# Getting started¶

## Library requirements¶

Arepo requires the following libraries for compilation:

Always needed:

1. mpi The Message Passing Interface, version 2.0 or higher. In practice Arepo is well-tested on the open source libraries MPICH <https://www.mpich.org> and OpenMPI <http://www.open-mpi.org>. For some machines, vendor supplied versions may exist and can also be used. Arepo relies exclusively on MPI for its parallelization.

2. gsl The GNU Scientific Library. This open-source library is available under <http://www.gnu.org/software/gsl>.

Needed in some cases:

1. fftw3 Fastest Fourier Transform in the West (<http://www.fftw.org>) This free software package for fast-fourier-transformations is used in the particle-mesh algorithm. Consequently, it is only needed if PMGRID is set in Config.sh.

2. hdf5 Hierarchical Data Format <https://support.hdfgroup.org/HDF5/doc/H5.intro.html> is a data format specification that is used for the input/output format 3 in Arepo, which is the de-facto standard I/O format (even though the old Gadget formats are supported as well). The library is needed if the HAVE_HDF5 flag is set in Config.sh.

3. hwloc : The hardware locality library is useful for allowing the code to probe the processor topology it is running on and enable a pinning of MPI threads to individual cores. This library is optional and only required if the IMPOSE_PINNING flag is set.

Arepo needs a working C compiler supporting at least the c11 standard, as well as GNU-Make and Python, which are used in the build process.

## Building the code¶

### Systype variable¶

The first thing after obtaining a local copy of the code is to set a SYSTYPE variable, either by typing:

export SYSYTPE="MySystype"


(assuming you run the bash-shell) or by copying the Template-Makefile.systype file:

cp Template-Makefile.systype Makefile.systype


and uncommenting the intended SYSTYPE variable in Makefile.systype. The SYSTYPE is a variable defining a specific machine. Each machine has its specific compiler settings and library paths defined in the Makefile. New SYSTYPE keywords can be added in the Makefile when needed.

### Config.sh – Compile time flags¶

The next step is to set the compile-time configuration of the code. This is done best by copying the Template-Config.sh file:

cp Template-Config.sh Config.sh


Note that the compile-time configuration file does not need to be called Config.sh, however, if it is not, this needs to be specified when calling the Makefile. For example, for the code examples this file is located in a different location, and they thus make use of this possibility.

The Config.sh file enables/disables possible modules of Arepo, such as, e.g. SELFGRAVITY, which switches on gravitational interactions between cells/particles. All flags not out-commented with a ‘#’ will be processed and cast into a #define MyModule in an automatically produced header file arepoconfig.h that is placed into the code build directory.

### Compiling the code¶

Arepo can be compiled with:

make


which will first execute a check on the compile time flags (see below) followed by the actual build. It is also possible to skip the check by typing:

make build


The compilation can be accelerated on multi-core machines by compiling files in parallel with an option like -j 8, which will use 8 threads for the compilation.

### Compile-time checks of flags¶

The normal make command will first execute a check using the python script check.py. This ensures that all flags used in the code via #if defined(MyFlag) or #ifdef MyFlag occur either in Template-Config.sh or defines_extra. The main reason for doing this is to prevent typos in the code such as #if defined(MyFlg), which are generally very hard to find because the code might compile normally, but the code followed by #if defined(MyFlg) just does not appear in the executable.

New flags that can be activated should be included to Template-Config.sh, while internally defined flags (such as header file guards) are listed in defines_extra in order to exempt them from the check.

## Starting the code¶

### param.txt – Runtime settings¶

After compiling Arepo, the executable Arepo should be present in the main directory. To start a simulation, Arepo needs be handed over the path to a file containing the runtime parameters, usually called param.txt. The settings that need to be set generally depend on the compile-time flags used. All possible parameters are listed under Parameterfile, including the compile-time flags they belong to. Starting Arepo with a parameter file that has too few or obsolete parameter settings will result in an output telling which parameters are required or still needed, respectively.

### Starting the simulation¶

A simulation is started with a command like:

mpirun -np 32 Arepo param.txt


which will start Arepo on 32 MPI ranks using the param.txt file for the run-time parameter settings. Note that on some systems, mpirun may instead be called mpiexec, or be another launch-command such as srun, with a slightly different syntax. We refer to the user guide of individual machines for such peculiarities.

## Interrupting a run¶

Arepo supports running a simulations in several consecutive installments, and resuming an already started simulation from so-called restart files (a mechanism sometimes called checkpointing), which are essentially memory dumps written to the output directory. The code is written in such a way that the results are completely independent of the number of restarts performed, which is a crucial aspect for debugging the code if necessary, as this ensures deterministic reproducability.

A (regular) code termination always results in the writing of such restart files, and is normally triggered when more than 85% of the specified maximum runtime has been reached, or when the final simulation time has been reached. It is important, in particular in larger simulations, that the code is able to complete the output of the restart file before, e.g. a job time-limit is reached. Therefore the specified maximum runtime should never exceed the runtime limits of the machine.

It is also possible to trigger a (regular) code interruption by introducing a file called stop in the output directory.:

touch stop


Furthermore, Arepo usually produces restart-files in regular, user specified time intervals to ensure runs can be resumed without a major loss of computation time even if a run was terminated abruptly by the operating system.

## Restarting a run¶

A simulation can be restarted in two ways in Arepo, which is indicated with the additional flags 1 or 2 in the execution call. The flag 1 is the restarting from a restart file and generally recommended if possible. This, however, has the restriction that a run needs to be resumed with exactly as many MPI ranks as it was originally started from. The restart-command then looks as follows:

mpirun -np 32 ./Arepo param.txt 1


Alternatively, a flag value of 2 signals that one wants to resume a simulation from a regular snapshot output. This is possible but discouraged because here the simulation results will not be binary identical to a run without this restart. In particular in runs where some fields (especially positions) are written in single precision, this can also have effects on the overall quality of the simulation. A command to restart Arepo from a snapshot looks like the following:

mpirun -np 32 ./Arepo param.txt 2 7


Here Arepo would read in the snapshot number 7 and resume the simulation from its corresponding time. This second method is only recommened if the regular start from restart files is not possible for some reason.

## Starting post-processing tasks¶

There are a number of post-processing options in the public version of Arepo. The main ones are

• to run the halo and/or subhalo finder in post-processing (flag 3)

• to convert a snapshot to another format (flag 6),

• to write the Voronoi-mesh data (flag 14)

• to calculate and output the gradients of hydrodynamical quantities (flag 17)

• to calculate and output the gravitational potential (flag 18)

Note that the non-continuity of the numbers originates from the goal to maintain consistency with the developer version, which has more options. Other flags are not supported in the current public version of Arepo.