0007. The chunk is the unit of dispatch

Context

A marked loop has an iteration space. The library could dispatch one task per iteration, or one task per contiguous range of iterations (a chunk). Per- iteration dispatch is the naive mapping and makes some analyses simpler, but it pays the dispatch, serialization, audit, and commit cost once per iteration. For a million-element loop that is a million times the overhead.

Every invariant Lucen holds constrains results, not the granularity of dispatch. So the granularity is free to be chosen for performance.

Decision

The chunk, a contiguous sub-range of the iteration space, is the single unit of dispatch, serialization, audit, and commit. Codegen emits one chunk function per block: a plain Python for loop over [start, stop). All Lucen overhead is paid once per chunk and amortizes toward zero per iteration.

The default chunk count is tuned per backend (more for threads, where dispatch is cheap; fewer for processes, where a dispatch costs an order of magnitude more), overridable with chunks=. A block must iterate a sized iterable, because chunking requires a known length; this restriction is load-bearing, not conservative.

Consequences

• A zero-clause proven-safe block compiles to a plain loop plus positional stores, with no per-iteration interception. This is the property the zero-interception test enforces.
• Correctness reasoning happens at chunk granularity: disjointness is audited between chunks, commit is ordered by chunk.
• Unsized iterables cannot be chunked and take the sequential fallback. A user who wants a generator parallelized materializes it to a list first.

Spec: technical specification sections 5.7.1, 5.16.