microuptime() went backwards

Matthew Seaman m.seaman at infracaninophile.co.uk
Fri Apr 23 05:41:26 PDT 2004

On Fri, Apr 23, 2004 at 01:13:11PM +0100, Jez Hancock wrote:
> On Fri, Apr 23, 2004 at 09:04:56AM +0300, hugle wrote:
> > SOmetimes I see such messages in dmesg.
> > 
> > perl# dmesg
> > uptime() went backwards (1574174.333073 -> 1573478.944788)
> > 
> > what they mean? and what causes them to appear ?
> > is it good or bad?? :)
> I'd always presumed these messages occured on my machine because the
> ntpd (network time protocol daemon) had adjusted the system clock.  I
> can't actually tell you for sure since the messages aren't logged by
> syslog here so there's no easy way of comparing the times to see if they
> correspond to the ntpd adjustments.  
> Check to see if you have ntpd running - if so that's probably the reason
> for the messages.

Actually, that shouldn't happen because of ntpd(8).  If ntpd detects
that your system clock is fast, it will make it run slightly slower
until it gradually comes back into synch.  It shouldn't ever jump the
system clock to the right time during normal operation, neither should
it ever cause the system clock to run backwards.

Of course, there is an exception: right after boot, it's usual to run
ntpdate(8), and fairly common to run that with the '-b' flag so that
the time gets stepped straight to the correct value.  The ntpd
developers have marked ntpdate for eventual retirement and have rolled
its functionality into the main ntpd(8) -- so 'ntpq -q' is meant to be
functionally equivalent to ntpdate.  Even so, it's not clear to me
that the 'step the clock' mode of operation is available from 'ntpd

The OP's original query about 'microuptime went backwards' is
something that has come up fairly frequently on various mailing lists.
Googling for that message returns a few hundred hits.  There has been
quite a lot of effort to eradicate it, but apparently not with
complete success yet.  Most of the time it was apparently due to
problems with apm on certain hardware, but it could be caused by other
factors.  With the switch to APCI in 5.x there have been far fewer
reports of these errors appearing.

Usually this is pretty innocuous.  If you're only getting these
messages occasionally, then you can probably just ignore them.  On the
other hand, if you've suddenly started to get floods of these messages
for no apparent reason, it may possibly indicate that you have
hardware which is starting to get a bit marginal.  Keep the system
under observation, backup religiously and check the log messages for
clues regularly.



Dr Matthew J Seaman MA, D.Phil.                       26 The Paddocks
                                                      Savill Way
PGP: http://www.infracaninophile.co.uk/pgpkey         Marlow
Tel: +44 1628 476614                                  Bucks., SL7 1TH UK
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