GRTeclyn

GRTeclyn (previously referred to as GRAMReX) is a new numerical relativity code that is currently under development. It is a port of the GRChombo code (based on the Chombo libraries) to the AMReX library in order to leverage AMReX's GPU offload capabilities and ongoing active development.

The AMReX documentation can be found here.

The name follows a similar pattern to GRChombo, namely "GR" + "<Tool in a foreign language>". In this case, "Teclyn" is a Welsh word for "Tool".

Feature status

Currently only the BinaryBH example has been [partially] ported.

Please see the table below for a list of features/capabilities which have been ported/implemented for this example. Note that this should be correct at the time this table was created but may not be correct at the time you read this.

Feature	Ported/Implemented	Notes
CCZ4 Evolution	✔️	Only fourth order spatial derivatives
Boundary Conditions	✔️ ❔	All except mixed BCs have been implemented
Plot/checkpoint files + restart	✔️	AMReX-specific, not HDF5
AMR/tagging criterion	✔️ ❔	Very basic tagging without puncture tracking
Diagnostic variables	✔️ ❔	(e.g. Constraints, $\Psi_4$) Implemented but needs refactoring to remove hardcoding and allow generalisation
AMR Interpolator	❌
GW extraction	❌	Requires AMR Interpolator
Puncture tracking	❌	Requires AMR Interpolator
TwoPunctures initial data	❌

Obtaining and building the code

Prerequisites

You will need the following software

• Git
• GNU Make >= 3.81
• Python >= 2.7
• A Unix-like environment with perl and sed commands
• C++ compiler with C++17 support (e.g. GCC >= 8, Clang >= 6, Intel Classic >= 19.14)
• MPI implementation (optional)

Note that the C++17 requirement means that older compilers you have used to build [GR]Chombo may not work with GRTeclyn but so long as you have a more recent compiler available, if you can build GRChombo, you can probably build GRTeclyn too.

Obtaining the code

First cd into a directory you are happy to clone the code into. For simplicity, we shall assume that is your home directory (so adjust any commands below accordingly if not).

The AMReX source code is hosted on GitHub. Clone this with a command such as

git clone https://github.com/AMReX-Codes/amrex.git

It will be cloned to the amrex directory.

Next, clone the GRTeclyn repository using a command such as

git clone https://github.com/GRChombo/GRTeclyn.git

Note We have assumed that you have cloned both of these repositories to the same directory so that the amrex and GRTeclyn directories share the same parent directory. If you want to clone AMReX elsewhere, make sure to set the AMREX_HOME environment variable appropriately e.g.

export AMREX_HOME=/path/to/amrex

Building the BinaryBH example

Warning See this page if you want to build on macOS using Homebrew GCC.

If you are on a cluster make sure you have loaded modules which provide you with the software listed under prerequisites (e.g. you might need a compiler newer than the default).

Now, navigate to the BinaryBH example directory

cd ~/GRTeclyn/Examples/BinaryBH

The default build options are set in Make.defaults. They are very similar to Chombo's, and, like Chombo, can be overriden on the command line. If want to consistently override these, it is best to create a Make.local-pre file at

amrex/Tools/GNUMake/Make.local-pre

which sets the build configuration variables as you wish. You might want to change some of the following:

• Set USE_OMP = TRUE to use OpenMP or USE_CUDA = TRUE to use CUDA GPU offload support.
• If you don't want to use the GNU compiler g++ , change COMP = gnu to COMP = intel-llvm (for the Intel LLVM-based C++ compiler icpx) or COMP = llvm (for LLVM clang++).
• If you don't have an MPI implementation available, set USE_MPI = FALSE.

For more detailed build configuration information, consult the AMReX documentation

Now start building AMReX and the BinaryBH example. For example, to build with 4 processes, do

make -j 4

A new tmp_build_dir directory will be created under ~/GRTeclyn to store the compiled object and auxiliary files. Assuming all is well, you should have an executable in the current directory of the form main<config>.ex e.g. main3d.gnu.MPI.OMP.ex.

Running the code

You can run the example as for GRChombo by passing the parameter file as the first argument. For example, for launching with MPI, you might do something like

mpiexec -n 4 ./main3d.gnu.MPI.OMP.ex ./<params_file>.txt

Unit tests

There are some tests in the Tests directory that are automatically run using GitHub actions when new commits are pushed to this repository. Please consult the Tests README on how to build and run them.

Note that only some GRChombo tests have been ported.

License

GRTeclyn is licensed under the BSD 3-Clause License. Please see LICENSE for details.