The Quiet Edit That Fixed GPU Routing: Threading gpu_index Through the Bellperson Layer

At first glance, message 477 in this opencode session appears almost trivial. It reads simply:

[edit] /tmp/czk/extern/bellperson/src/groth16/prover/supraseal.rs Edit applied successfully.

A single edit to a Rust file, confirmed successful. No diff shown, no explanation given, no fanfare. Yet this terse message represents a critical inflection point in a deep, multi-layered architectural fix — the second of two edits to the bellperson proving layer that together completed the Rust-side plumbing for proper multi-GPU routing in the CuZK proving engine. To understand why this message matters, one must understand the chain of failures, realizations, and corrections that led to it.

The Problem: Single-Circuit Proofs Always Hit GPU 0

The story begins with a seemingly unrelated crash. A SnapDeals workload of 16 identical partitions was running on a dual-GPU machine (an RTX 4000 Ada with 20 GB of VRAM per GPU), and it was consistently running out of memory. The initial diagnosis pointed to a data race: two Rust workers were entering the GPU proving code simultaneously, both contending for the same device. The first "fix" was a shared mutex that serialized all partition proofs onto GPU 0, effectively wasting the second GPU entirely. As the user astutely observed in [msg 444], this was "just a lazy hack."

The deeper investigation in [msg 445] and [msg 446] revealed the true root cause. Inside the C++ CUDA kernel launcher (groth16_cuda.cu), the number of GPUs to use was computed as n_gpus = min(ngpus(), num_circuits). For partitioned proofs, num_circuits is always 1 — each partition is proved as a separate single-circuit invocation. So n_gpus was always 1, and the GPU selection loop at line 883 always spawned a single thread with tid=0, which called select_gpu(0). Regardless of which Rust worker submitted the proof, regardless of which GPU that worker was assigned to, the C++ code always routed the work to GPU 0. The per-GPU mutexes that were supposed to provide isolation were never engaged because only one GPU thread was ever created.

The proper fix was clear: thread a gpu_index parameter through the entire call chain so that the C++ code uses the GPU assigned by the Rust engine instead of blindly defaulting to GPU 0. This required coordinated changes across five layers: the C++ CUDA kernel, the Rust FFI bindings in supraseal-c2, the bellperson prover library, the pipeline layer, and the engine's GPU worker code.

The Bellperson Layer: Where Abstraction Meets Hardware

Message 477 is the second of two edits to bellperson/src/groth16/prover/supraseal.rs. The bellperson library is the Rust proving abstraction that sits between the low-level FFI bindings (which call into C++ CUDA code) and the high-level pipeline orchestration (which manages GPU worker threads). It is the layer where generic type parameters like E (engine) and P (parameter source) meet concrete hardware concerns like GPU device selection.

The first edit in [msg 476] updated the prove_start function — the asynchronous entry point that launches GPU proving and returns a pending proof handle. This function is the heart of the interlocking pipeline design, where CPU preprocessing can overlap with GPU kernel execution. Adding gpu_index here meant threading it through the internal call to supraseal_c2::start_groth16_proof, the FFI wrapper that ultimately invokes the C++ generate_groth16_proofs_start_c.

Message 477 is the companion edit, completing the bellperson changes. While the exact diff is not shown in the message, the surrounding context reveals its purpose. After this edit, the assistant immediately reads the file again in [msg 478] to find and update prove_from_assignments — the synchronous counterpart. This suggests that message 477 likely updated shared type aliases, helper functions, or internal plumbing that both prove_start and prove_from_assignments depend on. It may have updated the GpuMutexPtr type alias, added a gpu_index field to an internal struct, or modified a shared helper that constructs the FFI call arguments.

Design Decisions Embedded in the Parameter

The choice of gpu_index: i32 with -1 meaning "auto" (use all available GPUs) reflects a deliberate design philosophy. For multi-circuit proofs (multiple circuits proved in a single batch), the existing auto-distribution logic remains correct — the C++ code can still spawn n_gpus threads and distribute circuits across them. But for single-circuit proofs (which is what partitioned proofs always are), the Rust engine now explicitly tells the C++ code which GPU to use. This preserves backward compatibility for non-engine callers (like tests and standalone tools) while enabling the engine to implement true load balancing.

The -1 sentinel is particularly elegant because it means that callers who don't care about GPU selection — such as the older generate_groth16_proofs function used outside the engine path — can simply pass -1 and get the old behavior. The engine workers, by contrast, pass the GPU ordinal they were assigned by the load-balancing logic in engine.rs.

What This Message Represents

Message 477 is a testament to the methodical, bottom-up approach the assistant took to this fix. Starting from the C++ CUDA code (the deepest layer), then moving up through the Rust FFI, then the bellperson abstraction, then the pipeline, and finally the engine — each layer was updated in sequence, with each edit building on the previous one. The two edits to the bellperson file (messages 476 and 477) represent the completion of the Rust-side plumbing at the abstraction boundary where generic proving logic meets hardware-specific GPU selection.

The message also embodies a correction of an earlier mistake. The shared mutex hack was a quick fix that "worked" in the sense of preventing crashes but fundamentally violated the architecture's intent. The CuZK proving engine was designed for multi-GPU operation, with per-GPU mutexes and an interlocking pipeline that overlaps CPU preprocessing with GPU kernel execution. The shared mutex serialized everything onto GPU 0, destroying both performance and correctness. Message 477 is part of restoring the architecture's original design intent — making the software actually use the hardware as it was meant to be used.

Conclusion

In the broader arc of this session, message 477 is a small but essential step. After this edit, the bellperson layer correctly accepts and propagates a gpu_index parameter. The subsequent edits to the pipeline and engine layers will complete the chain, and the final verification on the remote test host will show both GPUs active with proper load balancing — workers 0 and 1 on GPU 0, workers 2 and 3 on GPU 1, each with their own d_a_cache allocation. The SnapDeals OOM will be resolved not by serializing work onto a single GPU, but by distributing it across both GPUs as the architecture intended.

The quiet "[edit] ... Edit applied successfully" of message 477 is the sound of a fundamental architectural fix falling into place.