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% $Header: /u/gcmpack/manual/s_phys_pkgs/text/exch2.tex,v 1.28 2010/08/27 13:15:37 jmc Exp $ |
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% $Name: $ |
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%% * Introduction |
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%% o what it does, citations (refs go into mitgcm_manual.bib, |
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%% preferably in alphabetic order) |
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%% o Equations |
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%% * Key subroutines and parameters |
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%% * Reference material (auto generated from Protex and structured comments) |
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%% o automatically inserted at \section{Reference} |
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|
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\subsection{exch2: Extended Cubed Sphere \mbox{Topology}} |
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\label{sec:exch2} |
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|
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|
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\subsubsection{Introduction} |
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|
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The \texttt{exch2} package extends the original cubed sphere topology |
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configuration to allow more flexible domain decomposition and |
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parallelization. Cube faces (also called subdomains) may be divided |
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into any number of tiles that divide evenly into the grid point |
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dimensions of the subdomain. Furthermore, the tiles can run on |
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separate processors individually or in groups, which provides for |
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manual compile-time load balancing across a relatively arbitrary |
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number of processors. |
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|
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The exchange parameters are declared in |
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\filelink{pkg/exch2/W2\_EXCH2\_TOPOLOGY.h}{pkg-exch2-W2_EXCH2_TOPOLOGY.h} |
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and assigned in |
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\filelink{pkg/exch2/w2\_e2setup.F}{pkg-exch2-w2_e2setup.F}. The |
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validity of the cube topology depends on the \file{SIZE.h} file as |
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detailed below. The default files provided in the release configure a |
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cubed sphere topology of six tiles, one per subdomain, each with |
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32$\times$32 grid points, with all tiles running on a single processor. Both |
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files are generated by Matlab scripts in |
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\file{utils/exch2/matlab-topology-generator}; see Section |
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\ref{sec:topogen} \sectiontitle{Generating Topology Files for exch2} |
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for details on creating alternate topologies. Pregenerated examples |
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of these files with alternate topologies are provided under |
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\file{utils/exch2/code-mods} along with the appropriate \file{SIZE.h} |
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file for single-processor execution. |
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|
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\subsubsection{Invoking exch2} |
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|
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To use exch2 with the cubed sphere, the following conditions must be |
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met: |
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|
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\begin{itemize} |
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\item The exch2 package is included when \file{genmake2} is run. The |
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easiest way to do this is to add the line \code{exch2} to the |
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\file{packages.conf} file -- see Section \ref{sec:buildingCode} |
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\sectiontitle{Building the code} for general |
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details. |
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|
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\item An example of \file{W2\_EXCH2\_TOPOLOGY.h} and |
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\file{w2\_e2setup.F} must reside in a directory containing files |
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symbolically linked by the \file{genmake2} script. The safest place |
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to put these is the directory indicated in the \code{-mods=DIR} |
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command line modifier (typically \file{../code}), or the build |
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directory. The default versions of these files reside in |
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\file{pkg/exch2} and are linked automatically if no other versions |
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exist elsewhere in the build path, but they should be left untouched |
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to avoid breaking configurations other than the one you intend to |
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modify. |
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|
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\item Files containing grid parameters, named \file{tile00$n$.mitgrid} |
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where $n$=\code{(1:6)} (one per subdomain), must be in the working |
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directory when the MITgcm executable is run. These files are |
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provided in the example experiments for cubed sphere configurations |
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with 32$\times$32 cube sides -- please contact MITgcm support if you |
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want to generate files for other configurations. |
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|
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\item As always when compiling MITgcm, the file \file{SIZE.h} must be |
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placed where \file{genmake2} will find it. In particular for exch2, |
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the domain decomposition specified in \file{SIZE.h} must correspond |
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with the particular configuration's topology specified in |
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\file{W2\_EXCH2\_TOPOLOGY.h} and \file{w2\_e2setup.F}. Domain |
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decomposition issues particular to exch2 are addressed in Section |
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\ref{sec:topogen} \sectiontitle{Generating Topology Files for exch2} |
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and \ref{sec:exch2mpi} \sectiontitle{exch2, SIZE.h, and |
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Multiprocessing}; a more general background on the subject |
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relevant to MITgcm is presented in Section |
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\ref{sec:specifying_a_decomposition} |
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\sectiontitle{Specifying a decomposition}. |
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\end{itemize} |
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|
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At the time of this writing the following examples use exch2 and may |
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be used for guidance: |
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|
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\begin{verbatim} |
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verification/adjust_nlfs.cs-32x32x1 |
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verification/adjustment.cs-32x32x1 |
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verification/aim.5l_cs |
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verification/global_ocean.cs32x15 |
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verification/hs94.cs-32x32x5 |
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\end{verbatim} |
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|
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|
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|
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|
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\subsubsection{Generating Topology Files for exch2} |
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\label{sec:topogen} |
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|
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Alternate cubed sphere topologies may be created using the Matlab |
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scripts in \file{utils/exch2/matlab-topology-generator}. Running the |
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m-file |
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\filelink{driver.m}{utils-exch2-matlab-topology-generator_driver.m} |
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from the Matlab prompt (there are no parameters to pass) generates |
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exch2 topology files \file{W2\_EXCH2\_TOPOLOGY.h} and |
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\file{w2\_e2setup.F} in the working directory and displays a figure of |
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the topology via Matlab -- figures \ref{fig:6tile}, \ref{fig:18tile}, |
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and \ref{fig:48tile} are examples of the generated diagrams. The other |
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m-files in the directory are |
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subroutines called from \file{driver.m} and should not be run ``bare'' except |
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for development purposes. \\ |
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|
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The parameters that determine the dimensions and topology of the |
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generated configuration are \code{nr}, \code{nb}, \code{ng}, |
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\code{tnx} and \code{tny}, and all are assigned early in the script. \\ |
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|
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The first three determine the height and width of the subdomains and |
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hence the size of the overall domain. Each one determines the number |
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of grid points, and therefore the resolution, along the subdomain |
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sides in a ``great circle'' around each the three spatial axes of the cube. At the time |
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of this writing MITgcm requires these three parameters to be equal, |
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but they provide for future releases to accomodate different |
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resolutions around the axes to allow subdomains with differing resolutions.\\ |
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|
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The parameters \code{tnx} and \code{tny} determine the width and height of |
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the tiles into which the subdomains are decomposed, and must evenly |
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divide the integer assigned to \code{nr}, \code{nb} and \code{ng}. |
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The result is a rectangular tiling of the subdomain. Figure |
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\ref{fig:48tile} shows one possible topology for a twenty-four-tile |
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cube, and figure \ref{fig:6tile} shows one for six tiles. \\ |
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|
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\begin{figure} |
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\begin{center} |
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\resizebox{6in}{!}{ |
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% \includegraphics{s_phys_pkgs/figs/s24t_16x16.ps} |
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\includegraphics{s_phys_pkgs/figs/adjust_cs.ps} |
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} |
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\end{center} |
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|
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\caption{Plot of a cubed sphere topology with a 32$\times$192 domain |
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divided into six 32$\times$32 subdomains, each of which is divided |
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into eight tiles of width \code{tnx=16} and height \code{tny=8} for a |
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total of forty-eight tiles. The colored borders of the subdomains |
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represent the parameters \code{nr} (red), \code{ng} (green), and |
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\code{nb} (blue). |
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This tiling is used in the example |
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verification/adjustment.cs-32x32x1/ |
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with the option (blanklist.txt) to remove the land-only 4 tiles |
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(11,12,13,14) which are filled in red on the plot. |
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} \label{fig:48tile} |
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\end{figure} |
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|
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\begin{figure} |
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\begin{center} |
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\resizebox{6in}{!}{ |
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% \includegraphics{s_phys_pkgs/figs/s12t_16x32.ps} |
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\includegraphics{s_phys_pkgs/figs/polarcap.ps} |
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} |
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\end{center} |
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\caption{Plot of a non-square cubed sphere topology with |
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6 subdomains of different size (nr=90,ng=360,nb=90), |
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divided into one to four tiles each |
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(\code{tnx=90, tny=90}), resulting in a total of 18 tiles. |
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} \label{fig:18tile} |
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\end{figure} |
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|
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\begin{figure} |
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\begin{center} |
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\resizebox{4in}{!}{ |
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% \includegraphics{s_phys_pkgs/figs/s6t_32x32.ps} |
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\includegraphics{s_phys_pkgs/figs/s6t_32x32.ps} |
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} |
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\end{center} |
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\caption{Plot of a cubed sphere topology with a 32$\times$192 domain |
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divided into six 32$\times$32 subdomains with one tile each |
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(\code{tnx=32, tny=32}). This is the default configuration. |
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} |
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\label{fig:6tile} |
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\end{figure} |
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|
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|
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Tiles can be selected from the topology to be omitted from being |
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allocated memory and processors. This tuning is useful in ocean |
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modeling for omitting tiles that fall entirely on land. The tiles |
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omitted are specified in the file |
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\filelink{blanklist.txt}{utils-exch2-matlab-topology-generator_blanklist.txt} |
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by their tile number in the topology, separated by a newline. \\ |
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|
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|
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|
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|
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\subsubsection{exch2, SIZE.h, and Multiprocessing} |
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\label{sec:exch2mpi} |
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|
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Once the topology configuration files are created, the Fortran |
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\code{PARAMETER}s in \file{SIZE.h} must be configured to match. |
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Section \ref{sec:specifying_a_decomposition} \sectiontitle{Specifying |
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a decomposition} provides a general description of domain |
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decomposition within MITgcm and its relation to \file{SIZE.h}. The |
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current section specifies constraints that the exch2 package imposes |
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and describes how to enable parallel execution with MPI. |
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|
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As in the general case, the parameters \varlink{sNx}{sNx} and |
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\varlink{sNy}{sNy} define the size of the individual tiles, and so |
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must be assigned the same respective values as \code{tnx} and |
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\code{tny} in \file{driver.m}. |
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|
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The halo width parameters \varlink{OLx}{OLx} and \varlink{OLy}{OLy} |
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have no special bearing on exch2 and may be assigned as in the general |
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case. The same holds for \varlink{Nr}{Nr}, the number of vertical |
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levels in the model. |
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|
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The parameters \varlink{nSx}{nSx}, \varlink{nSy}{nSy}, |
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\varlink{nPx}{nPx}, and \varlink{nPy}{nPy} relate to the number of |
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tiles and how they are distributed on processors. When using exch2, |
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the tiles are stored in the $x$ dimension, and so |
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\code{\varlink{nSy}{nSy}=1} in all cases. Since the tiles as |
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configured by exch2 cannot be split up accross processors without |
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regenerating the topology, \code{\varlink{nPy}{nPy}=1} as well. |
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|
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The number of tiles MITgcm allocates and how they are distributed |
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between processors depends on \varlink{nPx}{nPx} and |
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\varlink{nSx}{nSx}. \varlink{nSx}{nSx} is the number of tiles per |
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processor and \varlink{nPx}{nPx} is the number of processors. The |
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total number of tiles in the topology minus those listed in |
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\file{blanklist.txt} must equal \code{nSx*nPx}. Note that in order to |
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obtain maximum usage from a given number of processors in some cases, |
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this restriction might entail sharing a processor with a tile that |
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would otherwise be excluded because it is topographically outside of |
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the domain and therefore in \file{blanklist.txt}. For example, |
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suppose you have five processors and a domain decomposition of |
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thirty-six tiles that allows you to exclude seven tiles. To evenly |
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distribute the remaining twenty-nine tiles among five processors, you |
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would have to run one ``dummy'' tile to make an even six tiles per |
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processor. Such dummy tiles are \emph{not} listed in |
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\file{blanklist.txt}. |
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|
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The following is an example of \file{SIZE.h} for the six-tile |
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configuration illustrated in figure \ref{fig:6tile} |
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running on one processor: |
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|
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\begin{verbatim} |
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PARAMETER ( |
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& sNx = 32, |
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& sNy = 32, |
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& OLx = 2, |
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& OLy = 2, |
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& nSx = 6, |
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& nSy = 1, |
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& nPx = 1, |
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& nPy = 1, |
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& Nx = sNx*nSx*nPx, |
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& Ny = sNy*nSy*nPy, |
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& Nr = 5) |
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\end{verbatim} |
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|
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The following is an example for the forty-eight-tile topology in |
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figure \ref{fig:48tile} running on six processors: |
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|
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\begin{verbatim} |
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PARAMETER ( |
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& sNx = 16, |
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& sNy = 8, |
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& OLx = 2, |
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& OLy = 2, |
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& nSx = 8, |
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& nSy = 1, |
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& nPx = 6, |
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& nPy = 1, |
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& Nx = sNx*nSx*nPx, |
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& Ny = sNy*nSy*nPy, |
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& Nr = 5) |
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\end{verbatim} |
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|
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|
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\subsubsection{Key Variables} |
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|
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The descriptions of the variables are divided up into scalars, |
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one-dimensional arrays indexed to the tile number, and two and |
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three-dimensional arrays indexed to tile number and neighboring tile. |
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This division reflects the functionality of these variables: The |
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scalars are common to every part of the topology, the tile-indexed |
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arrays to individual tiles, and the arrays indexed by tile and |
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neighbor to relationships between tiles and their neighbors. \\ |
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|
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Scalars: |
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|
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The number of tiles in a particular topology is set with the parameter |
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\code{NTILES}, and the maximum number of neighbors of any tiles by |
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\code{MAX\_NEIGHBOURS}. These parameters are used for defining the |
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size of the various one and two dimensional arrays that store tile |
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parameters indexed to the tile number and are assigned in the files |
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generated by \file{driver.m}.\\ |
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|
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The scalar parameters \varlink{exch2\_domain\_nxt}{exch2_domain_nxt} |
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and \varlink{exch2\_domain\_nyt}{exch2_domain_nyt} express the number |
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of tiles in the $x$ and $y$ global indices. For example, the default |
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setup of six tiles (Fig. \ref{fig:6tile}) has |
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\code{exch2\_domain\_nxt=6} and \code{exch2\_domain\_nyt=1}. A |
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topology of forty-eight tiles, eight per subdomain (as in figure |
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\ref{fig:48tile}), will have \code{exch2\_domain\_nxt=12} and |
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\code{exch2\_domain\_nyt=4}. Note that these parameters express the |
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tile layout in order to allow global data files that are tile-layout-neutral. |
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They have no bearing on the internal storage of the arrays. The tiles |
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are stored internally in a range from \code{\varlink{bi}{bi}=(1:NTILES)} in the |
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$x$ axis, and the $y$ axis variable \varlink{bj}{bj} is assumed to |
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equal \code{1} throughout the package. \\ |
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|
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Arrays indexed to tile number: |
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|
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The following arrays are of length \code{NTILES} and are indexed to |
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the tile number, which is indicated in the diagrams with the notation |
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\textsf{t}$n$. The indices are omitted in the descriptions. \\ |
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|
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The arrays \varlink{exch2\_tnx}{exch2_tnx} and |
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\varlink{exch2\_tny}{exch2_tny} express the $x$ and $y$ dimensions of |
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each tile. At present for each tile \texttt{exch2\_tnx=sNx} and |
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\texttt{exch2\_tny=sNy}, as assigned in \file{SIZE.h} and described in |
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Section \ref{sec:exch2mpi} \sectiontitle{exch2, SIZE.h, and |
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Multiprocessing}. Future releases of MITgcm may allow varying tile |
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sizes. \\ |
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|
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The arrays \varlink{exch2\_tbasex}{exch2_tbasex} and |
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\varlink{exch2\_tbasey}{exch2_tbasey} determine the tiles' |
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Cartesian origin within a subdomain |
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and locate the edges of different tiles relative to each other. As |
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an example, in the default six-tile topology (Fig. \ref{fig:6tile}) |
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each index in these arrays is set to \code{0} since a tile occupies |
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its entire subdomain. The twenty-four-tile case discussed above will |
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have values of \code{0} or \code{16}, depending on the quadrant of the |
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tile within the subdomain. The elements of the arrays |
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\varlink{exch2\_txglobalo}{exch2_txglobalo} and |
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\varlink{exch2\_txglobalo}{exch2_txglobalo} are similar to |
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\varlink{exch2\_tbasex}{exch2_tbasex} and |
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\varlink{exch2\_tbasey}{exch2_tbasey}, but locate the tile edges within the |
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global address space, similar to that used by global output and input |
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files. \\ |
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|
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The array \varlink{exch2\_myFace}{exch2_myFace} contains the number of |
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the subdomain of each tile, in a range \code{(1:6)} in the case of the |
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standard cube topology and indicated by \textbf{\textsf{f}}$n$ in |
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figures \ref{fig:6tile} and |
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\ref{fig:48tile}. \varlink{exch2\_nNeighbours}{exch2_nNeighbours} |
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contains a count of the neighboring tiles each tile has, and sets |
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the bounds for looping over neighboring tiles. |
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\varlink{exch2\_tProc}{exch2_tProc} holds the process rank of each |
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tile, and is used in interprocess communication. \\ |
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|
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|
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The arrays \varlink{exch2\_isWedge}{exch2_isWedge}, |
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\varlink{exch2\_isEedge}{exch2_isEedge}, |
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\varlink{exch2\_isSedge}{exch2_isSedge}, and |
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\varlink{exch2\_isNedge}{exch2_isNedge} are set to \code{1} if the |
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indexed tile lies on the edge of its subdomain, \code{0} if |
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not. The values are used within the topology generator to determine |
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the orientation of neighboring tiles, and to indicate whether a tile |
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lies on the corner of a subdomain. The latter case requires special |
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exchange and numerical handling for the singularities at the eight |
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corners of the cube. \\ |
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|
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|
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Arrays Indexed to Tile Number and Neighbor: |
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|
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The following arrays have vectors of length \code{MAX\_NEIGHBOURS} and |
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\code{NTILES} and describe the orientations between the the tiles. \\ |
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|
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The array \code{exch2\_neighbourId(a,T)} holds the tile number |
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\code{Tn} for each of the tile number \code{T}'s neighboring tiles |
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\code{a}. The neighbor tiles are indexed |
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\code{(1:exch2\_nNeighbours(T))} in the order right to left on the |
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north then south edges, and then top to bottom on the east then west |
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edges. \\ |
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|
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The \code{exch2\_opposingSend\_record(a,T)} array holds the |
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index \code{b} of the element in \texttt{exch2\_neighbourId(b,Tn)} |
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that holds the tile number \code{T}, given |
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\code{Tn=exch2\_neighborId(a,T)}. In other words, |
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\begin{verbatim} |
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exch2_neighbourId( exch2_opposingSend_record(a,T), |
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exch2_neighbourId(a,T) ) = T |
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\end{verbatim} |
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This provides a back-reference from the neighbor tiles. \\ |
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|
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The arrays \varlink{exch2\_pi}{exch2_pi} and |
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\varlink{exch2\_pj}{exch2_pj} specify the transformations of indices |
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in exchanges between the neighboring tiles. These transformations are |
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necessary in exchanges between subdomains because a horizontal dimension |
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in one subdomain |
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may map to other horizonal dimension in an adjacent subdomain, and |
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may also have its indexing reversed. This swapping arises from the |
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``folding'' of two-dimensional arrays into a three-dimensional |
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cube. \\ |
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|
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The dimensions of \code{exch2\_pi(t,N,T)} and \code{exch2\_pj(t,N,T)} |
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are the neighbor ID \code{N} and the tile number \code{T} as explained |
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above, plus a vector of length \code{2} containing transformation |
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factors \code{t}. The first element of the transformation vector |
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holds the factor to multiply the index in the same dimension, and the |
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second element holds the the same for the orthogonal dimension. To |
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clarify, \code{exch2\_pi(1,N,T)} holds the mapping of the $x$ axis |
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index of tile \code{T} to the $x$ axis of tile \code{T}'s neighbor |
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\code{N}, and \code{exch2\_pi(2,N,T)} holds the mapping of \code{T}'s |
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$x$ index to the neighbor \code{N}'s $y$ index. \\ |
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|
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One of the two elements of \code{exch2\_pi} or \code{exch2\_pj} for a |
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given tile \code{T} and neighbor \code{N} will be \code{0}, reflecting |
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the fact that the two axes are orthogonal. The other element will be |
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\code{1} or \code{-1}, depending on whether the axes are indexed in |
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the same or opposite directions. For example, the transform vector of |
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the arrays for all tile neighbors on the same subdomain will be |
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\code{(1,0)}, since all tiles on the same subdomain are oriented |
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identically. An axis that corresponds to the orthogonal dimension |
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with the same index direction in a particular tile-neighbor |
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orientation will have \code{(0,1)}. Those with the opposite index |
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direction will have \code{(0,-1)} in order to reverse the ordering. \\ |
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|
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The arrays \varlink{exch2\_oi}{exch2_oi}, |
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\varlink{exch2\_oj}{exch2_oj}, \varlink{exch2\_oi\_f}{exch2_oi_f}, and |
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\varlink{exch2\_oj\_f}{exch2_oj_f} are indexed to tile number and |
425 |
neighbor and specify the relative offset within the subdomain of the |
426 |
array index of a variable going from a neighboring tile \code{N} to a |
427 |
local tile \code{T}. Consider \code{T=1} in the six-tile topology |
428 |
(Fig. \ref{fig:6tile}), where |
429 |
|
430 |
\begin{verbatim} |
431 |
exch2_oi(1,1)=33 |
432 |
exch2_oi(2,1)=0 |
433 |
exch2_oi(3,1)=32 |
434 |
exch2_oi(4,1)=-32 |
435 |
\end{verbatim} |
436 |
|
437 |
The simplest case is \code{exch2\_oi(2,1)}, the southern neighbor, |
438 |
which is \code{Tn=6}. The axes of \code{T} and \code{Tn} have the |
439 |
same orientation and their $x$ axes have the same origin, and so an |
440 |
exchange between the two requires no changes to the $x$ index. For |
441 |
the western neighbor (\code{Tn=5}), \code{code\_oi(3,1)=32} since the |
442 |
\code{x=0} vector on \code{T} corresponds to the \code{y=32} vector on |
443 |
\code{Tn}. The eastern edge of \code{T} shows the reverse case |
444 |
(\code{exch2\_oi(4,1)=-32)}), where \code{x=32} on \code{T} exchanges |
445 |
with \code{x=0} on \code{Tn=2}. \\ |
446 |
|
447 |
The most interesting case, where \code{exch2\_oi(1,1)=33} and |
448 |
\code{Tn=3}, involves a reversal of indices. As in every case, the |
449 |
offset \code{exch2\_oi} is added to the original $x$ index of \code{T} |
450 |
multiplied by the transformation factor \code{exch2\_pi(t,N,T)}. Here |
451 |
\code{exch2\_pi(1,1,1)=0} since the $x$ axis of \code{T} is orthogonal |
452 |
to the $x$ axis of \code{Tn}. \code{exch2\_pi(2,1,1)=-1} since the |
453 |
$x$ axis of \code{T} corresponds to the $y$ axis of \code{Tn}, but the |
454 |
index is reversed. The result is that the index of the northern edge |
455 |
of \code{T}, which runs \code{(1:32)}, is transformed to |
456 |
\code{(-1:-32)}. \code{exch2\_oi(1,1)} is then added to this range to |
457 |
get back \code{(32:1)} -- the index of the $y$ axis of \code{Tn} |
458 |
relative to \code{T}. This transformation may seem overly convoluted |
459 |
for the six-tile case, but it is necessary to provide a general |
460 |
solution for various topologies. \\ |
461 |
|
462 |
|
463 |
|
464 |
Finally, \varlink{exch2\_itlo\_c}{exch2_itlo_c}, |
465 |
\varlink{exch2\_ithi\_c}{exch2_ithi_c}, |
466 |
\varlink{exch2\_jtlo\_c}{exch2_jtlo_c} and |
467 |
\varlink{exch2\_jthi\_c}{exch2_jthi_c} hold the location and index |
468 |
bounds of the edge segment of the neighbor tile \code{N}'s subdomain |
469 |
that gets exchanged with the local tile \code{T}. To take the example |
470 |
of tile \code{T=2} in the forty-eight-tile topology |
471 |
(Fig. \ref{fig:48tile}): \\ |
472 |
|
473 |
\begin{verbatim} |
474 |
exch2_itlo_c(4,2)=17 |
475 |
exch2_ithi_c(4,2)=17 |
476 |
exch2_jtlo_c(4,2)=0 |
477 |
exch2_jthi_c(4,2)=33 |
478 |
\end{verbatim} |
479 |
|
480 |
Here \code{N=4}, indicating the western neighbor, which is |
481 |
\code{Tn=1}. \code{Tn} resides on the same subdomain as \code{T}, so |
482 |
the tiles have the same orientation and the same $x$ and $y$ axes. |
483 |
The $x$ axis is orthogonal to the western edge and the tile is 16 |
484 |
points wide, so \code{exch2\_itlo\_c} and \code{exch2\_ithi\_c} |
485 |
indicate the column beyond \code{Tn}'s eastern edge, in that tile's |
486 |
halo region. Since the border of the tiles extends through the entire |
487 |
height of the subdomain, the $y$ axis bounds \code{exch2\_jtlo\_c} to |
488 |
\code{exch2\_jthi\_c} cover the height of \code{(1:32)}, plus 1 in |
489 |
either direction to cover part of the halo. \\ |
490 |
|
491 |
For the north edge of the same tile \code{T=2} where \code{N=1} and |
492 |
the neighbor tile is \code{Tn=5}: |
493 |
|
494 |
\begin{verbatim} |
495 |
exch2_itlo_c(1,2)=0 |
496 |
exch2_ithi_c(1,2)=0 |
497 |
exch2_jtlo_c(1,2)=0 |
498 |
exch2_jthi_c(1,2)=17 |
499 |
\end{verbatim} |
500 |
|
501 |
\code{T}'s northern edge is parallel to the $x$ axis, but since |
502 |
\code{Tn}'s $y$ axis corresponds to \code{T}'s $x$ axis, \code{T}'s |
503 |
northern edge exchanges with \code{Tn}'s western edge. The western |
504 |
edge of the tiles corresponds to the lower bound of the $x$ axis, so |
505 |
\code{exch2\_itlo\_c} and \code{exch2\_ithi\_c} are \code{0}, in the |
506 |
western halo region of \code{Tn}. The range of |
507 |
\code{exch2\_jtlo\_c} and \code{exch2\_jthi\_c} correspond to the |
508 |
width of \code{T}'s northern edge, expanded by one into the halo. \\ |
509 |
|
510 |
|
511 |
\subsubsection{Key Routines} |
512 |
|
513 |
Most of the subroutines particular to exch2 handle the exchanges |
514 |
themselves and are of the same format as those described in |
515 |
\ref{sec:cube_sphere_communication} \sectiontitle{Cube sphere |
516 |
communication}. Like the original routines, they are written as |
517 |
templates which the local Makefile converts from \code{RX} into |
518 |
\code{RL} and \code{RS} forms. \\ |
519 |
|
520 |
The interfaces with the core model subroutines are |
521 |
\code{EXCH\_UV\_XY\_RX}, \code{EXCH\_UV\_XYZ\_RX} and |
522 |
\code{EXCH\_XY\_RX}. They override the standard exchange routines |
523 |
when \code{genmake2} is run with \code{exch2} option. They in turn |
524 |
call the local exch2 subroutines \code{EXCH2\_UV\_XY\_RX} and |
525 |
\code{EXCH2\_UV\_XYZ\_RX} for two and three-dimensional vector |
526 |
quantities, and \code{EXCH2\_XY\_RX} and \code{EXCH2\_XYZ\_RX} for two |
527 |
and three-dimensional scalar quantities. These subroutines set the |
528 |
dimensions of the area to be exchanged, call \code{EXCH2\_RX1\_CUBE} |
529 |
for scalars and \code{EXCH2\_RX2\_CUBE} for vectors, and then handle |
530 |
the singularities at the cube corners. \\ |
531 |
|
532 |
The separate scalar and vector forms of \code{EXCH2\_RX1\_CUBE} and |
533 |
\code{EXCH2\_RX2\_CUBE} reflect that the vector-handling subroutine |
534 |
needs to pass both the $u$ and $v$ components of the physical vectors. |
535 |
This swapping arises from the topological folding discussed above, where the |
536 |
$x$ and $y$ axes get swapped in some cases, and is not an |
537 |
issue with the scalar case. These subroutines call |
538 |
\code{EXCH2\_SEND\_RX1} and \code{EXCH2\_SEND\_RX2}, which do most of |
539 |
the work using the variables discussed above. \\ |
540 |
|
541 |
\subsubsection{Experiments and tutorials that use exch2} |
542 |
\label{sec:pkg:exch2:experiments} |
543 |
|
544 |
\begin{itemize} |
545 |
\item{Held Suarez tutorial, in tutorial\_held\_suarez\_cs verification directory, |
546 |
described in section \ref{sec:eg-hs} } |
547 |
\end{itemize} |