On Mon, 2018-02-12 at 17:29 -0700, Alan Somers wrote:
> On Tue, Jan 23, 2018 at 8:40 AM, Alan Somers <asom...@freebsd.org> wrote:
> 
> > 
> > On Tue, Jan 23, 2018 at 3:48 AM, Mike Pumford <micha...@bsquare.com>
> > wrote:
> > 
> > > 
> > > On 22/01/2018 17:07, Alan Somers wrote:
> > > 
> > > > 
> > > > Since upgrading my jail server to 11.1-RELEASE, the clock occasionally
> > > > jumps backwards by 5-35 minutes for no apparent reason.  Has anybody 
> > > > seen
> > > > something like this?
> > > > 
> > > > Details
> > > > =====
> > > > 
> > > > * Happens about once a day on my jail server, and has happened at least
> > > > once on a separate bhyve server.
> > > > 
> > > > * The jumps almost always happen between 1 and 3 AM, but I've also seen
> > > > them happen at 06:30 and 20:15.
> > > > 
> > > > That's the window when the period scripts are run which if you have a
> > > default configuration and a lot of jails will put the system under a lot 
> > > of
> > > stress.
> > > 
> > That did not fail to escape my notice.  However, none of the jails'
> > periodic jobs involve the clock in any way.  And I wouldn't think that a
> > high CPU load could cause clock drift, could it?  This isn't Windows XP,
> > after all.
> > 
> > 
> > > 
> > > * I'm using the default ntp.conf file.
> > > > 
> > > > 
> > > > Are you running ntpd inside the jail or on the jail host? On my jail
> > > systems (which are 10.3 and 11.1) I run ntpd out the jail host (outside 
> > > all
> > > jails) and not inside the jails and the jails then get the accurate time 
> > > as
> > > the underlying host has accurate time.
> > > 
> > Only on the host.
> > 
> > New info: there is a possibility that my NFS server is hanging for
> > awhile.  That would explain my problem's timing.  However, ntpd shouldn't
> > be accessing any NFS shares, and I wouldn't think that a hung NFS server
> > should be able to pause the clock.  I'm doing a new experiment that should
> > be more informative.  But I'll have to wait until the problem recurs to
> > learn anything.
> > 
> I have a little more data now.  The problem happens much more frequently
> than I originally realized, but usually for just a few seconds at a time.
> It looks like the system is hanging for awhile and then recovering.  Or at
> least, the clocks are hanging.  The only other possibility would be for
> both the realtime _and_ monotonic clocks to jump backwards.  In any case,
> the problem is not ntpd's fault.  I don't know what could cause a system to
> hang for up to 30 minutes without crashing, and I'm not sure how to tell
> unless it happens during working hours.  I'll send another update if I
> learn more.
> 
> -Alan

Under the hood, CLOCK_REALTIME and CLOCK_MONOTONIC are the same
hardware; if one stops, both do.  CLOCK_REALTIME is simply offset
somewhat from MONOTONIC, and the offset can change (which is why
REALTIME isn't monotonic).

If you want to take a snapshot of some independently-running clocks,
you can use sysctl kern.timecounter.tc.<clockname>.counter, but that is
a snapshot of the raw hardware counter for the clock, which usually
rolls over pretty quickly (.mask tells you how many bits the counter
has, that and .frequency can be used to figure out how fast it rolls
over).

-- Ian

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