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% $Name$ |
% $Name$ |
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\section{Using MITgcm Packages} |
\section{Using MITgcm Packages} |
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\label{sec:pkg:using} |
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\begin{rawhtml} |
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<!-- CMIREDIR:package_using: --> |
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\end{rawhtml} |
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The set of packages that will be used within a partiucular model can |
The set of packages that will be used within a partiucular model can |
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be configured using a combination of both ``compile--time'' and |
be configured using a combination of both ``compile--time'' and |
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For run--time package control, MITgcm uses flags set through a |
For run--time package control, MITgcm uses flags set through a |
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\texttt{data.pkg} file. While some packages (\textit{eg.} |
\texttt{data.pkg} file. While some packages (\textit{eg.} |
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\texttt{debug}, \texttt{mnc}, \texttt{cost}) may have their own |
\texttt{debug}, \texttt{mnc}, \texttt{exch2}) may have their own usage |
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conventions, most follow a simple flag naming convention of the |
conventions, most follow a simple flag naming convention of the form: |
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form: |
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\begin{verbatim} |
\begin{verbatim} |
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usePackageName=.TRUE. |
usePackageName=.TRUE. |
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\end{verbatim} |
\end{verbatim} |
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where the \texttt{usePackageName} variable can activate or disable the |
where the \texttt{usePackageName} variable can activate or disable the |
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package at runtime. |
package at runtime. As mentioned previously, packages must be |
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included in order to be activated. Generally, such mistakes will be |
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detected and reported as errors by the code. However, users should |
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still be aware of the dependency. |
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%\subsection{Modifying or Creating Packages} |
\subsection{Package Coding Standards} |
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The following sections describe how to modify and/or create new MITgcm |
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packages. |
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\subsubsection{Packages are Not Libraries} |
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To a beginner, the MITgcm packages may resemble libraries as used in |
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myriad software projects. While future versions are likely to |
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implement packages as libraries (perhaps using FORTRAN90/95 syntax) |
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the current packages (FORTRAN77) are \textbf{not} based upon any |
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concept of libraries. |
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\subsubsection{File Inclusion Rules} |
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Instead, packages should be viewed only as directories containing |
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``sets of source files'' that are built using some simple mechanisms |
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provided by \texttt{genmake2}. Conceptually, the build process adds |
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files as they are found and proceeds according to the following rules: |
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\begin{enumerate} |
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\item \texttt{genmake2} locates a ``core'' or main set of source files |
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(the \texttt{-standarddirs} option sets these locations and the |
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default value contains the directories \texttt{eesupp} and |
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\texttt{model}). |
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\item \texttt{genmake2} then finds additional source files by |
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inspecting the contents of each of the package directories: |
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\begin{enumerate} |
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\item As the new files are found, they are added to a list of source |
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files. |
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\item If there is a file name ``collision'' (that is, if one of the |
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files in a package has the same name as one of the files |
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previously encountered) then the file within the newer (more |
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recently visited) package will superseed (or ``hide'') any |
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previous file(s) with the same name. |
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\item Packages are visited (and thus files discovered) {\it in the |
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order that the packages are enabled} within \texttt{genmake2}. |
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Thus, the files in \texttt{PackB} may superseed the files in |
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\texttt{PackA} if \texttt{PackA} is enabled before \texttt{PackB}. |
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Thus, package ordering can be significant! For this reason, |
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\texttt{genmake2} honors the order in which packages are |
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specified. |
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\end{enumerate} |
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\end{enumerate} |
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These rules were adopted since they provide a relatively simple means |
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for rapidly including (or ``hiding'') existing files with modified |
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versions. |
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\subsubsection{Conditional Compilation and \texttt{PACKAGES\_CONFIG.h}} |
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Given that packages are simply groups of files that may be added or |
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removed to form a whole, one may wonder how linking (that is, FORTRAN |
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symbol resolution) is handled. This is the second way that |
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\texttt{genmake2} supports the concept of packages. Basically, |
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\texttt{genmake2} creates a \texttt{Makefile} that, in turn, is able |
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to create a file called \texttt{PACKAGES\_CONFIG.h} that contains a set |
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of C pre-processor (or ``CPP'') directives such as: |
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\begin{verbatim} |
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#undef ALLOW_KPP |
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#undef ALLOW_LAND |
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... |
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#define ALLOW_GENERIC_ADVDIFF |
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#define ALLOW_MDSIO |
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... |
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\end{verbatim} |
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These CPP symbols are then used throughout the code to conditionally |
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isolate variable definitions, function calls, or any other code that |
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depends upon the presence or absence of any particular package. |
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An example illustrating the use of these defines is: |
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\begin{verbatim} |
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#ifdef ALLOW_GMREDI |
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IF (useGMRedi) CALL GMREDI_CALC_DIFF( |
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I bi,bj,iMin,iMax,jMin,jMax,K, |
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I maskUp, |
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O KappaRT,KappaRS, |
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I myThid) |
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#endif |
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\end{verbatim} |
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which is included from the file |
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\filelink{calc\_diffusivity.F}{model-src-calc_diffusivity.F} |
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and shows how both the compile--time \texttt{ALLOW\_GMREDI} flag and the |
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run--time \texttt{useGMRedi} are nested. |
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There are some benefits to using the technique described here. The |
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first is that code snippets or subroutines associated with packages |
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can be placed or called from almost anywhere else within the code. |
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The second benefit is related to memory footprint and performance. |
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Since unused code can be removed, there is no performance penalty due |
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to unnecessary memory allocation, unused function calls, or extra |
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run-time \texttt{IF (...)} conditions. The major problems with this |
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approach are the potentially difficult-to-read and difficult-to-debug |
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code caused by an overuse of CPP statements. So while it can be done, |
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developers should exerecise some discipline and avoid unnecesarily |
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``smearing'' their package implementation details across numerous |
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files. |
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\subsubsection{Package Startup or Boot Sequence} |
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Calls to package routines within the core code timestepping loop can |
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vary. However, all packages should follow a required "boot" sequence |
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outlined here: |
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{\footnotesize |
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\begin{verbatim} |
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1. S/R PACKAGES_BOOT() |
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: |
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CALL OPEN_COPY_DATA_FILE( 'data.pkg', 'PACKAGES_BOOT', ... ) |
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2. S/R PACKAGES_READPARMS() |
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: |
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#ifdef ALLOW_${PKG} |
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if ( use${Pkg} ) |
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& CALL ${PKG}_READPARMS( retCode ) |
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#endif |
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3. S/R PACKAGES_INIT_FIXED() |
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: |
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#ifdef ALLOW_${PKG} |
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if ( use${Pkg} ) |
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& CALL ${PKG}_INIT_FIXED( retCode ) |
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#endif |
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4. S/R PACKAGES_CHECK() |
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: |
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#ifdef ALLOW_${PKG} |
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if ( use${Pkg} ) |
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& CALL ${PKG}_CHECK( retCode ) |
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#else |
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if ( use${Pkg} ) |
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& CALL PACKAGES_CHECK_ERROR('${PKG}') |
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#endif |
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5. S/R PACKAGES_INIT_VARIABLES() |
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: |
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#ifdef ALLOW_${PKG} |
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if ( use${Pkg} ) |
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& CALL ${PKG}_INIT_VARIA( ) |
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#endif |
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6. S/R DO_THE_MODEL_IO |
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#ifdef ALLOW_${PKG} |
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if ( use${Pkg} ) |
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& CALL ${PKG}_DIAGS( ) |
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#endif |
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7. S/R PACKAGES_WRITE_PICKUP() |
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#ifdef ALLOW_${PKG} |
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if ( use${Pkg} ) |
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& CALL ${PKG}_WRITE_PICKUP( ) |
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#endif\end{verbatim} |
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} |
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\subsubsection{Adding a package to PARAMS.h and packages\_boot()} |
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An MITgcm package directory contains all the code needed for that package apart |
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from one variable for each package. This variable is the {\tt use\$\{Pkg\} } |
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flag. This flag, which is of type logical, {\bf must} be declared in the |
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shared header file {\it PARAMS.h} in the {\it PARM\_PACKAGES} block. This |
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convention is used to support a single runtime control file {\it data.pkg} |
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which is read by the startup routine {\it packages\_boot()} and that sets a |
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flag controlling the runtime use of a package. This routine needs to be able to |
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read the flags for packages that were not built at compile time. Therefore |
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when adding a new package, in addition to creating the per-package directory |
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in the {\it pkg/} subdirectory a developer should add a {\tt use\$\{Pkg\} } |
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flag to {\it PARAMS.h} and a {\tt use\$\{Pkg\} } entry to the |
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{\it packages\_boot()} {\it PACKAGES} namelist. |
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The only other package specific code that should appear outside the individual |
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package directory are calls to the specific package API. |
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