ADR 0004: Execution layer for 0.2 — minimal homegrown, no Kokkos

Status

Accepted (2026-07-23). Supersedes the “ease later Kokkos adoption” rationale in kernel/execution/view.hpp.

Context

The audit (OPENPFC_ARCHITECTURE_AUDIT.md §6, §17.1) found two parallel execution abstractions:

  • a Kokkos-facsimile in include/openpfc/kernel/execution/ (View, memory/execution spaces, RangePolicy/MDRangePolicy, parallel_for, deep_copy, create_mirror), whose device parallel_for was a serial host loop (Pre-M0 PB turned that into a compile error) and which no application uses; and

  • the real device layer — DataBuffer<Tag,T>, hand-written CUDA/HIP kernels, device halo exchangers — which every GPU app actually uses.

The facsimile’s only stated benefit was making a future switch to Kokkos mechanical. That benefit is preserved by deleting the half-implementation rather than maintaining it, and the hard part (halo/MPI interplay) is homegrown either way.

Decision

  1. For the entire 0.2 series, OpenPFC keeps a minimal homegrown execution layer: DataBuffer<MemorySpace,T> as the single storage primitive plus a single-sourced device kernel layer (M3). Kokkos is not adopted in 0.2.

  2. The Kokkos-facsimile above DataBuffer (View, parallel_for, policies, create_mirror, layout/memory-trait scaffolding, and the vendor view_*/parallel_*/execution_space_* headers) is removed in M3. DataBuffer, the memory-space tags, and deep_copy on buffers survive.

  3. Kokkos adoption is deferred to a possible 0.3, to be reconsidered only if kernel diversity grows beyond stencil-shaped loops (reductions, scans, irregular/unstructured access) where a mature backend earns its dependency cost alongside HeFFTe.

Consequences

  • M3 (single-source GPU runtime) proceeds against DataBuffer + a vendor shim, not against a portability framework.

  • No new dependency is added; build/packaging surface stays as audited.

  • Device parallel_for/parallel_reduce/View are not part of the public 0.2 API; models use DataBuffer and the runtime kernels.

  • If 0.3 adopts Kokkos, DataBuffer is the seam to replace with Kokkos::View; this ADR is revisited then.