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

Andrew M.A. Cater amacater at galactic.demon.co.uk
Thu Jan 8 17:53:52 EST 2004


On Thu, Jan 08, 2004 at 02:05:57PM -0500, Robert G. Brown wrote:
> On Thu, 8 Jan 2004, Jim Lux wrote:
> 
> > > The second approach is hardware, which seems perfectly "legal" but which
> > > requires an additional investment and which pushes one a bit away from
> > > the "commodity" ideal and into the "hardware hacker" ideal which is not
> > > everybody's cup of tea.  GPS is (as was discussed) a good way to get a
> > > very accurate time base, with the caveat that GPS signals are very weak
> > > (my handheld GPS won't work indoors, or even inside a car, let alone in
> > > a basement server room underneat a few hundred tons of steel and
> > > concrete).
> > 
> > I think that one can probably do it with no mods to the motherboard or
> > outside hardware. For instance, the NTP servers can bring in the 1pps on one
> > of the serial port lines. And, "legacy free" notwithstanding, most
> > motherboards still have a parallel printer port.  Perhaps something along
> > the lines of PAPERS?
> 
> Again, one has to navigate the device driver for the peripheral and
> likely hack the kernel or suffer random delays.  If you're going this
> route one might as well just use the network and work harder, as either
> way you'll need the same techniques to overcome the various sources of
> random latency and refine an average to the desired resolution.
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.

> 
> > > GPS is getting to be VERY cheap -- almost all cell phones now apparently
> > > have built in GPS,
> > 
> > In the GPS mfrs dreams maybe!... Soon.  Motorola just released their latest
> > widget, a GPS receiver that's about 10x15 mm, and can do what's called
> > "assisted" fixes, where the cell site does a lot of the work in terms of
> > keeping track of satellite ephemerides, and so forth. (Greatly reducing the
> > processing, and power, in the phone).
> > It's all part of the E911 mandate which requires cellphone providers to give
> > locations within a few meters when someone makes a 911 call on their
> > cellphone.  (Such position information is also quite commercially valuable..
> > they could send you customized SMS text messages: "Looking for Starbucks? 2
> > blocks ahead on the left?")
> 
> I was pretty surprised, but then my Christmas present in 2002 was a
> cell-phone sized GPS that cost about $100, complete with nifty screen,
> built in maps, leedle controls -- and it is still mostly battery, as I
> take it the GPS signals from the various satellites are quite weak and
> require a lot of amplification.

Correlation of the satellite constellation is what takes the processing
power.
> 
> >  and for $45 or so I bought a GPS-sync'd clock from
> > > Wal-Mart (which won't work inside the house because GPS signals are so
> > > weak, sigh).  GPS thus seems like something that would actually be
> > > fairly tricky to get working inside a lot of server room environments
> > > unless an external antenna were used and piped down to the device, which
> > > would add to the expense and hassle and be moderately inefficient on a
> > > per-system basis.
> > 
> > That clock is probably not GPS, but uses the 60 kHz WWVB broadcast (at least
> > most of the widely advertised "atomic clocks" do that...)
At 60kHz, the clocks you get in the States have warnings on - not 
necessarily reliable on the Eastern seaboard because of co-channel 
interference from Rugby, UK.  Here in Europe, we also have/had DCF77
on approx 77kHz from Germany.

> 
> 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 ... :)
> 
> Frankly, it would probably work better for SETI as well.
> 
> Maybe even tabletop relativity experiments as well.
> 
> 
> > > While we are at it sailing around in the blue sky, can we also put an
> > > entropy generator on the add-on card?  Something that can generate
> > > random bits (in parallel) at oh, 32 GHz, buffer them internally, and
> > > deliver the entire buffer to memory via DMA on a single interrupt?  My
> > > systems get SO tired of generating pseudorands...:-)
> > 
> 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 :)
> Lots of effort is actually expended here in corporate america as random
> numbers are a key element of cryptography, and some devices are on the
> market.  They're just expensive and slow and in some cases not terribly
> high quality -- "unpredictable" but not necessarily "random".
> Unpredictable can be made random, sort of, sometimes -- but only via
> transformations that slow the generator down.  Right now I don't know of
> any process or device that can deliver uniform deviates as fast or
> faster (or of any better quality) than the faster/better pseudorandom
> generators, although there may be some under development or out there as
> the world keeps changing.
> 
>    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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