlege but will see what I can do about this. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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