# Quick Start

This chapter describes how to get up-and-running quickly with Flash-X with an example simulation, the Sedov explosion. We explain how to configure a problem, build it, run it, and examine the output using IDL.

## System requirements

You should verify that you have the following:

• A copy of the Flash-X source code distribution. To obtain a copy of the source code, please visit https://github.com/Flash-X and request access to the Flash-X repository as a collaborator.

• A FORTRAN compiler, a C compiler, and a C++ compiler, and Python 3 or above.

• An installed copy of the Message-Passing Interface (MPI) library, and the HDF5 library.

• To use AMReX for AMR, it must be installed. AMReX requires a different version each for 1, 2 or 3 dimensional application configurations. (for details see …..). Paramesh is included in the source code.

• AMReX is compatible with PETSc and Hypre math libraries, and Paramesh is compatible with Hypre. To use either of these libraries they must be installed on the platform and their location should be made known to AMReX .

• The GNU make utility

• A copy of the Python language, version 3.0 or later is required.

## Obtaining and configuring Flash-X for quick start

To begin, clone the Flash-X source code distribution.

The code of the PARAMESH library for Flash-X is provided as a git submodule by a separate code repository named PARAMESH-for-Flash-X. After successful initialization of the submodule the code will appear as a subtree under Grid/GridMain/AMR in the PM4_package directory. The Flash-X setup and build mechanisms will take care of compiling this code (if needed for a simulation configuration); PARAMESH code is not organized or built as a separate library.

#Git with Submodules

To prepare for building simulations that use libraries whose code must be retrieved from a separate git repository, the following modified git commands can be used:

• git pull –recurse-submodules=yes (in place of the usual git pull)

• git submodule update –init (additionally, after git pull)

• git submodule update –init source/Grid/GridMain/AMR/Paramesh4/PM4_package (variant of the previous item, in case you want to only get the Paramesh package submodule)

This quick start example uses Paramesh for AMR and does not depend on PETSc or Hypre. It also does not use accelerators or other devices on the node, neither does it exercise advanced features of the Orchestration System. It requires MPI and HDF5 installed, and runs using one CPU per MPI rank.

Once you have the source code and all the necessary libraries installed configure the Flash-X source tree for the Sedov explosion problem using the setup command Type

./setup Sedov -auto

This configures Flash-X for the 2d Sedov problem using the default hydrodynamic solver, equation of state, Grid unit, and I/O format defined for this problem, linking all necessary files into a new directory, called object. For the purpose of this example, we will use the default I/O format, serial HDF5. In order to compile a problem on a given machine Flash-X allows the user to create a file called Makefile.h which sets the paths to compilers and libraries specific to a given platform. This file is located in the directory sites/mymachine.myinstitution.mydomain/. The setup script will attempt to see if your machine/platform has a Makefile.h already created, and if so, this will be linked into the object directory. If one is not created the setup script will use a prototype Makefile.h with guesses as to the locations of libraries on your machine. The current distribution includes prototypes for *Linux, OSX operating systems. In any case, it is advisable to create a Makefile.h specific to your machine. See for details.

Type the command cd object to enter the object directory which was created when you setup the Sedov problem, and then execute make. This will compile the Flash-X code.

cd object
make


If you have problems and need to recompile, make clean will remove all object files from the object directory, leaving the source configuration intact; make realclean will remove all files and links from object. After ‘make realclean’, a new invocation of setup is required before the code can be built. The building can take a long time on some machines; doing a parallel build (make -j for example) can significantly increase compilation speed, even on single processor systems.

Assuming compilation and linking were successful, you should now find an executable named flashx in the object directory. You may wish to check that this is the case.

If compilation and linking were not successful, here are a few common suggestions to diagnose the problem:

• Make sure the correct compilers are in your path, and that they produce a valid executable.

• The default Sedov problem uses HDF5 in serial. Make sure you have HDF5 installed. If you do not have HDF5, you can still setup and compile Flash-X, but you will not be able to generate either a checkpoint or a plot file. You can setup Flash-X without I/O by typing

./setup Sedov -auto +noio

• Make sure the paths to the MPI and HDF libraries are correctly set in the Makefile.h in the object directory.

• Make sure your version of MPI creates a valid executable that can run in parallel.

Flash-X by default expects to find a text file named flash.par in the directory from which it is run. This file sets the values of various runtime parameters that determine the behavior of Flash-X. If it is not present, Flash-X will abort; flash.par must be created in order for the program to run (note: all of the distributed setups already come with a flash.par which is copied into the object directory at setup time). There is command-line option to use a different name for this file, described in the next section. Here we will create a simple flash.par that sets a few parameters and allows the rest to take on default values. With your text editor, edit the flash.par in the object directory so it looks like .

# runtime parameters

basenm = “sedov_

lrefine_max = 5 refine_var_1 = “dens” refine_var_2 = “pres”

restart = .false. checkpointFileIntervalTime = 0.01

nend = 10000 tmax = 0.05

gamma = 1.4

xl_boundary_type = “outflow” xr_boundary_type = “outflow”

yl_boundary_type = “outflow” yr_boundary_type = “outflow”

plot_var_1 = “dens” plot_var_2 = “temp” plot_var_3 = “pres”

sim_profFileName = “/dev/null”

This example first instructs Flash-X to name output files appropriately (through the basenm parameter). Flash-X is then instructed to use up to five levels of adaptive mesh refinement (AMR) (through the lrefine_max parameter), with the actual refinement adapted based on the density and pressure of the solution (parameters refine_var_1 and refine_var_2). We will not be starting from a checkpoint file (“restart = .false.” — this is the default, but here it is explicitly set for clarity). Output files are to be written every 0.01 time units (checkpointFileIntervalTime) and will be created until $$t=0.05$$ or 10000 timesteps have been taken (tmax and nend respectively), whichever comes first. The ratio of specific heats for the gas (gamma = $$\gamma$$) is taken to be 1.4, and all four boundaries of the two-dimensional grid have outflow (zero-gradient or Neumann) boundary conditions (set via the [xyz][lr]_boundary_type parameters).

Note the format of the file – each line is of the form variable = value, a comment (denoted by a hash mark, #), or a blank. String values are enclosed in double quotes (“). Boolean values are indicated in the FORTRAN style, .true. or .false.. Be sure to insert a carriage return after the last line of text. A full list of the parameters available for your current setup is contained in the file setup_params located in the object directory, which also includes brief comments for each parameter. If you wish to skip the creation of a flash.par, a complete example is provided in the source/Simulation/SimulationMain/Sedov/ directory.

## Running Flash-X

We are now ready to run Flash-X. To run Flash-X on N processors, type

mpirun -np N ./flashx

remembering to replace N with the appropriate values. Some systems may require you to start MPI programs with a different command; use whichever command is appropriate for your system. The Flash-X executable accepts an optional command-line argument for the runtime parameters file. If “-par_file filename” is present, Flash-X reads the file specified on command line for runtime parameters, otherwise it reads flash.par.

You should see a number of lines of output indicating that Flash-X is initializing the Sedov problem, listing the initial parameters, and giving the timestep chosen at each step. After the run is finished, you should find several files in the current directory:

• sedov.log echoes the runtime parameter settings and indicates the run time, the build time, and the build machine. During the run, a line is written for each timestep, along with any warning messages. If the run terminates normally, a performance summary is written to this file.

• sedov.dat contains a number of integral quantities as functions of time: total mass, total energy, total momentum, etc. This file can be used directly by plotting programs such as gnuplot; note that the first line begins with a hash (#) and is thus ignored by gnuplot.

• sedov_hdf5_chk_000* are the different checkpoint files. These are complete dumps of the entire simulation state at intervals of checkpointFileIntervalTime and are suitable for use in restarting the simulation.

• sedov_hdf5_plt_cnt_000* are plot files. In this example, these files contain density, temperature, and pressure in single precision. If needed, more variables can be dumped in the plotfiles by specifying them in flash.par. They are usually written more frequently than checkpoint files, since they are the primary output of Flash-X for analyzing the results of the simulation. They are also used for making simulation movies. Checkpoint files can also be used for analysis and sometimes it is necessary to use them since they have comprehensive information about the state of the simulation at a given time. However, in general, plotfiles are preferred since they have more frequent snapshots of the time evolution. Please see for more information about IO outputs.

Flash-X is intended to be customized by the user to work with interesting initial and boundary conditions. In the following sections, we will cover in more detail the algorithms and structure of Flash-X and the sample problems and tools distributed with it.