1 |
|
2 |
# $Header: /u/gcmpack/MITgcm/pkg/ex3/README.txt,v 1.1 2005/10/14 05:49:42 edhill Exp $ |
3 |
# $Name: $ |
4 |
|
5 |
======= |
6 |
EX3 |
7 |
======= |
8 |
|
9 |
|
10 |
===== WARNING ===== |
11 |
|
12 |
THIS PACKAGE IS *** UNFINISHED *** AND CONTAINS SHARP, JAGGED BITS |
13 |
THAT ARE LIKELY TO LEAVE NASTY CUTS !!! |
14 |
|
15 |
HANDLE IT AT YOUR OWN RISK ! |
16 |
|
17 |
===== WARNING ===== |
18 |
|
19 |
|
20 |
This is the "ex3" package which stands for "EXchange v3" and it has |
21 |
been designed to do everything that exch2 does with the following |
22 |
advantages: |
23 |
|
24 |
0) EX3 works on more general topologies -- not just cubes! |
25 |
|
26 |
1) A simplified topology "encoding" is used that expresses how the |
27 |
tiles are connected to each other only in terms of cell indicies. |
28 |
It allows for all of the topologies within exch2 plus it has the |
29 |
following advantages: |
30 |
|
31 |
a) Only the cell indicies need to be input by a user -- all the |
32 |
"offsets" needed for exchanging quantities at cell corners, |
33 |
cell faces, etc. are calculated by the exchange routines |
34 |
themselves based upon the simpler cell indicies. |
35 |
|
36 |
b) Any number [n=0,1,2,3,4,...] of tiles may be adjacent at |
37 |
corner points with sufficient information supplied for the |
38 |
vorticity path integrals, |
39 |
|
40 |
c) The array-to-buffer and buffer-to-array copying have been |
41 |
almost completely separated from the logic and data structures |
42 |
which determine where the data sources/sinks are located. |
43 |
This should allow the "encoding" to change without altering |
44 |
the copying routines. |
45 |
|
46 |
d) Tiles with "internal" or "overlapping" connections are now |
47 |
easily represented. |
48 |
|
49 |
2) A more efficient exchange algorithm: |
50 |
|
51 |
a) For vector quantities and scalars defined at vorticity points, |
52 |
a simple "N+1" exchange is first performed to fill the values |
53 |
at the sNx+1 and sNy+1 points. A second pass then fills all |
54 |
the remaining points within the exchange regions. This |
55 |
correctly populates the values around the corners for any |
56 |
number of tiles meeting at a corner point. This is also in |
57 |
contrast to the three-pass algorithm required by exch2. |
58 |
|
59 |
b) Vectors are packed into a single buffer, reducing the number |
60 |
of MPI messages. |
61 |
|
62 |
|
63 |
The main disadvantage of the ex3 package is the removal of support for |
64 |
MDSIO "global" files. With the arrival of non-cube topologies, the |
65 |
cube-oriented MDSIO global file option is not practical since it is |
66 |
(1) not a dense encoding of the data and (2) will require a re-working |
67 |
of the existing MDSIO code. |
68 |
|