/[MITgcm]/manual/s_phys_pkgs/text/exch2.tex
ViewVC logotype

Diff of /manual/s_phys_pkgs/text/exch2.tex

Parent Directory Parent Directory | Revision Log Revision Log | View Revision Graph Revision Graph | View Patch Patch

revision 1.3 by afe, Thu Jan 29 15:39:49 2004 UTC revision 1.5 by afe, Thu Jan 29 21:03:53 2004 UTC
# Line 18  Line 18 
18    
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 of size {\em NTILES}, are indexed to the tile number,
66    and the indices are 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 variables
76    are used to relate the location of the edges of the tiles to each other.  As
77    an example, in the default six-tile topology (the degenerate case)
78    each index in these arrays are
79    set to 0.  The twenty-four, 32x32 cube face case discussed above will have
80    values of 0 or 16, depending on the quadrant the tile falls within the
81    subdomain.  {\em exch2\_myFace} contains the number of the
82    cubeface/subdomain of each tile, numbered 1-6 in the case of the standard
83    cube topology.  
84    
85    The arrays {\em exch2\_txglobalo} and {\em exch2\_txglobalo} are similar to
86    {\em exch2\_tbasex} and {\em exch2\_tbasey}, but locate the tiles within
87    the global address space, similar to that used by global files.  
88    
89    The arrays {\em exch2\_isWedge}, {\em exch2\_isEedge}, {\em exch2\_isSedge},
90    and {\em exch2\_isNedge} are set to 1 if the indexed tile lies on the edge
91    of a subdomain, 0 if not.  The values are used within the topology generator
92    to determine the orientation of neighboring tiles and to indicate whether
93    a tile lies on the corner of a subdomain.  The latter case indicates
94    special exchange and numerical handling for the singularities at the eight
95    corners of the cube.  {\em exch2\_isNedge} contains a count of how many
96    neighboring tiles each tile has, and is used for setting bounds for looping
97    over neighboring tiles.  {\em exch2\_tProc} holds the process rank of each tile,
98    and is used in interprocess communication.
99    
100    \subsubsection{Two-Dimensional Arrays}
101    
102    The following arrays are all of size {\em MAX\_NEIGHBOURS}x{\em NTILES} and
103    describe the orientations between the the tiles.
104    
105    The array {\em exch2\_neighbourId(a,T)} holds the tile number $T_{n}$ for each tile
106    {\em T}'s neighbor tile {\em a}, and {\em exch2\_opposingSend\_record(a,T)} holds
107    the index c in {\em exch2\_neighbourId(b,$T_{n}$)} that holds the tile number T.
108    In other words,
109    
110    \begin{verbatim}  
111    exch2_neighbourId( exch2_opposingSend_record(a,T), exch2_neighbourId(a,T) ) = T
112    \end{verbatim}
113    
114    {\em exch2\_neighbourId(exch2\_opposingSend\_record(a,T),exch2\_neighbourId(a,T))=T}.
115    This is to provide a backreference from the neighbor tiles.
116    
117    
118    //
119    
120    \begin{verbatim}
121    
122    
123    C      exch2_neighbourId :: Tile number for each neighbour entry.        
124    C      exch2_opposingSend_record :: Record for entry in target tile send
125    C                                :: list that has this tile and face    
126    C                                :: as its target.                      
127    C      exch2_pi          :: X index row of target to source permutation
128    C                        :: matrix for each neighbour entry.            
129    C      exch2_pj          :: Y index row of target to source permutation
130    C                        :: matrix for each neighbour entry.            
131    C      exch2_oi          :: X index element of target to source
132    C                        :: offset vector for cell-centered quantities  
133    C                        :: of each neighbor entry.                    
134    C      exch2_oj          :: Y index element of target to source
135    C                        :: offset vector for cell-centered quantities  
136    C                        :: of each neighbor entry.                    
137    C      exch2_oi_f        :: X index element of target to source
138    C                        :: offset vector for face quantities          
139    C                        :: of each neighbor entry.                    
140    C      exch2_oj_f        :: Y index element of target to source
141    C                        :: offset vector for face quantities          
142    C                        :: of each neighbor entry.                    
143    \end{verbatim}
144    
145    
146    
Legend:
Removed from v.1.3  
changed lines
  Added in v.1.5
  ViewVC Help
Powered by ViewVC 1.1.22