[LEAPSECS] Lets get REAL about time.

[Gerard Ashton]

On 1/20/2012 2:23 PM, Rob Seaman wrote, in part:

> There is plenty of prior art to consult here on both the CS side (early Crays already minimized cable runs to mitigate light speed constraints, it is commonplace today) and on the physics side (e.g., Planck units*).  Workflow logistics and systems engineering are still needed to figure out the appropriate trade-offs for whatever purpose.

>

> There is unlikely to be one timekeeping methodology appropriate for all purposes, for instance this amazing video:

>

> 	http://web.mit.edu/newsoffice/2011/trillion-fps-camera-1213.html

>

> Rob

> --

>

> * The Planck length is 25 orders of magnitude smaller than twice the Bohr radius (i.e., the diameter of a hydrogen atom).  The Planck time is 5.391 x 10^-44 s, FWIW.


The time scale which a computer must keep track of while operating is 
different than the time scale which must
be considered while designing the computer. The former is more likely to 
be time-of-day or time-since-an-epoch
while the latter is more likely to be duration. I have been involved in 
the design of computers with a 500 ps clock
period but design calculations were carried out with a resolution of 
about 1 ps.

A gross example of being aware of the location of various parts of a 
computer system would be an auction web site where bids
are considered received when they arrive at any of the web site's 
servers. One solution would be to time-stamp the bids
at the first server they arrive at; an alternate approach would be to 
subtract the transmission delay from the first-received
server to the server that decides who won. It's difficult to predict 
what applications might require awareness of much
shorter transmission delays.

Gerard Ashton