Re: [chrony-users] makestep in Chrony

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Fri, May 11, 2018 at 12:30:30AM +0000, Hei Chan wrote:
>  Hi Bill,
> Sorry that I wasn't clear.
> What I tried to do is to call clock_gettime() and rdtsc(p) as soon as chrony finishes synch so that I can get the best estimate when I try to derive time from (invariant) tsc.

Ok, so the assumption here is that once the system clock is
"synchronized" by chronyd there will be a linear function between the
tsc and system time? And the goal is to have a clock that can be read
in constant time and it doesn't have to be very accurate, but still
track the real time?

I'm not sure if that's possible. The tsc is the direct source for the
CLOCK_MONOTONIC_RAW clock. Its frequency doesn't change with chronyd's
adjustments, i.e. it's sensitive to temperature changes etc. The
constants of the linear function would have to be periodically updated
and then you would need to deal with locking, which would increase the
maximum latency in the reading of the clock.

> Ideally, I have a C application that calls chrony's API (if there is one) similar to "chronyd -q" to block till it finishes or gets a callback.
> Any suggestion?

There is no C API for chrony (yet). Instead, you could use adjtimex()
and check the frequency and maxerror fields. The maxerror value
increases slowly and drops only when chronyd updates the clock. When
it drops below a threshold and the frequency didn't change
significantly, the system clock could be considered to be
synchronized.

-- 
Miroslav Lichvar

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