[ntp:questions] how do I lock in average frequency correction

unruh unruh at invalid.ca
Wed Feb 15 20:18:32 UTC 2012


On 2012-02-15, Dennis Ferguson <dennis.c.ferguson at gmail.com> wrote:
>
> On 14 Feb, 2012, at 22:04 , Garrett Wollman wrote:
>> All of these have issues, starting with the fact that they run at
>> inconvenient rates.
>
> For a clock implementation which works well in the presence of time
> synchronization software there are (i.e. must be) no inconvenient
> rates.  That is, even when the frequency written on the oscillator's
> package is a nice round number the actual frequency of the oscillator
> is going to be different and is unlikely to be "convenient" at all.
> Yet if you want the clock to keep accurate time you must be able to
> program the clock to operate at the oscillator's actual rate no matter
> what that is.
>
>> Modern operating systems are moving towards "tickless" operation to
>> improve power efficiency.  This requires an on-board timing device
>> that has sufficient range and stability to accurately determine the
>> amount of time that has elapsed between two non-periodic interrupts,
>> so that the clock can be advanced by the correct amount.
>
> I'm having trouble with this because it implies that the system's
> clock must be advanced in an interrupt service routine which samples
> a counter.  If the system provides a continuously incrementing counter


> then I think the last thing you want to do with it is to use it
> in a way which incorporates the jitter and other inaccuracies that

Anything based on the system will incorporate that jitter, whether timer
interrupts or cpu counters. You need something with an independent
constant tick rate-- rtc, hpet, ...-- to give the clock time. 
Thus on a tickless system, the timer interrupt, the cpu instruction
counter, etc are all terrible clocks. 

> come from interrupt processing into the system clock.  System clocks
> maintained by counting interrupts should be reserved for situations
> where there are is no other means of measuring the advance of time.
> If you have a continuously incrementing counter you are way better
> off just deriving the system clock directly from that (which I guess

It depends on whether or not that counter increments at a constant rate
in time. 


> makes the system clock "tickless" as well).  There is nothing you can
> do in an interrupt service routine which helps, in fact trying to do
> anything there usually just makes the result worse.

The primary purpose of the timer interrupt is scheduling, not keeping
time. The computer really does not care what rate it runs at.
soundcards do. video system might. 

>
> I have had very good results with the following procedure.  If `tc'
> is the 64 bit output of a counter which is continuously incremented
> at the rate of some oscillator then the time can be computed as follows:

It depends on whether or not that oscillator ticks at a consistant rate. 

>
>     t = r * (tc << s) + c
>
> The value of `s' is selected so that a second's worth of advance of
> tc causes a change to bit 32 of (tc << s); that is, `s' is a constant

It cannot. Because the clock rate is never a multiple of 2. 

But I understand what you are trying to do.


> pre-computed from the nominal frequency of tc's counter and an assumption
> about how far from the actual frequency of the oscillator that might
> be.  The multiply (r * (tc << s)), where `r' is a 64 bit value, returns
> the high order 64 bits of the 128 bit product.  In my case, `t' (and
> hence 'c') was picked to be the time of day computed in the NTP fixed
> point format, i.e. a 32 bit integer seconds part and a 32 bit fractional
> part, so given a tc from the counter the above converts it directly to
> a time of day.  The computation itself takes nanoseconds, or less, on
> a modern processor.
>
> Clearly the adjustment interface operates to produce values of `r' and
> `c' that minimize the error in the result.  Phase adjustments are implemented
> by adding an offset to 'c', and hence have a precision of 233 picoseconds.
> Rate (i.e. frequency) adjustments are implemented by modifying the value
> of 'r', with a corresponding change to 'c' to ensure phase continuity at
> the time of the change, and have a precision of better than 10^-19.  Slews,
> a la adjtime(), can be implemented by making a rate change for a fixed
> interval, at the end of which `r' is returned to its previous value with
> 'c' precisely offset.  The only time `r' and `c' change is when the time
> synchronization software makes a system call to adjust the clock; there
> is no need to do anything in an interrupt service routine (the latter
> assumes that the system call interface is not ntp_adjtime() or adjtimex();
> while that code can be supported as well there are good arguments for
> why you can expect better results, even from ntpd, if you don't include
> that code in the kernel, though that's a different topic).  If your TSC
> counter is reliable then making the sets of (s, r, c) constants available
> to user space on a shared page allows user space processes to obtain
> accurate time without system calls.  There are no "inconvenient" clock
> rates and the clock precision and jitter is the same as the precision
> and jitter of the underlying hardware counter.

Well, no. BEcause whatever drives that counter can have different rates
and jitter. 

>
> If you want a "tickless" kernel the best way to do that might be to
> start with a "tickless" system clock.  If you have a continuously
> incrementing counter there is nothing that an interrupt service
> routine needs to do.

But that counter must be driven by something whose rate is
(approximately to PPM level) constant. Not varying around. 

>
> Dennis Ferguson



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