[Beowulf] ...Re: Benchmark results

Jim Lux James.P.Lux at jpl.nasa.gov
Thu Jan 8 19:48:16 EST 2004


At 04:48 PM 1/8/2004 -0500, Andrew Piskorski wrote:
>On Thu, Jan 08, 2004 at 11:34:26AM -0500, Robert G. Brown wrote:
> > On Thu, 8 Jan 2004, Josip Loncaric wrote:
>
> > > I love NTP, but in its current state it does not deliver microsecond
> > > level synchronization.
>
> > > NTP may need to be streamlined and tuned for clusters...
> >
> > I'm not sure that the problem is NTP itself -- from what I recall
> > reading in its documentation it is CAPABLE of achieving 1 usec
> > synchronization, over time.  However, in order to do that it needs
> > better tools than the current gettimeofday (and other time calls) which
> > seem to deliver no better than perhaps millisecond resolution.  It also
> > very likely needs to be done not by a userspace daemon but by the
> > kernel, as only the kernel can decide to do something in a true "single
>
>Would DJB's clockspeed, taiclock, etc. stuff be any better than ntp
>for that?  His website mentions using CPU hardware tick or nanosecond
>counters, but doesn't define "precise" when it says that taiclock is
>"... suitable for precise time synchronization over a local area
>network ...":
>
>   http://cr.yp.to/clockspeed.html
>   http://cr.yp.to/libtai.html


Nope.. from what I can see in the source code, DJB doesn't have a hardware 
"tick" interface.  I suppose, over a long enough time, the uncertainty 
introduced by grabbing the time from NTP, etc., would average out, but 
there's still a sync problem.





>--
>Andrew Piskorski <atp at piskorski.com>
>http://www.piskorski.com/
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James Lux, P.E.
Spacecraft Telecommunications Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875

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