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SubjectRe: Extreme time jitter with suspend/resume cycles
On Wed, Oct 04, 2017 at 05:16:31PM -0700, John Stultz wrote:
> On Wed, Oct 4, 2017 at 9:11 AM, Gabriel Beddingfield <gabe@nestlabs.com> wrote:
> > We found that the problem is an interaction between the NTP code and
> > what I call the "delta_delta hack." (see [1] and [2]) This code
> > allocates a static variable in a function that contains an offset from
> > the system time to the persistent/rtc clock. It uses that time to
> > fudge the suspend timestamp so that on resume the sleep time will be
> > compensated. It's kind of a statistical hack that assumes things will
> > average out. It seems to have two main assumptions:
> >
> > 1. The persistent/rtc clock has only single-second precision
> > 2. The system does not frequently suspend/resume.
> > 3. If delta_delta is less than 2 seconds, these assumptions are "true"
> >
> > Because the delta_delta hack is trying to maintain an offset from the
> > system time to the persistent/rtc clock, any minor NTP corrections
> > that have occurred since the last suspend will be discarded. However,
> > the NTP subsystem isn't notified that this is happening -- and so it
> > causes some level of instability in its PLL logic.

This is interesting. What polling interval was ntpd using? If I
understand it correctly, with a high-resolution persistent clock the
delta-delta compensation should be very small and shouldn't disrupt
ntpd. Does this instability disappear when ntpd is not controlling the
clock (i.e. "disable ntp" in ntp.conf)?

> We should also figure out how to best handle ntpd in userspace dealing
> with frequent suspend/resume cycles. This is problematic, as the
> closest analogy is trying driving on the road while frequently falling
> asleep. This is not something I think ntpd handles well. I suspect
> our options are that any ntp adjustments being made might be made for
> far too long (causing potentially massive over-correction) or not at
> all, and not at all seems slightly better in my mind.

Yeah, controlling the clock in such conditions will be difficult. The
kernel/ntp PLL requires periodic updates. There is some code in
ntp_update_offset() that reduces the frequency adjustment when PLL
updates are missing, but I'm not actually sure if it works correctly
with suspend.

--
Miroslav Lichvar

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