The Plumbing That Makes Multi-GPU Proving Real

Message 469: "Now update the FFI call inside start_groth16_proof:"

A Single Line of Progress in a Multi-Layer Refactoring

At first glance, message 469 in this opencode session appears almost trivial. The assistant writes: "Now update the FFI call inside start_groth16_proof:" followed by an edit confirmation. There is no code diff shown, no lengthy explanation, no dramatic revelation. Yet this single message sits at a critical juncture in one of the most consequential architectural refactorings in the entire conversation: the threading of a gpu_index parameter through five layers of software — C++, Rust FFI, bellperson prover functions, pipeline orchestration, and engine worker code — to fix a fundamental multi-GPU load-balancing defect in the CuZK proving engine.

To understand why this message exists, we must first understand the crisis that precipitated it.

The Crisis: A Lazy Hack Exposed by SnapDeals

The story begins with a partitioned proof failure. The CuZK proving engine, a sophisticated GPU-accelerated zero-knowledge proving system, was crashing on WindowPoSt proofs. The initial diagnosis revealed a data race: the C++ GPU proving code in groth16_cuda.cu contained the line n_gpus = std::min(ngpus(), num_circuits) (see [msg 445]), which for partitioned proofs — where num_circuits=1 — always evaluated to n_gpus=1, causing the code to call select_gpu(0) regardless of which GPU the Rust worker had been assigned. On a multi-GPU system, two workers would both target GPU 0, creating a data race on the GPU's memory and kernel execution resources.

The initial "fix" was a shared mutex that serialized all partition proofs onto GPU 0. It prevented the crash but at a terrible cost: the second GPU sat entirely idle, and the engine's carefully designed interlocking pipeline — where CPU preprocessing on one worker overlaps with GPU kernels on another — was reduced to sequential execution.

The user called this out in no uncertain terms at [msg 444]: "Why is GPU prove for the second GPU not running.. on the second GPU? That's the whole point. CuZK is meant to be a fairly sophisticated proving engine, so it must support multiple GPUs, gpu memory management, and GPU workers are supposed to be interlocking two phases of data transfer to gpu vs compute. Isn't the shared lock just a lazy hack?"

The user was right. And the proof came when a SnapDeals workload (16 identical partitions) was deployed to a 20 GB RTX 4000 Ada host. Two workers entered the GPU code simultaneously on GPU 0, and the VRAM budget for a single SnapDeals partition — with its 4096 MiB d_a_cache allocation plus NTT scratch, MSM bases, and batch-add buffers — was too large to allow concurrent execution on the same device. The result was an out-of-memory panic (cudaMallocAsync failed: "out of memory" at [msg 440]).

The Proper Fix: Threading gpu_index Through Five Layers

The proper solution, as the assistant articulated at [msg 452], was to "thread a gpu_index parameter through the entire call chain so the C++ code uses select_gpu(gpu_index) instead of select_gpu(0) for single-circuit proofs." This would make the C++ code actually use the GPU that the Rust engine assigned, allowing per-GPU mutexes to work correctly and enabling the 16 SnapDeals partitions to naturally load-balance across both GPUs.

The implementation followed a careful bottom-up strategy:

1. C++ layer (groth16_cuda.cu): Add int gpu_index parameter to generate_groth16_proofs_start_c and generate_groth16_proofs_c. When gpu_index >= 0, force n_gpus = 1 and use select_gpu(gpu_index) instead of select_gpu(tid) where tid=0. When gpu_index == -1, fall back to the original auto-distribution logic. (Completed at [msg 457] through [msg 461])
2. Rust FFI layer (supraseal-c2/src/lib.rs): Update the extern "C" declarations for both C++ functions to accept the new gpu_index: i32 parameter. (Completed at <msg id=466-467>)
3. Rust public API (supraseal-c2/src/lib.rs): Update start_groth16_proof to accept and forward gpu_index. (Completed at [msg 468])
4. FFI call inside start_groth16_proof: This is where message 469 fits in. The assistant updates the actual call site — the line of code that invokes the C function through the FFI boundary — to pass the gpu_index parameter.
5. Sync wrapper and non-engine callers: Update generate_groth16_proof and generate_groth16_proofs to accept gpu_index (with -1 as default for backward compatibility). (Completed at [msg 470])

Why Message 469 Matters

Message 469 is the moment where the abstract parameter plumbing becomes concrete. The assistant had already updated the function signature of start_groth16_proof at [msg 468], adding gpu_index: i32 as a parameter. But a function signature change is just a declaration — it creates a contract without fulfilling it. The FFI call inside the function body is where that contract is executed: the Rust value is marshaled across the language boundary into a C-compatible type and passed to the foreign function.

This is a particularly delicate operation in Rust. FFI calls are inherently unsafe — the compiler cannot verify that the types, layouts, and calling conventions match between Rust and C++. A mistake here (wrong parameter order, incorrect type size, mismatched calling convention) would not produce a compile error but would cause undefined behavior at runtime, potentially corrupting memory or crashing in ways that are extremely difficult to debug. The assistant's confidence in making this edit — and the fact that it compiled cleanly — reflects a thorough understanding of both the Rust FFI semantics and the C++ function signatures.

Assumptions and Knowledge Required

To understand and execute this edit correctly, the assistant needed:

• Knowledge of the C++ function signature: The exact parameter types and order for generate_groth16_proofs_start_c, including that the new gpu_index parameter is an int (which maps to Rust's i32).
• Knowledge of Rust FFI conventions: That C int corresponds to Rust i32, that the extern "C" declaration must match the C++ function's mangled name (or use extern "C" linkage in C++), and that the parameter must be passed by value.
• Knowledge of the call chain: That start_groth16_proof is the public API entry point that eventually calls the C++ function, and that the gpu_index value must flow through without transformation.
• Knowledge of backward compatibility: The -1 sentinel value convention, where -1 means "auto-detect GPUs" (the old behavior) and non-negative values mean "use this specific GPU." This allows callers that don't care about GPU selection — such as the non-engine generate_groth16_proofs function used by older code paths — to pass -1 and get the original behavior.
• Knowledge of the d_a_cache global state: The assistant recognized at [msg 457] that the global g_d_a_cache singleton would thrash between GPUs under concurrent access, but deferred that fix as a separate issue, correctly prioritizing the functional correctness of GPU routing over optimal memory management.

The Thinking Process Visible in the Sequence

What is most revealing about message 469 is not what it says but where it sits in the sequence of edits. The assistant is working methodically from the bottom of the stack upward — a classic systems-engineering approach. Each layer is completed before the next layer is touched, ensuring that at every point the codebase is in a compilable state (or as close to it as possible during a multi-file refactoring).

The assistant's todo list (visible at <msg id=452-453>) shows the discipline: each item is marked "pending" initially, then "in_progress" when work begins, and implicitly "completed" when the edit succeeds. Message 469 represents the transition of item 3 ("Add gpu_index to supraseal-c2 Rust FFI wrappers") from "in_progress" to nearly complete — the extern declarations and the public function signature have been updated, and now the critical FFI call site is being modified.

What This Message Creates

The output of message 469 is not just an edited file — it is a correctly wired FFI call that connects the Rust public API to the C++ GPU selection logic. This edit, combined with the preceding and following changes, creates the architectural foundation for proper multi-GPU load balancing in the CuZK proving engine. Without it, the gpu_index parameter would exist in the function signature but never reach the C++ code that actually calls select_gpu(). The parameter would be dead code — declared but unused.

With this edit, the parameter becomes live: when the Rust engine assigns a worker to GPU 1, the gpu_index value of 1 flows through start_groth16_proof, across the FFI boundary, into the C++ function, and ultimately to select_gpu(1), which calls cudaSetDevice(1) to activate the correct GPU. The two-GPU system finally works as designed.

Conclusion

Message 469 is a testament to the fact that in complex systems engineering, the most critical work often happens in the smallest, most mundane-looking edits. A single line changed in an FFI call site — "now pass gpu_index to the C function" — is the difference between a system that wastes half its hardware capacity and one that delivers on its architectural promise. The message itself is brief, but the reasoning, the context, and the stakes behind it are anything but.