The Pivot Point: A Single Edit That Unlocks Pipelined Groth16 Proving

Message: [assistant] [edit] /home/theuser/curio/extern/bellperson/src/groth16/prover/mod.rs Edit applied successfully.

At first glance, this message is almost invisible — a terse confirmation that a file was edited. It contains no analysis, no reasoning, no code diff. Yet this single line represents the culmination of dozens of prior messages spanning thousands of words of investigation, and it marks the moment when a months-long research project crossed from design into implementation. The message is the first concrete code change of Phase 2 of the cuzk proving engine, a pipelined SNARK proving daemon for Filecoin storage proofs. Understanding why this particular edit matters requires unpacking the entire trajectory that led to it.

The Context: From Memory Bottleneck to Pipelined Architecture

The cuzk project was born from a brutal constraint: Filecoin's Groth16 proof generation for 32 GiB sectors consumes approximately 200 GiB of peak memory when using the existing supraseal C2 pipeline. This memory footprint makes it impossible to run multiple proving jobs concurrently on commodity hardware, creates idle GPU time during CPU synthesis phases, and forces expensive SRS (Structured Reference String) parameter reloading between proofs. The earlier optimization proposals (Proposals 1–5) had identified nine structural bottlenecks and proposed three composable optimizations — Sequential Partition Synthesis, Persistent Prover Daemon, and Cross-Sector Batching — but these remained theoretical designs.

The cuzk project (segments 3–7) translated these proposals into a concrete architecture: a gRPC-based proving daemon with priority scheduling, multi-GPU worker pools, and a pipelined execution model. Phase 0 implemented the daemon skeleton, gRPC API, and end-to-end proof submission pipeline. Phase 1 added support for all four Filecoin proof types (PoRep, WinningPoSt, WindowPoSt, SnapDeals), multi-GPU isolation, and a benchmark command. By the end of Phase 1, the daemon could accept proof requests and dispatch them to GPU workers, but each proof still ran monolithically — CPU synthesis and GPU proving were fused into a single blocking call.

The Discovery: The Split Already Exists

The critical insight that made Phase 2 possible emerged during a deep analysis of bellperson's internals ([msg 398]). Bellperson is the Rust library that implements the Groth16 proving system for Filecoin, and its supraseal backend (the CUDA-accelerated variant) already contained a clean internal separation between two phases:

1. synthesize_circuits_batch() — a CPU-only function (~60 lines) that runs circuit.synthesize() for each circuit in parallel via Rayon, producing ProvingAssignment<Scalar> structures containing the a, b, and c evaluation vectors along with density trackers.
2. The GPU phase (~70 lines after synthesis) — packs these assignments into raw pointer arrays and calls supraseal_c2::generate_groth16_proof(), which performs NTT (Number Theoretic Transform), MSM (Multi-Scalar Multiplication), and proof assembly on CUDA. The boundary between these phases is clean: synthesis produces data structures in host memory, and the GPU phase consumes them. But both functions were private to bellperson's internal module hierarchy. The ProvingAssignment struct was crate-private. synthesize_circuits_batch was private. There was no way for external code to call synthesis without immediately proceeding to GPU proving.

The Reasoning: Why Fork at All?

The assistant faced a strategic fork-choice problem ([msg 399]). Four options existed:

• Strategy A: Fork only bellperson, expose the split API, and call through the existing filecoin-proofs-api layer for circuit construction.
• Strategy B: Fork bellperson plus storage-proofs-core, filecoin-proofs, and filecoin-proofs-api — four forks total.
• Strategy C: Fork only bellperson and replicate the glue code from filecoin-proofs directly in cuzk-core, calling CompoundProof::circuit() (a public trait method) to build circuit instances.
• Strategy D: Fork nothing and use the monolithic API as-is, losing all pipelining benefits. The assistant systematically evaluated each strategy against the project's core constraint: minimal changes, maximal impact. Strategy B was rejected as too invasive — four forks would create a maintenance nightmare and diverge from upstream too aggressively. Strategy D was rejected because it would not achieve the pipelining goals. Strategy A was initially appealing but required understanding whether CompoundProof::circuit() was public. A follow-up investigation ([msg 400]) confirmed the critical facts: CompoundProof::circuit() is a public trait method, StackedCompound and other compound types are public, and SuprasealParameters::new(path) is public for loading SRS parameters directly. This validated Strategy C: fork only bellperson, expose the internal split, and let cuzk-core call CompoundProof::circuit() directly to build circuits, then call synthesize_circuits_batch() for CPU synthesis, and later call prove_from_assignments() for GPU proving.

The Design Document: 791 Lines of Memory-Aware Architecture

Before touching any code, the assistant wrote a comprehensive Phase 2 design document (cuzk-phase2-design.md, 791 lines) covering the per-partition pipeline strategy, memory budget analysis, SRS manager design, and a 7-step implementation plan ([msg 403]). The key architectural insight was the per-partition pipeline: instead of synthesizing all 10 PoRep partitions at once (producing ~136 GiB of intermediate state), the pipeline processes one partition at a time, reducing peak intermediate state to ~13.6 GiB. This makes it feasible to run the pipelined prover on machines with 128 GiB of RAM while still keeping the GPU continuously fed.

The design document also included a detailed memory breakdown, SRS residency analysis (the 20.5% speedup measured in Phase 0 from keeping SRS loaded in GPU memory), and a comparison of the monolithic call chain versus the pipelined call chain. The assistant then verified constraint counts per circuit ([msg 404]) to ensure the memory calculations were accurate, and updated the design document with the corrected numbers ([msg 405]).

The Subject Message: First Edit to prover/mod.rs

The subject message (index 414) is the first of three edits to the bellperson fork. It targets /home/theuser/curio/extern/bellperson/src/groth16/prover/mod.rs, the module that defines the prover's core types and orchestrates the synthesis and proving phases.

Based on the plan outlined in [msg 413], this edit makes two changes:

1. Makes ProvingAssignment<Scalar> and all its fields pub — This struct was previously crate-private, meaning only code within bellperson could construct or inspect it. Making it public allows cuzk-core to receive the synthesized assignments, store them in a queue, and later pass them to the GPU phase. The struct contains the a, b, and c evaluation vectors (each a Vec<Scalar>), density trackers for the MSM computation, and metadata about the circuit's constraint structure.
2. Makes synthesize_circuits_batch() pub — This function was previously private to the prover module. Making it public allows external code to invoke the CPU synthesis phase independently. The function takes a vector of circuit references and SRS parameters, runs all circuit syntheses in parallel via Rayon, and returns the ProvingAssignment vectors along with extracted input and auxiliary assignments. The edit also likely makes the supraseal submodule pub (it was previously mod supraseal; under a feature gate), so that the prove_from_assignments() function added in a subsequent edit is accessible from outside the prover module.

The Assumptions Behind the Edit

Several assumptions underpin this seemingly simple edit:

Assumption 1: The internal API boundary is stable. The assistant assumes that the existing synthesize_circuits_batch() function and ProvingAssignment struct will not change significantly in future versions of bellperson. This is a reasonable assumption because the function has been stable since its introduction and is a natural architectural boundary, but it carries the risk that upstream changes could require updating the fork.

Assumption 2: Exposing internal types does not break encapsulation. Making ProvingAssignment public exposes its internal field layout. If upstream bellperson later refactors these fields, the fork must be updated. The assistant mitigates this by keeping the fork minimal — only ~130 lines changed across three files — and by not modifying any logic, only visibility modifiers.

Assumption 3: The GPU phase can be cleanly extracted. The prove_from_assignments() function (added in a subsequent edit) extracts the GPU-phase code from create_proof_batch_priority_inner. The assistant assumes this extraction is straightforward because the boundary between synthesis and GPU work is already clean in the existing code. This is validated by reading the source: after synthesize_circuits_batch() returns, the remaining code packs raw pointers and calls the CUDA backend with no further CPU-intensive work.

Assumption 4: The [patch.crates-io] mechanism works for transitive dependencies. The bellperson fork needs to be picked up not just by cuzk-core directly, but also by filecoin-proofs-api and its transitive dependencies (which depend on bellperson through storage-proofs-core). The assistant initially used version 0.26.0-cuzk.1 but discovered that semver matching requires the version to satisfy the dependency constraint bellperson = "0.26.0" — a pre-release suffix doesn't match. This was corrected in [msg 422] by reverting to version 0.26.0.

Input Knowledge Required

To understand this message, one needs:

• Groth16 proof system architecture: Understanding that SNARK proving splits into circuit synthesis (CPU-bound constraint generation) and proof computation (GPU-bound NTT/MSM operations).
• Filecoin proof pipeline: Knowing that PoRep C2 for 32 GiB sectors involves 10 partitions, each with ~2.5 million constraints, and that the supraseal backend uses CUDA for the heavy linear algebra.
• Bellperson's module structure: The groth16::prover module contains both native and supraseal backends, with ProvingAssignment as the core intermediate data structure.
• Rust visibility system: Understanding pub, pub(super), and crate-private visibility, and how the [patch.crates-io] mechanism works for local forks.
• The cuzk project's history: The earlier phases, the design documents, the memory budget analysis, and the commitment to a minimal-fork philosophy.

Output Knowledge Created

This edit produces:

• A forked bellperson with an exposed split API: External code can now call synthesize_circuits_batch() to run CPU synthesis, store the resulting ProvingAssignment in a queue, and later call prove_from_assignments() for GPU proving.
• The foundation for pipelined proving: The entire Phase 2 architecture — per-partition pipeline, SRS manager, priority scheduling across synthesis and GPU phases — depends on this split being externally accessible.
• A reusable artifact: The bellperson fork is committed to the repository at extern/bellperson/ and can be maintained independently, with clear documentation of what was changed and why.

The Thinking Process: Minimalism as a Design Principle

The most striking aspect of the assistant's reasoning is the consistent commitment to minimalism. Every decision is evaluated against the question: "What is the smallest change that achieves the goal?" The assistant explicitly rejects forking four crates (Strategy B) in favor of forking one. The changes to bellperson are described as "~30 lines of code... No logic changes, just visibility" ([msg 398]). When the version number causes a patch resolution failure, the fix is a single-line change to revert to the matching version ([msg 422]).

This minimalism is not laziness — it is a deliberate architectural strategy. By keeping the fork as close to upstream as possible, the assistant preserves the ability to merge upstream changes, reduces the maintenance burden, and minimizes the surface area for bugs. The split API is already present in bellperson's internal architecture; the fork merely unlocks the door that was already there.

The Broader Significance

The subject message, for all its brevity, is the pivot point of the entire cuzk project. Before this edit, pipelined proving was a design document and a set of proposals. After this edit, it becomes a concrete possibility. The next messages in the conversation confirm this: the fork compiles cleanly ([msg 423]), all 8 existing tests pass ([msg 424]), and the milestone is committed as f258e8c7 with a detailed commit message ([msg 428]). The project is now positioned to implement the pipelined prover in cuzk-core, with the bellperson fork providing the necessary API surface.

In the broader context of Filecoin infrastructure, this edit represents a shift from optimizing individual proof generation toward architecting a continuous, memory-efficient proving pipeline. The 20.5% speedup from SRS residency (measured in Phase 0) and the ~10× reduction in peak intermediate state from per-partition pipelining are not incremental improvements — they change the economic calculus of running proving hardware in heterogeneous cloud rental markets. A proving daemon that can keep multiple GPUs saturated while fitting in 128 GiB of RAM is dramatically cheaper to operate than one that requires 256 GiB+ machines and reloads SRS parameters for every proof.

Conclusion

A single edit that makes a struct public and a function public — seven characters changed (pub added in a few places) — is the difference between a monolithic proving pipeline and a pipelined one. The subject message is a testament to the power of understanding existing architecture deeply before modifying it. The assistant did not rewrite bellperson's internals; it simply exposed the boundary that was already there. The result is a fork so minimal that it could be upstreamed with a few visibility changes, yet so impactful that it unlocks an entirely new proving architecture.