[ntp:questions] What prevents continuous time within an operating system ?

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Fri Jan 22 14:57:00 UTC 2016


On 2016-01-22 05:05, Charles Elliott wrote:
>> Affinity is best set to CPU (nprocs-1) to minimize cache thrashing, and
> ensure that time stamps >are consistent, as not all systems sync their
> clocks across all CPUs at startup, leading to skews >between time stamps
> from different CPUs.
>
> My bosses will not let me do this, citing:
>
> 1.  There is little if any evidence that limiting NTPD to one CPU is
> beneficial.
>
> 2.  NTPD's main loop experiences between 1 and 7 context switches a second.
> Two other threads run
>      sporadically, but apparently at no fixed frequency, one of them very
> often.  Two threads, on
>      Windows, just sit there.  One can see this with Process Explorer.  This
> implies two
>      conclusions:
>    A. Occasionally, NTPD needs more than one CPU simultaneously.
>    B. When NTPD requests a CPU, and another process at a lower priority owns
> NTPD's assigned CPU,
>       then NTPD has to wait one time slice for access.  It is called priority
> inversion and is the
>       bane of systems everywhere.
>
> In addition, it is not clear to me that limiting NTPD to one CPU is NTP
> official policy.
>
> Do you have any hard evidence that limiting NTPD to one CPU has any
> beneficial impact?

I have managed to keep a Windows system running stable for a couple of years now
with a Garmin 18xLVC and NMEA user mode PPS, as PPSAPI unsupported on PCI serial,
and approx stats:
		median	max
	offset	<1us	50us
	jitter	15us	30us
	drift	1ppm	1.01ppm
	wander	0.2ppb	0.4ppb.

Prior to that I kept the same system with only network sources within ~20us,
once poll stabilized at 1024s, but the ref clock bug hit fairly soon.
Power management and spread spectrum frequency fuzzing is switched off in the BIOS,
and the OS runs a performance power management policy.
I have so far resisted becoming a time nut with Rb and Cs clocks requiring use
of a soldering iron.

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada


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