I will discuss recent and ongoing work (mostly with J. Zou).

Speaker Tobias Weinzierl will be talking about: 'Modern tasking approaches to simulate black holes (and other interesting phenomena): How can we make them fit to modern hardware?'

Over the past decade, my team has developed a simulation code for binary black hole mergers that runs on dynamically adaptive Cartesian meshes. 
Its dynamic adaptivity, coupled with multiple numerical schemes operating at different scales and non-deterministic loads from puncture sources, makes task-based parallelisation a natural choice:
Task stealing across fine-grained work units balances the load across many CPU cores, while treating tasks as atomic compute units should---in theory---allow us to deploy seamlessly to accelerators.

In practice, it is far from straightforward.

Fine-grained tasks clash with accelerators, which thrive on large, homogeneous data access patterns;
task bursts on the CPU overwhelm tasking systems and produce suboptimal execution schedules;
and when tasks span address spaces, expensive memory movements kill performance.
Surprisingly, many mainstream tasking frameworks even lack the features our domain demands, i.e. to express key task concepts.

Our application serves as a powerful lens for examining these challenges. 
While our code base extends to other wave phenomena, Lagrangian techniques, and multigrid solvers, they all reveal the same fundamental tension: 
modern hardware increasingly struggles to accommodate modern HPC concepts, and it even challenges the notion that one solution fits all hardware components.

The talk proposes practical workarounds and solutions to these shortcomings, while all solutions are designed, wherever possible, to be upstreamed into mainstream software building blocks or at least decoupled from our particular PDE solver, making them broadly applicable to the community.

This talk is hosted by Rutherford Appleton Laboratory and will take place @ Harwell Campus, Didcot, OX11 0QX