The Final Link: Updating the GPU Worker for Cross-Sector Batch Proving

"Now I need to update the GPU worker to handle batched results — splitting proof bytes back into per-sector groups and notifying multiple callers."

This single sentence, issued as an edit command to engine.rs in message [msg 677], represents the culminating moment of a complex architectural transformation. The assistant is connecting the last wire in Phase 3 of the cuzk pipelined SNARK proving engine — cross-sector batching. The message is deceptively short, but it sits at the apex of a multi-hour engineering effort involving reverse-engineering Filecoin's Groth16 proof pipeline, forking bellperson, designing a batch collector, and implementing multi-sector circuit synthesis. Understanding why this particular edit matters requires tracing the entire chain of reasoning that led to it.

The Cross-Sector Batching Architecture

Phase 3's goal was to improve throughput by batching multiple sectors' proof requests together. Instead of proving one sector at a time (synthesize → GPU prove → return result, then repeat), the engine would accumulate multiple same-type requests (PoRep, SnapDeals), synthesize all their circuits in a single combined pass, send the combined intermediate state to the GPU for a single proving operation, and then split the resulting proof bytes back into per-sector groups.

The architecture had been built piece by piece across the preceding messages:

1. [msg 668]: batch_collector.rs — a new module that accumulates same-circuit-type proof requests, flushing when max_batch_size is reached or max_batch_wait_ms expires. Non-batchable proof types (WinningPoSt, WindowPoSt) preempt-flush any pending batch and process immediately, ensuring no latency impact on priority-critical proofs.
2. [msg 670]: synthesize_porep_c2_multi() in pipeline.rs — takes N sectors' C1 outputs, builds all N×10 partition circuits, and performs a single combined synthesis pass via bellperson::synthesize_circuits_batch().
3. [msg 671]: split_batched_proofs() in pipeline.rs — a unit-tested function that separates concatenated proof bytes back into per-sector results using the known proof size for each circuit type.
4. [msg 674]: The SynthesizedJob type in engine.rs was extended with a batch_info field carrying the list of ProofRequests and their per-sector result channels, so the GPU worker could notify each caller individually.
5. [msg 675]: The synthesis task in engine.rs was rewritten to pull from the scheduler, feed into the batch collector, and dispatch batch or single-sector jobs to the GPU channel.
6. [msg 676]: The process_batch async function was added — the core synthesis processing that handles both single and multi-sector batches. But there was still a missing piece. The GPU worker — the async task that receives SynthesizedJobs from the synthesis channel, calls the GPU proving functions, and returns results to callers — still operated in single-sector mode. It would take a SynthesizedJob, prove it, and notify exactly one caller. With batching, a single SynthesizedJob could contain multiple sectors' worth of intermediate state, and the GPU would produce a single concatenated proof output. The worker needed to split that output and notify multiple callers, each with their own timing and result channel.

The Edit: What Changed

Message [msg 677] applies the edit that completes this chain. The GPU worker's proof-completion logic was modified to:

• Check whether the SynthesizedJob carries batch information (multiple ProofRequests with multiple result channels).
• If single-sector (no batch info), proceed as before: notify the single caller with the proof bytes.
• If multi-sector (batch info present), call split_batched_proofs() on the concatenated proof output, then iterate over each sector's proof bytes and corresponding result channel, sending each caller their individual proof with accurate per-sector timing information. This is the critical "demux" step that makes batching transparent to callers. From the perspective of a Curio worker submitting a single-sector proof request, the API is identical whether the engine processes it individually or as part of a batch. The batching happens entirely inside the engine, invisible to the client.

Assumptions and Design Decisions

Several assumptions underpin this edit, each carrying engineering risk:

Deterministic ordering: The edit assumes that the GPU produces proofs in exactly the same order as the sectors were submitted to synthesize_porep_c2_multi(). If the GPU reorders partitions internally, the split would assign wrong proofs to wrong sectors. This assumption is validated by the fact that synthesize_circuits_batch() in bellperson processes circuits in FIFO order, and the GPU proving functions (prove_from_assignments) preserve that order in the output.

Proof size constancy: The edit relies on split_batched_proofs() knowing the exact byte size of each proof type (PoRep, SnapDeals). If proof sizes vary (e.g., due to variable-length elements), the splitting logic would produce garbage. The assumption is that Groth16 proofs for a given circuit are fixed-size — a property of the protocol.

Independent caller notification: The edit assumes that notifying multiple callers can happen sequentially without blocking the GPU worker for excessive time. Each ProofResponse is sent via a tokio::sync::oneshot::Sender, which is a non-blocking operation. However, if a caller has dropped their receiver, the send fails silently — the edit must handle this gracefully without crashing the worker.

Timing accuracy: The edit must assign accurate per-sector timing. The GPU proves the entire batch in one operation, so the "GPU time" is shared. The edit likely assigns the same GPU duration to each sector or divides it proportionally. The synthesis time is already per-sector since synthesize_porep_c2_multi() can track individual sector synthesis within the combined pass.

Input Knowledge Required

To understand this message, one needs:

• Groth16 proof structure: Knowledge that a Groth16 proof is a fixed-size byte sequence (for BLS12-381, proofs are typically 192 bytes or similar), and that concatenating proofs is valid because they are self-delimiting by size.
• Bellperson internals: Understanding that synthesize_circuits_batch() produces a single combined SynthesizedProof containing all circuits' assignments, and that prove_from_assignments() produces a single concatenated proof output. This was discovered through extensive reverse-engineering in Phase 2 ([msg 661]).
• Tokio async patterns: The GPU worker runs in a tokio::task::spawn-ed async context, receiving jobs from a mpsc::Receiver and sending results via oneshot::Sender channels. The edit must correctly handle the batch-info variant without deadlocking or dropping notifications.
• The cuzk engine architecture: Understanding that Engine owns a Scheduler (which holds pending ProofRequests), a BatchCollector (which accumulates them), a synthesis channel (connecting synthesis task to GPU worker), and multiple GPU worker tasks (one per GPU, isolated by CUDA_VISIBLE_DEVICES).
• Proof type distinctions: PoRep and SnapDeals are batchable (same circuit type, multiple sectors). WinningPoSt and WindowPoSt are non-batchable (priority-critical, different circuit structure). The GPU worker must handle both paths.

Output Knowledge Created

This message produces:

• A complete cross-sector batching pipeline: The GPU worker now correctly handles batched SynthesizedJobs, splitting proofs and notifying multiple callers. This is the final integration point that makes Phase 3 functional end-to-end.
• Backward compatibility: Single-sector jobs (batch_size=1, or non-batchable types) continue to work identically to Phase 2. The batch-info check is a simple conditional that preserves the existing code path.
• A foundation for benchmarking: With the GPU worker handling batching, the E2E validation (documented in the chunk summary) becomes possible. The RTX 5070 Ti tests showing 1.46x throughput improvement with batch_size=2 rely on this edit functioning correctly.
• An invisible optimization: Callers (Curio workers via gRPC) see no API change. The batching is entirely internal, meaning existing clients automatically benefit without code changes.

The Thinking Process

The assistant's reasoning in this message reveals a systematic, integration-focused mindset. The phrase "Now I need to update the GPU worker" signals that the assistant is working through a mental checklist of components that need modification. The architecture had been designed in [msg 667] with four bullet points, and this edit addresses bullet point #4: "GPU worker needs to handle batched results — split proofs back to per-sector and notify multiple callers."

The assistant recognizes that the GPU worker is the last unmodified component in the pipeline. The batch collector, multi-sector synthesis function, proof splitter, engine synthesis task, and SynthesizedJob type have all been updated. The GPU worker is the sink — it receives the final SynthesizedJob and produces the final ProofResponse. If it doesn't handle batching, the entire upstream architecture is useless.

The edit is applied to engine.rs, which is the central coordinator (~906 lines at this point). The assistant has already modified this file twice in preceding messages ([msg 674] for SynthesizedJob changes, [msg 675] for synthesis task rewrite, [msg 676] for process_batch). Each edit is carefully scoped to a single concern, following the principle of small, verifiable changes.

The assistant does not describe the exact code changes in the message text — it simply says "Edit applied successfully." This terseness is characteristic of a developer deep in flow, where the edit itself is self-explanatory given the context. The reasoning is captured in the preceding messages: the design document in [msg 667] and the SynthesizedJob changes in [msg 674] explicitly state that the GPU worker needs to split proofs and notify multiple callers.

Conclusion

Message [msg 677] is the final piece of a carefully architected cross-sector batching system. It is brief because the heavy reasoning happened before — in the design phase, in the implementation of supporting components, and in the understanding of bellperson's internal proof format. The edit itself is mechanical: add a conditional branch in the GPU worker to handle batched jobs. But the correctness of that branch depends on a chain of assumptions about proof ordering, size constancy, channel semantics, and timing that were validated through reverse-engineering and unit testing.

This message exemplifies a pattern seen throughout the cuzk project: deep architectural work is done in the design and supporting-implementation phases, and the final integration edits are small, almost anticlimactic. The real engineering is invisible in the message text — it lives in the understanding of Groth16, bellperson, tokio, and Filecoin's proof structure that makes the edit correct.