PMG and PFASST

Scientific area

Time-parallel methods, space-time multilevel algorithms, parallel multigrid.

Short description

The PMG+PFASST code provides a space-time parallel solver for systems of ODEs with linear stiff terms, stemming e.g. from method of lines discretizations of PDEs.

The "parallel full approximation scheme in space and time" (PFASST) joins Parareal-like iterations for time-parallel integration with multilevel spectral deferred correction sweeps in a space-time multigrid fashion. With innovative coarsening strategies in space and time, parallel efficiency can be significantly increased compared to classical Parareal.

For the solution of linear systems in space, which arise from implicit/stiff parts of the ODE, the parallel multigrid methods PMG is applied. The PFASST library primarily uses MPI for communication in time, an experimental on-node implementation with Pthreads is available as well. PMG, on the other hand, relies on MPI for the spatial distribution of degrees of freedom, but first tests with additional OpenMP pragmas show promising results. With the coupling of PFASST and PMG for the benchmark problem, the already impressive strong-scaling capabilities of the spatial solver alone can be increased by a factor of three on the full IBM Blue Gene/Q installation.

2D shear layer instability at three different time-steps (horizontal-axis) with three-level coarsening in space and time (vertical-axis) using reduction of temporal nodes, reduction of degrees of freedom in space and reduced discretization order.

Scalability

• 458,752 cores on BlueGene/Q (JUQUEEN)
• 262,144 cores on BlueGene/P (JUGENE)
• 16,384 cores on Cray XE6

Space-time parallel scaling of PMG+PFASST on JUQUEEN. PMG alone already scales to the full machine using 511^3 degrees of freedom, while the application of PFASST as parallel time stepper gives an additional speedup of 3 on 448K cores.

Programming language and model

• Fortran 2003 and C
• MPI and MPI+Pthreads

Tested on platforms

• BlueGene/Q and /P
• Cray XE6
• x86

Application developers and contact