Using Go for NTPsec

[Achim Gratz]

Eric S. Raymond via devel writes:
> Talk to me about what you think the effect of very occasional
> stop-the-world pauses of 600 microseconds or less would be on sync
> accuracy. By "very occasionally" let's say once every ten minutes or
> so, that being what I think is a *very* pessimistic estimate of GC
> frequency for a program with NTP's memory-usage pattern.

It's hard to say what that will look like without having an actual
statistics.  The trouble with stalls is that they introduce bias since
they alway shift in the same direction.  Once every ten minutes would
likely not make much of a difference for most systems even if you could
not filter these events out.

> What I want to understand - and have others understand - is whether
> pauses of that size and frequency would mess with sync accuracy
> enough that heroic measures are required to avoid them.  What kind
> of distortion would they introduce in comparison with other
> components of the error budget?

NTP already filters out values that fall out of the ordinary statistic.
For the control loop I tend to think that eventually it would make sense
to drop the assumption of a regular interval at which the control
interventions happen.

> Mind you, heroic measures are available.  The simplest would be to
> run with GC off by default and schedule times to perform a GC when it
> can reasonably be expected not to collide with the next polling
> action.

I tend to organize timer triggered code in a way that any computations
that take a long or indeterminate time happen after the time-critical
section that uses either very short computations or precomputed values
from the last period.  That assumes that the resulting lag until the
next timer invocation can be taken into account (which should usually be
possible).  So if things get organized like that, as long as whatever GC
does is done until the next trigger would not matter at all.



Regards,
Achim.
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