Making the MeshConvert utility and using it the help produce scalable mesh loading

What it's for

When you run Chaste on large clusters or supercomputers the part of the simulation that might scale least well is the initial mesh load.

• Traditional mesh files come in ASCII format with a variable amount of characters per line.
• Because random access into ASCII files is difficult each process must spool through unwanted lines
• This means each file is read in its entirety by all processes causing file contention

This page shows you how to convert from a given 3D mesh format (Tetgen/Memfem/CARP/VTK) to our a special binary version of the Triangle/Tetgen format.

• This allows for random access when running in parallel
• This also produces a "node connectivity list" (.ncl) file which is a map from node indices to containing element indices. The presence of this file cuts the calculation cost at mesh load time.

Making the executable(s)

It is assumed that as you are running on a parallel cluster or supercomputer you have access to the Chaste source code. You can produce all executables (including the standalone Chaste program) using:

scons chaste_libs=1 compile_only=1 exe=1 build=GccOpt apps

Binaries Chaste, MeshConvert... are made in apps/src.

You may get a speed improvement from using a "no debug" build-type like build=GccOpt_ndebug, but this is at the expense of doing less error checking.

Running the converter

You may want to download and save the attached file MeshConvert.tgz and unpack the files for testing purposes. Either use an Archive Manager to extract the content to disk or save it and then unpack it with

tar xvfz  MeshConvert.tgz

These files are just ASCII mesh data for the UCSD ventricular mesh. Run the converter like so:

./apps/src/MeshConvert MeshConvert/simple_cube

The argument MeshConvert/simple_cube should be the full relative path to the place where you unpacked the mesh files. Note that no suffix is given. If you have compiled with VTK enabled then you will be able to convert a single .vtu file into a binary-format file.

You will see the usual Chaste Copyright notice and information about the compilation details followed by:

...
Note: for VTK reading, give the full file path (including '.vtu' extension)
Opening simple_cube mesh file(s).
Writing  simple_cube_bin.node etc. mesh file in [PATH_TO_OUTPUT]
Done.

You can control the location of the folder for the output using $CHASTE_TEST_OUTPUT: The space labelled [PATH_TO_OUTPUT] in the above will be taken from the environmental variable $CHASTE_TEST_OUTPUT, if it is set, but will be ./testoutput otherwise.

ls ./testoutput
simple_cube_bin.ele  simple_cube_bin.face  simple_cube_bin.ncl  simple_cube_bin.node

Running in parallel

If you run the program in parallel then the original mesh is loaded (in a non-scalable way) and then partitioned before writing back out. This means that the original node ordering may be permuted. The advantage to pre-processing by running this converter in parallel (on n processes) is that mesh partitioning may be faster when the Chaste code is run live (also on n processes).