Messages in this thread | | | Date | Wed, 8 Dec 2004 12:14:02 -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:
> > With the improved scaling factor one should be able to come very close to > > ntp scaled time without invoking ntp_scale itself. Tick processing will > > then update time to be ntp scaled by fine tuning the scaling factor (with > > the bit shifting we can get very fine tuning) and eventually skip a few > > nanoseconds. Its basically some piece of interpolator logic in there so > > that the heavyweight calculations can just be done once in a while. > > No. I agree ntp_scale() is a performance concern. However I'm not sure > how your suggestion of just slowing or tweaking the timesource > mult/shift frequency values will allow us to implement the expected > behavior of adjtimex(). We need to be able to implement the following > adjustments within a single tick: > > 1. Adjust the frequency by 500ppm for 10usecs > 2. After that adjust the frequency by 30ppm for the rest of the tick.
Frequency adjustments just means an adjustment of the scaling factor. Am I missing something?
> We can see how much of this can be fudged or generalized, but I'm > hesitant to be too flippant about changing the NTP behavior for fear > that the astronomers who so dearly care about leap seconds and minute > time adjustments will "forget" to mention the asteroid heading towards > my home. :)
I am not sure what NTP behavior needs to be fudged. Sorry about my limited NTP knowledge. Could you elaborate on what the problem is? > > I may have asked this before, but w/ 32 bit mult and shifts, how > granular can these adjustments be?
Yes. 128bit would be great for this. 64bit is fine though as far as I can see and allows granularity up to fractions of nanoseconds if applied between 1ms intervals.
> Also additional complications arise when we have multiple things (like > cpufreq) playing with the timesource frequency values as well.
I think these could all be taken into account by a scaling factor off a certain base established at a tick-like event that does the ntp scaling. The scaling between tick-like event needs to be just a scaling factor for performance reasons.
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