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The 2DECOMP&FFT library provides access to a slabs and pencil decompositions as well as FFTs.

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# 2DECOMP&FFT

[![DOI](https://joss.theoj.org/papers/10.21105/joss.05813/status.svg)](https://doi.org/10.21105/joss.05813)

This README contains basic instructions for building and installing the 2DECOMP&FFT library, more
detailed instructions about installation and linking to the library within an external project 
can be found in the [install section](INSTALL.md).
Please have a look at [HOWTO.md](HOWTO.md) and at the examples [examples](examples/README.md) for how to use the library with your application

## Building

The build system is driven by `CMake`. It is good practice to directly point to the MPI Fortran wrapper 
that you would like to use to guarantee consistency between Fortran compiler and MPI. 
This can be done by setting the default Fortran environmental variable 
```
$ export FC=my_mpif90
```
The build system can then be generated by running
```
$ cmake -S $path_to_sources -B $path_to_build_directory -DOPTION1 -DOPTION2 ...
```
for many users a configuration line
```
$ cmake -S . -B build
```
run from the 2DECOMP&FFT root directory will be sufficient.
If the build directory does not exist it will be generated and it will contain the configuration files.
By default a ``RELEASE`` build will built for CPU using MPI and the ``generic`` FFT backend included 
with 2DECOMP&FFT, please see [INSTALL.md](INSTALL.md) for instructions on changing the build, 
including debugging builds, building for GPUs and selecting external FFT libraries.

Once the build system has been configured, you can build 2DECOMP&FFT by running
```
$ cmake --build $path_to_build_directory -j <nproc>
```
appending `-v` will display additional information about the build, such as compiler flags.

After building the library can be tested. Please see section [Testing and examples](#testing-and-examples)

Finally, the build library can be installed by running 
```
$ cmake --install $path_to_build_directory
```
The default location for `libdecomp2d.a` is `$path_to_build_directory/opt/lib`or  `$path_to_build_directory/opt/lib64` unless the variable `CMAKE_INSTALL_PREFIX` is modified.
The module files generated by the build process will similarly be installed to `$path_to_build_directory/opt/install`, users of the library should add this to the include paths for their program.

Occasionally a clean build is required, this can be performed by running
```
$ cmake --build $path_to_build_directory --target clean
```

### GPU compilation

The library can perform multi GPU offoloading using the NVHPC compiler suite for NVIDIA hardware. 
The implementation is based on CUDA-aware MPI and NVIDIA Collective Communication Library (NCCL).
The FFT is based on cuFFT.

For details of how to configure 2DECOMP&FFT for GPU offload, see the GPU compilation section in
[INSTALL.md](INSTALL.md).

## Testing and examples

By default building of the tests is deactivated. 
To activate the testing the option `-DBUILD_TESTING=ON` can be added or 
alternativey the option can be activated in the GUI interface `ccmake`.
After building the library can be tested by running
```
ctest --test-dir $path_to_build_directory
```
which uses the `ctest` utility. By default tests are performed in serial, 
but more than 1 rank can be used by setting `MPIEXEC_MAX_NUMPROCS` under `ccmake` utility.
It is also possible to specify the decomposition by setting 
`PROW` and `PCOL` parameters at the configure stage or using `ccmake`. 
During the configure stage users should ensure that the number of MPI tasks `MPIEXEC_MAX_NUMPROCS` 
is equal to the product of PROW times PCOL. 
Mesh resolution can also be imposed using the parameters `NX`, `NY` and `NZ`.

For the GPU implementation please be aware that it is based on a single MPI rank per GPU. 
Therefore, to test multiple GPUs, use the maximum number of available GPUs 
on the system/node and not the maximum number of MPI tasks.

## Profiling

The 2DECOMP&FFT library has integrated profiling support via external libraries, see the Profiling
section of [INSTALL.md](INSTALL.md) for instructions on configuring a profiling build.
Currently, support for profiling is provided by the `caliper` library.
A generic profiler is also available.

One can tune the profiler before calling `decomp_2d_init` using the subroutine
`decomp_profiler_prep`. 
The input argument for this subroutine is a logical array of size 4. 
Each input allow activation / deactivation of the profiling as follows :

1. Profile transpose operations (default : true for caliper, false otherwise)
2. Profile IO operations (default : true for caliper, false otherwise)
3. Profile FFT operations (default : true for caliper, false otherwise)
4. Profile decomp_2d init / fin subroutines (default : true for caliper, false otherwise)

## FFT backends

The library provides a built-in FFT engine and supports various FFT backends : 
FFTW, Intel oneMKL, Nvidia cuFFT. 
The FFT engine selected during compilation is available through the variable `D2D_FFT_BACKEND` 
defined in the module `decomp_2d_fft`. The expected value is defined by the integer constants
```
integer, parameter, public :: D2D_FFT_BACKEND_GENERIC = 0   ! Built-in engine
integer, parameter, public :: D2D_FFT_BACKEND_FFTW3 = 1     ! FFTW
integer, parameter, public :: D2D_FFT_BACKEND_FFTW3_F03 = 2 ! FFTW (Fortran 2003)
integer, parameter, public :: D2D_FFT_BACKEND_MKL = 3       ! Intel oneMKL
integer, parameter, public :: D2D_FFT_BACKEND_CUFFT = 4     ! Nvidia cuFFT
```
exported by the module `decomp_2d_constants`.
The external code can use the named variables to check the FFT backend used in a given build.

### OVERWRITE flag

- The generic backend supports the OVERWRITE flag but it can not perform in-place transforms
- The FFTW3 and FFTW3_F03 backends support the OVERWRITE flag and can perform in-place complex 1D fft
- The oneMKL backend supports the OVERWRITE flag and can perform in-place complex 1D fft
- The cuFFT backend supports the OVERWRITE flag and can perform in-place complex 1D fft

## Miscellaneous

### Print the log to a file or to stdout

Before calling `decomp_2d_init`, the external code can modify the variable `decomp_log` 
to change the output for the log. The expected value is defined by the integer constants
```
integer, parameter, public :: D2D_LOG_QUIET = 0       ! No logging output
integer, parameter, public :: D2D_LOG_STDOUT = 1      ! Root rank logs output to stdout
integer, parameter, public :: D2D_LOG_TOFILE = 2      ! Root rank logs output to the file "decomp_2d_setup.log"
integer, parameter, public :: D2D_LOG_TOFILE_FULL = 3 ! All ranks log output to a dedicated file
```
exported by the `decomp_2d_constants` module.
Although their values are shown here, users should not rely on these and are recommended to prefer to use the named variables `D2D_LOG_QUIET`, etc. instead.
The default value used is `D2D_LOG_TOFILE` for the default build and `D2D_LOG_TOFILE_FULL` for a debug build.

### Change the debug level for debug builds

Before calling `decomp_2d_init`, the external code can modify the variable `decomp_debug` 
to change the debug level. The user can also modify this variable using the environment 
variable `DECOMP_2D_DEBUG`. Please note that the environment variable is processed only for debug builds.
The expected value for the variable `decomp_debug` is some integer between 0 and 6, bounds included.

### Code formatting

The code is formatted using the `fprettify` program (available via `pip`), 
to ensure consistency of use there is a script file `scripts/format.sh` 
which will run `fprettify` across the 2decomp&fft source, 
you can also use the `format` build target to run the script.
It is recommended that you should format the code before making a pull request.

### Versioning

The development of 2DECOMP&FFT occurs on Github, with release versions on the `main` branch.
New features will be implemented on the `dev` branch 
and merged into `main` once a new release
is ready.
For example, starting from `v2.0.0` the `main` branch will only be updated to receive fixes giving
`v2.0.1`, etc. until the next release (either `v2.1.0` or `v3.0.0` depending on the magnitude of the
change is ready).

### Contributing

If you would like to contribute to the development of the 2DECOMP&FFT library or report a bug please refer to 
the [Contributing section](Contribute.md)

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