How to customize the OpenIFS build

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:

Optimized build ('opt') - this is uses higher optimization and will generate the faster running model. Note that we do not always recommend the very highest optimization levels as this can sometimes cause the model to fail. The optimizations options are chosen conservatively to ensure the model will run successfully on a wide range of hardware and compiler versions. OpenMP is enabled in this configuration only. The user is free to increase optimization levels once the model is running successfully and we welcome feedback on best choice of compiler options.

Non-optimized build ('noopt') - this is intended for development and debugging. Optimization is set to -O0 and OpenMP is disabled. The model will run significantly slower. We recommend that when compiling OpenIFS for the first time, this build configuration is used as it will compile faster and be more stable.

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.

Build cfg 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:

OIFS_ARCH  - This sets the architecture of the computer. By default, the FCM build configuration assumes x86_64 (i.e. Linux).

OIFS_COMP - This sets the choice of compiler. The default is 'gnu' which means the gfortran/gcc compilers will be used.

OIFS_BUILD - This sets the type of build. The default is 'noopt'.

These variables and their defaults are set in the fcmcfg/oifs.cfg file. e.g.

$OIFS_ARCH{?} = x86_64
$OIFS_COMP{?} = gnu
$OIFS_BUILD{?} = noopt

The {?} syntax (often seen in the configuration files) means that if an environment variable of the same name exists, that value will be used instead of the value assigned. This provides a way to change many of the settings in the configuration files. For example, using:

export OIFS_BUILD=opt

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.

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

export OIFS_ARCH=darwin
export OIFS_COMP=intel
export OIFS_BUILD=opt

FCM would then read the file fcmcfg/oifs.cfg which would expect to find a file called: darwin-intel-opt.cfg. This file would need to be created by the user.

Compiler options

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.

The following compilation variables in the configuration files can be overriden by environment variables:

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.
          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:
          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.

Recommendations

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

• Optimised
• Non-optimized
• Full debugging (nansC)

The compilation of OpenIFS can be changed is several ways:

1. Setting the OIFS_ARCH, OIFS_COMP and OIFS_BUILD environment variables.
   This changes the compilation to use one of the supplied FCM configuration files: <arch>-<comp>-<build>.cfg.
   
   
2. Set additional environment variables to override the settings in the supplied configuration files.
   e.g. changing OIFS_FFLAGS to alter the optimization level.
   
   
3. Creating new .cfg files and set OIFS_ARCH, OIFS_COMP & OIFS_BUILD accordingly. Often best when testing is finished and different compiler options have been chosen.

For temporary changes when testing or debugging, setting environment variables (2) to override the compiler options is perhaps the simplest.

Once testing is finished and permanent changes are required, or if unsupported compilers are required, then copy an existing .cfg file and create your own. Any names can be used. For instance, if you wanted to have 2 configurations for the Intel compiler, one with the Intel MPI, the other with OpenMPI, then create files: x86_64-intel_impi-opt.cfg and x86_64-intel_ompi-opt.cfg and use the environment variable OIFS_COMP to switch between them. e.g. OIFS_COMP=intel_ompi (it's up to the user whether they want to create .cfg files for each build type; 'opt', 'noopt', 'nansC').