/[MITgcm]/manual/s_phys_pkgs/text/exch2.tex
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19  The exch2 package is an extension to the original cubed sphere exchanges  The exch2 package is an extension to the original cubed sphere exchanges
20  to allow more flexible domain decomposition and parallelization.  Cube faces  to allow more flexible domain decomposition and parallelization.  Cube faces
21  (subdomain) may be divided into whatever number of tiles that divide evenly  (subdomains) may be divided into whatever number of tiles that divide evenly
22  into the grid point dimensions of the subdomain.  Furthermore, the individual  into the grid point dimensions of the subdomain.  Furthermore, the individual
23  tiles may be run on different processors in any combination, (tone this down  tiles may be run on separate processors in different combinations,
24  a bit), and whether exchanges between particular tiles occur between different  and whether exchanges between particular tiles occur between different
25  processors is decided at runtime.  processors is determined at runtime.
26    
27    The exchange parameters are declared in {\em W2\_EXCH2\_TOPOLOGY.h} and
28    assigned in {\em w2\_e2setup.F}, both in the
29    {\em pkg/exch2} directory.  The validity of the cube topology depends
30    on the {\em SIZE.h} file as detailed below.  Both files are generated by
31    Matlab scripts and
32    should not be edited.  The default files provided in the release set up
33    a cube sphere arrangement of six tiles, one per subdomain, each with 32x32 grid
34    points, running on a single processor.  
35    
36    \subsection{Key Variables}
37    
38    The descriptions of the variables are divided up into scalars,
39    one-dimensional arrays indexed to the tile number, and two-dimensional
40    arrays indexed to tile number and neighboring tile.  This division
41    actually reflects  the functionality of these variables, not just the
42    whim of some FORTRAN enthusiast.
43    
44    \subsubsection{Scalars}
45    
46    The number of tiles in a particular topology is set with the parameter
47    {\em NTILES}, and the maximum number of neighbors of any tiles by
48    {\em MAX\_NEIGHBOURS}.  These parameters are used for defining the size of
49    the various one and two dimensional arrays that store tile parameters
50    indexed to the tile number.
51    
52    The scalar parameters {\em exch2\_domain\_nxt} and
53    {\em exch2\_domain\_nyt} express the number of tiles in the x and y global
54    indices.  For example, the default setup of six tiles has
55    {\em exch2\_domain\_nxt=6} and {\em exch2\_domain\_nyt=1}.  A topology of
56    twenty-four square (in gridpoints) tiles, four (2x2) per subdomain, will
57    have {\em exch2\_domain\_nxt=12} and {\em exch2\_domain\_nyt=2}.  Note
58    that these parameters express the tile layout to allow global data files that
59    are tile-layout-neutral and have no bearing on the internal storage of the
60    arrays.  The tiles are internally stored in a range from {\em 1,bi} (in the
61    x axis) and y-axis variable {\em bj} is generally ignored within the package.
62    
63    \subsubsection{One-Dimensional Arrays}
64    
65    The following arrays are indexed to the tile number, and the indices are
66    omitted in their descriptions.
67    
68    The arrays {\em exch2\_tnx} and {\em exch2\_tny}
69    express the x and y dimensions of each tile.  At present for each tile
70    {\em exch2\_tnx = sNx}
71    and {\em exch2\_tny = sNy}, as assigned in {\em SIZE.h}.  Future releases of
72    MITgcm are to allow varying tile sizes.
73    
74    The location of the tiles' Cartesian origin within a subdomain are determined
75    by the arrays {\em exch2\_tbasex} and {\em exch2\_tbasey}.  These
76    
77    \subsubsection{Two-Dimensional Arrays}
78    
79    
80    //
81    
82    \begin{verbatim}
83    C      NTILES            :: Number of tiles in this topology
84    C      MAX_NEIGHBOURS    :: Maximum number of neighbours any tile has.
85    C      exch2_domain_nxt  :: Total domain length in tiles.
86    C      exch2_domain_nyt  :: Maximum domain height in tiles.
87    C      exch2_tnx         :: Size in X for each tile.                  
88    C      exch2_tny         :: Size in Y for each tile.                  
89    C      exch2_tbasex      :: Tile offset in X within its sub-domain (cube face)
90    C      exch2_tbasey      :: Tile offset in Y within its sub-domain (cube face)
91    C      exch2_tglobalxlo  :: Tile base X index within global index space.
92    C      exch2_tglobalylo  :: Tile base Y index within global index space.
93    C      exch2_isWedge     :: 0 if West not at domain edge, 1 if it is.  
94    C      exch2_isNedge     :: 0 if North not at domain edge, 1 if it is.  
95    C      exch2_isEedge     :: 0 if East not at domain edge, 1 if it is.  
96    C      exch2_isSedge     :: 0 if South not at domain edge, 1 if it is.  
97    C      exch2_myFace      :: Cube face number used for I/O.              
98    C      exch2_nNeighbours :: Tile neighbour entries count.              
99    C      exch2_tProc       :: Rank of process owning tile                
100    C                        :: (filled at run time).                      
101    C      exch2_neighbourId :: Tile number for each neighbour entry.        
102    C      exch2_opposingSend_record :: Record for entry in target tile send
103    C                                :: list that has this tile and face    
104    C                                :: as its target.                      
105    C      exch2_pi          :: X index row of target to source permutation
106    C                        :: matrix for each neighbour entry.            
107    C      exch2_pj          :: Y index row of target to source permutation
108    C                        :: matrix for each neighbour entry.            
109    C      exch2_oi          :: X index element of target to source
110    C                        :: offset vector for cell-centered quantities  
111    C                        :: of each neighbor entry.                    
112    C      exch2_oj          :: Y index element of target to source
113    C                        :: offset vector for cell-centered quantities  
114    C                        :: of each neighbor entry.                    
115    C      exch2_oi_f        :: X index element of target to source
116    C                        :: offset vector for face quantities          
117    C                        :: of each neighbor entry.                    
118    C      exch2_oj_f        :: Y index element of target to source
119    C                        :: offset vector for face quantities          
120    C                        :: of each neighbor entry.                    
121    \end{verbatim}
122    
123    
124    
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