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Compiling with FCM

The Flexible Configuration Management (FCM) software, from the UK Meteorological Office, is used to compile the model. FCM offers a number of advantages: configuration files are easy to understand, model object files are compiled 'out-of-source', source code changes can be 'out-of-source', fortran interface blocks can be generated automatically. FCM replaces the traditionally 'make' or 'gmake' command and is specifically written for compiling large fortran or mixed fortran/C codes like OpenIFS.

A quick introduction to FCM for OpenIFS users is available. A more detailed description of the 'fcm make' command is also available.

Build types

The supplied configuration files support 3 different build types:

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Full debugging ('nansC') - this configuration is intended for debugging only. As well as setting the lowest optimization, -O0, it also enables array bound checking and initialisation of variables with special values that will trigger 'not-a-number' exceptions useful for trapping variables used before initialized. This will cause the model to run very slowly and is normally only used for short runs for tracing bugs.

Config files and variables

The FCM software uses configuration files identified by the suffix '.cfg'. They can be found in the directory oifs/make/fcmcfg. The master configuration file for OpenIFS is in fcmcfg/oifs.cfg. It sets general configuration options for FCM, lets FCM know where the source code is and reads a single architecture and compiler specific configuration file. No compiler options are contained in the oifs.cfg file - these are always in the architecture specific configuration files.

The naming convention for these architecture specific files is: <architecture>-<compiler>-<build type>.cfg. For example, the file fcmcfg/x86_64-gnu-opt.cfg would be for Linux 64bit with the GNU compilers and the optimized build. The user can copy and rename the files provided or create their own using an existing one as a template.

How to change build type.

Before compiling the OpenIFS model, the build configuration must first be set by several environment variables:

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changes the build type to the 'optimized' build. This means instead the file 'fcmcfg/x86_64-gnu-opt.cfg' will be included by fcmcfg/oifs.cfg instead of the file fcmcfg/x86_64-gnu-noopt.cfg.

Create your own config files

Suppose we wanted to run the model on MacOS X with the intel compiler and a optimized setting. We could set:

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% cd oifs/make
% mkdir mycfg
% ln -s fcmcfg/oifs.cfg mycfg/oifs.cfg           #  the oifs.cfg must exist in the same directory as your custom .cfg file.
% cp fcmcfg/x86_64-intel-opt.cfg mycfg/darwin-intel_v13-opt.cfg
(edit darwin-intel_v13-opt.cfg)
% export OIFS_ARCH=darwin
% export OIFS_COMP=intel_v13
% export OIFS_BUILD=opt
% fcm make -f mycfg/oifs.cfg

Change compiler options globally

All the compiler flags and options are contained in the build specific files (e.g. x86_64-intel-opt.cfg). As above, a number of environment variables can be defined to override the default options set in these files. As a general rule, any variable with a {?} can be overriden by setting an environment variable.

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Examples

1. Set minimum required options.
   The only two compilation variables that need to be changed before building OpenIFS are the location of the grib_api installation and the location of the LAPACK and BLAS libraries.
   e.g.
   
   

   Code Block
   export OIFS_GRIB_API_DIR=/home/me/ecmwf/grib_api export OIFS_LAPACK_LIB="/opt/apps/lapack/current/LP64 -llapack -lblas" fcm make -f fcmcfg/oifs.cfg

   
   
   Would override the values of these variables in the FCM config files. In this example, FCM would use the x86_64-gnu-noopt.cfg build as this is the default specified in the oifs.cfg file (in the oifs/make/fcmcfg directory).
   
   

2. Changing compiler and compiler options.
   In this example, the build type is changed and the choice of underlying fortran compiler is altered along with some compiler options:

   Code Block
   export OIFS_COMP=gnu export OIFS_BUILD=opt export OIFS_GRIB_DIR=/opt/local/ecmwf/grib_api_xe6 export OIFS_LAPACK_LIB="-L/opt/apps/scal/lib -llapack -lblas" export OIFS_FC="mpif90 --fc=crayftn" export OIFS_FFLAGS="-g -O1 -m64 -convert big-endian"

   
   In this example, the compiler has been changed although FCM will still use the x86_64-gnu-opt.cfg configuration file. This might be because a user wants to temporarily try a different compiler. However, it's recommended that each compiler has its own set of .cfg files rather than alter it this way.
   
   Another example is where an optimized build is required but the optimization level is dropped to -O1 instead of -O2.

Changing options per file

The description above covers changing the compiler options globally, that is, for all files in the compilation. There may be instances when options for one or a few files need to be changed. For instance, in debugging where array bound checking is required for a handful of changed subroutines.

Changing options per file cannot be done using environment variables. There are several ways in which it can be done:

Edit the oifs.cfg file

The oifs.cfg file is the place to put all file specific options. Some can be found in this file already e.g.

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Using the 'include' statement

FCM configuration files support an 'include' statement. The oifs.cfg file uses this to include architecture and compiler specific information. If you need to include several lines changing the options for multiple files then another option would be to add an additional 'include' statement to point to a new file containing the build.prop lines you need. For example:

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Inherited build

A third way to customize at the file level is through FCM's 'inherited build' mechanism. This simple but powerful facility is described in more detail in the HowTo modify and add new code. In brief this allows you to leave the current configuration and source untouched and 'use' or 'inherit' the OpenIFS configuration in a completely separate directory. This makes it easier to update the OpenIFS code for new releases and keeps any changes you make completely separate.

The reader is referred to the separate HowTo modify and add new code for examples on how to use it.

Recommendations

As described above, the build environment uses 3 types of build:

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