Chunk 19.0

## Summary In this session, we implemented the **Phase 6 slotted partition pipeline** for the cuzk SNARK proving engine, completing the core architecture change described in `c2-optimization-proposal-6.md`. The implementation involved several coordinated changes across the codebase: refactoring C1 JSON deserialization out of `synthesize_porep_c2_partition()` into a shared `ParsedC1Output` struct to avoid redundant 51 MB parses per slot; adding a `ProofAssembler` struct for collecting per-slot proof bytes; implementing the main `prove_porep_c2_slotted()` function in `pipeline.rs` using `std::thread::scope` with a bounded `sync_channel(1)` to overlap synthesis and GPU proving; adding `slot_size` configuration to `PipelineConfig`; wiring the slotted pipeline into the engine's `process_batch` for PoRep C2 when `slot_size > 0`; and adding a `SlottedBench` subcommand to `cuzk-bench` with GPU utilization tracking. The benchmark results validated the design predictions: **slot_size=2** achieved **42.3s total time** (36.7s synth + 5.6s GPU) with **54 GiB peak RSS**, compared to the batch-all baseline (slot_size=10) at **63.4s total** with **228 GiB peak RSS** — a **1.50× speedup** and **4.2× memory reduction**. **slot_size=1** reached **39.1s** with only **27 GiB** memory, matching the design doc's predicted 38.9s. The GPU utilization was lower than expected (10–27%) because the single-proof benchmark doesn't reach steady-state overlap; the design doc predicted 80–88% for single-proof but 96% in steady-state with multiple proofs queued. A bug was identified in the overlap calculation (showing 1.00x instead of the actual synth/GPU overlap factor), and slot_size=1 was slower than expected due to rayon parallelism limits when synthesizing only one circuit at a time, suggesting slot_size=2 as the recommended default.

The Slotted Pipeline: How a 4.2× Memory Reduction Was Engineered in the cuzk SNARK Proving Engine 3336 words

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