On Fri, Sep 23, 2005 at 01:09:46AM +0200, Roman Zippel wrote:
> On Thu, 22 Sep 2005, Thomas Gleixner wrote:
[ . . . ]
> > > The main difference between them is that the latter is user
> > > programmable.
> >
> > wallclock is reprogrammable too and it introduces a bunch of horrible
> > functions in posix-timers.c. grep for abs_list. I explained why its
> > horrible already.
>
> I said _user_ programmable, wallclock time is usually NTP controlled.
I believe Thomas is concerned about workloads that need a short-term
stable timebase. For example, a process-control application might need
to accurately measure a (say) 1500-millisecond time interval. Both
user-programmability and NTP adjustments to a given timebase could
destroy the needed measurement accuracy.
Such a workload does not need the long-term tie to wallclock time that
NTP provides, but it does need the accurate short-term timekeeping that
NTP cannot provide -- NTP sacrifices short-term accuracy in order to
adjust the clock as needed to gain long-term stability.
Thomas, John, please jump in if I am missing the point here.
Thanx, Paul
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