Introduction

This article describes how the compilation of the OpenIFS model is controlled and how to modify the compilation. It provides more detailed information than the page 5.5 Building OpenIFS. There are a number of ways in which the compilation can be changed

.

Various compiler are supported for OpenIFS and configuration files are supported for OpenIFS which are tested and known to work on systems we have access to . Specific optimisation options have generally been avoided because they often depend on specific compiler options and/or hardware. (see 5.1 Supported systems).

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 straightforward 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 traditional '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.

Configuration files and variables

The FCM software uses configuration files identified by the suffix '.cfg'. They can be found in the directory oifs/make/cfg. The master configuration file for OpenIFS is in cfg/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: <compiler>-<build type>.cfg. For example, the file cfg/gnu-opt.cfg would be for 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_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 cfg/oifs.cfg file. e.g.

$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 'cfg/gnu-opt.cfg' will be included by cfg/oifs.cfg instead of the file cfg/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:

export OIFS_COMP=osx_intel
export OIFS_BUILD=opt

FCM would then expect to find a file called: osx_intel-opt.cfg in the make/cfg directory. This file would need to be created by the user. We suggest copying one of the existing .cfg files as a template and modifying it to suit your purpose.

Your own .cfg files do not need to be kept in the make/cfg directory. You might like to create a directory, say, make/mycfg and store them in there. Use the -f option for the fcm make command to load the new files:

% cd oifs/make
% mkdir mycfg
% ln -s cfg/oifs.cfg mycfg/oifs.cfg                                 #  the oifs.cfg must exist in the same directory as your custom .cfg file.
% cp cfg/intel-opt.cfg mycfg/osx_intel_v13-opt.cfg
(edit osx_intel_v13-opt.cfg)
% export OIFS_COMP=osx_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. 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.
   
   

   Code Block
   title 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 cfg/oifs.cfg

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

build.prop{fc.flags}[algor/internal/fourier]       = $OIFS_FFIXED $OIFS_FFLAGS
build.prop{fc.flags}[algor/internal/lanczos/i2x.F] = $OIFS_FFIXED $OIFS_FFLAGS

More information on the configuration file syntax for FCM can be found on the FCM Users Guide from the UK Met Office.

The first line above sets the build property 'fc.flags' (ie. fortran compiler flags) for the directory algor/internal/fourier (under the 'src' directory). Note this sets compiler flags for all files in that directory.

The second line set the build property, compiler flags, for the specific file, algor/internal/lanczos/i2x.F. Note that all other files in this directory, unless changed elsewhere, will use the default compiler options as described above.

These statements are used to ensure F77 files are compiled correctly.

Example: you need to reduce the optimisation and include array bound checking on a specific file for testing. We suggest you make a copy of the oifs.cfg and add the line:

build.prop{fc.flags}[ifs/module/yomtrans.F90] = -g -O0 -m64 -C -fconvert=big-endian

and then compile using this new file:

fcm make -f cfg/oifs_copy.cfg

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:

include = $HERE/debug.cfg

and then create the file cfg/debug.cfg with the 'build.prop' lines you need.

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:

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

The compilation of OpenIFS can be changed is several ways:

1. Setting the OIFS_COMP and OIFS_BUILD environment variables.
   This changes the compilation to use one of the supplied FCM configuration files: <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_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: intel_impi-opt.cfg and 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').