DisCoTec: Distributed Combination Technique Framework

What is DisCoTec?

This project contains DisCoTec, a code for running the distributed sparse grid combination technique with MPI parallelization. While it originates from the excellent SGpp project, all the parallelization makes it a very different code, such that it has become its own project.

DisCoTec is designed as a framework that can run multiple instances of a (black-box) grid-based PDE solver implementation. The most basic example we use is a mass-conserving FDM/FVM constant advection upwinding solver. An example of a separate, coupled solver is SeLaLib.

Sparse Grid Combination Technique with Time Stepping

The sparse grid combination technique (Griebel et al. 1992, Garcke 2013, Harding 2016) can be used to alleviate the curse of dimensionality encountered in high-dimensional simulations. Instead of using your PDE solver on a single structured full grid (where every dimension is finely resolved), you would use it on many different structured full grids (each of them differently resolved). We call these coarsely-resolved grids component grids. Taken together, all component grids form a sparse grid approximation, which can be explicitly obtained by a linear superposition of the individual grid functions, with the so-called combination coefficients.

In this two-dimensional combination scheme, all combination coefficients are 1 and -1, respectively. Figure originally published in (Pollinger 2024).

Between time steps, the grids exchange data through a multi-scale approach, which is summarized as the "combination" step in DisCoTec. Assuming a certain smoothness in the solution, this allows for a good approximation of the finely-resolved function, while achieving drastic reductions in compute and memory requirements.

Parallelism in DisCoTec

The DisCoTec framework can work with existing MPI parallelized PDE solver codes operating on structured grids. In addition to the parallelism provided by the PDE solver, it adds the combination technique's parallelism. This is achieved through process groups (pgs): MPI_COMM_WORLD is subdivided into equal-sized process groups (and optionally, a manager rank).

The image describes the two ways of scaling up: One can either increase the size or the number of process groups. Figure originally published in (Pollinger 2024).

Combining the two ways of scaling up, DisCoTec's scalability was demonstrated on several machines, with the experiments comprising up to 524288 cores:

We see the timings (in seconds) for the advection solver step and the combination step, respectively. This weak scaling experiment used four OpenMP threads per rank, and starts with one pg of four processes in the upper left corner. The largest parallelization is 64 pgs of 2048 processes each. Figure originally published in (Pollinger 2024).

Find a more detailed discussion in the docs.

There are only few codes that allow weak scaling up to this problem size: a size that uses most of the available main memory of the entire system.

When to Use DisCoTec?

If you are using a structured grid PDE solver and want to increase its accuracy while not spending additional compute or memory resources on it, DisCoTec may be a viable option. The codes most likely in this situation are the ones that solve high-dimensional problems and thus suffer the curse of dimensionality, such as the 4-6D discretizations occurring in plasma physics or computational quantum chemistry. But if you have a "normal" 2-3D problem and find yourself resource constrained, DisCoTec could be for you, too! Use its multiscale benefits without worrying about any multiscale yourself 😊

Why not try it with your own PDE solver?

What Numerical Advantage Can I Expect?

Depends on your problem! Figure 3.6 here shows a first-order accurate 2D PDE solver achieving approximately second-order accuracy with the Combination Technique considering the total number of DOF. (Figure omitted due to licensing, first published here.)

When Not to Use DisCoTec?

1. If memory and/or time constraints are not your limiting factor; you can easily achieve the numerical accuracy you need with your resources.
2. If your PDE solver just does not fit the discretization constraints imposed by DisCoTec:
   • a rectilinear (or mapped to rectilinear) domain
   • structured rectilinear grids in your main data structure (=typically the unknown function), stored as a linearized array
   • numbers of values per dimension that can be chosen as various powers of two, and where any power of two is a coarsened version of the discretization achieved with the next power of two ("nested discretization").
   • if distributed-memory parallelism is used, it must be MPI
   • currently, DisCoTec does not support Discontinuous Galerkin schemes, but it could be part of future versions (through Alpert multiwavelets). Let us know in case you are interested!

Installing

DisCoTec can be installed via spack, which handles all dependencies. We recommend the spack dev-build workflow:

Clone both spack and DisCoTec to find or build the dependencies and then compile DisCoTec:

```bash git clone git@github.com:spack/spack.git # use https if ssh is not set up on github ./spack/bin/spack external find # find already-installed packages ./spack/bin/spack compiler find # find compilers present on system ./spack/bin/spack info discotec@main # shows DisCoTec's variants

shows DisCoTec's dependency tree and which parts are already found

./spack/bin/spack spec discotec@main

git clone git@github.com:SGpp/DisCoTec.git cd DisCoTec ../spack/bin/spack dev-build -b install discotec@main ```

This will first build all dependencies, and then build DisCoTec inside the cloned folder. The executables are placed in the respective example and test folders.

To use DisCoTec in another CMake project, you can then add the line

CMake add_subdirectory(DisCoTec/src)

in your project's CMake files.

Here are Docs for CMake options and further Spack customization hints.

Read The Full Documentation

DisCoTec documentation here!

SeLaLib public source and SeLaLib documentation

For the current GENE documentation, you need to apply for access at genecode.org.