Time and synchronization, was Re: [Beowulf] ...Re: Benchmark results

Robert G. Brown rgb at phy.duke.edu
Fri Jan 9 09:06:52 EST 2004


On Thu, 8 Jan 2004, Andrew M.A. Cater wrote:

> Maplin electronics did a kit - cheaply - to run off the time code from
> 60kHz clocks and put it into the serial/parallel port.  Know any
> radio hams - a lot are taking a temperature controlled crystal 
> oscillator and phase locking to WWWV or equivalent.

This is useful to know.  I'd really like to set up a local ntp server in
my house and it wouldn't hurt to have one in the department at Duke, but
not for hundreds of dollars in either case.  A 60 KHz signal really
sounds like one should be able to tune it and amplify it and deliver it
as a pulse accurate within an easy millisecond with a five dollars worth
of electronics (coil, cap, rectifier, amplifier and maybe a serial
interface chip).  I just don't have the time to do a lot of hobby
electronics -- I have a really nifty thermal sensor kit upstairs that
I've had for six months now without having had time to build it, for
example.

> > nsec syncronization on a global scale has lots of benefits, some of
> > which could enable projects that would dwarf SETI at home.  The heck with
> > turning every cellular tower in the country into a giant radiotelescope
> > array -- turn every suitably equipped PC in the country into a receiver
> > in a really enormous radiotelescope array.  Now there is a project for a
> > GRID!
> 
> Haven't you heard - the Govt. is using them to track individuals by 
> their individual radar fingerprint.  They can watch me wherever I walk 
> and they're using the system to mess with my mind ... :)

Time to take your medication, there, there, it will be all right...;-)

> > Unfortunately, a lot of "random" physical sources turn
> > out not be random at all, just to have a relatively short
> > autocorrelation function and to have distributions that are not uniform
> > as well. A lot of the noise-based generators fall into this category,
> > but quantum generators can as well (one has to look at second and higher
> > order correlation functions, for example -- even resonance fluorescence,
> > a textbook example of a "random quantum process" turns out to be photon
> > antibunched, and there is also the venerable work of Hanbury Brown and
> > Twiss.  It turns out not to be horribly easy to build a hardware
> > generator that passes diehard or the NIST suite, let alone one that
> > works fast.
> > 
> 
> The Knuth random number generator that converged to a value - the first
> example in Seminumerical Algorithms springs to mind :)

It is certainly worth remembering that in a deterministic universe (and
we DO live in a deterministic universe according to physics, regardless
of what one is tempted to conclude by looking at a quantum system that
is "inside" a generalized master equation and is analyzed with
non-relativistic quantum as well) there are no random number generators
or random events, period.  There is only entropy, which the way I
learned it is the log of the missing information.  Missing information
is an expression of our degree of ignorance of true state given a
limited/projective view of a large, multivariate space, not a statement
that the information is in any sense not there.

I do wish that more quantum classes taught the GME and quantum
statistics properly, I do I do...

Marsaglia apparently shares this view.  IIRC it is him that I'm
semiquoting when I say that hardware RNG's often aren't, at least
according to diehard.  Even with entropy generators and thermal noise,
the RNG has to be BOTH unpredictable AND uniform at the bit level to all
orders, and the latter is actually not terribly easy to accomplish
without a pure random binary process at hand.

I've written this nifty random number generator rating program (as
always, alas, functional but unfinished in terms of original design
goals) that when finished will encapsulate all the diehard tests and all
the NIST STS/FIPS tests in an shell that permits them to be
automagically applied to any GSL-wrapped RNG.  It also permits the RNGs
to be timed for benchmarking purposes (using the cpu_rate timer I
published on list a few days ago).  Leaving ppm effects aside :-) one
discovers that the linux /dev/random entropy generator is pretty good
but slow as molasses (not surprising).  The Mersenne twister rocks.  GSL
rocks -- there are over 50 RNG's pre-wrapped and ready to go, including
lots of the old/famous failures.  It's kind of fun to test some of the
original rand()'s and watch them fail all over the place...

   rgb

-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu



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