[evla-sw-discuss] Delay models to station boards

[Barry Clark]

It seems to me that we need to do, in sequence,
1.  Reaffirm the long term direction we want to take.
2.  Decide how the short term implementation is to proceed.
3.  Establish shared use of the code repositories, especially for schema.
4.  Determine the details of the records to be passed.

This message applies to #1.

Do we want delay models to cross the VCI?  

They have very different properties from other messages.  In particular, 
the data rates are comparatively high.  As I mentioned in an earlier 
message on this exploder, the most difficult case called for in the EVLA 
specs is on-the-fly mapping at high frequency, which requires new delay 
models at least five times per second.  For 27 antennas and 4 station boards 
per antenna this runs to about 600 models per second.

Some timing tests I did a year ago indicated that the current mchost 
computer would not quite be able to achieve this, but was not too slow
by a very large factor.

By looking at the code, it seems to me that the computation in the executor
will go mainly to CALC and to conversion of data into ASCII messages.
The CALC part of the code requires three relatively complicated calculations
of earth orientation and then, for each antenna, three relatively simple
path calculations.  The ASCII conversions will be a message to the antenna
MIB for pointing and the XML transmission of the delay models.  I have no
idea of the relative split of computation between the arithmetic of CALC
and the ASCII conversions.  It would not surprise me if they were equal 
drains on the CPU.  Decoding the messages will be a comparable effort to
encoding them.  If indeed we are limited by how fast Executor can produce
these messages, and all messages go through the VCI, then the MCCC will 
have to spend essentially all of its CPU receiving these messages, decoding
them, splitting them out and sending them to the REST address of the station
boards.

Another difference between delay models and other messages to the correlator
is that it is not very serious if a message is lost.  Another one will be 
along in a few seconds anyway (and, in the middle of an observation, the
correlator should just coast almost unnoticeably if one is missing).

So an alternative to putting the messages through the VCI is to simply 
multicast them.  It would be easy to arrange that each antenna has its
own socket to broadcast over, so the receiver on the station board would
get messages only for itself and for the other three station boards of
the antenna.  It would be a relatively small job for it to decode and 
use these messages.