The 12 GiB Shadow: A Lesson in Rust Memory Ownership During GPU Proof Generation

Introduction

In high-performance GPU proving systems, memory is the invisible governor. No matter how fast your kernels run, how clever your pipeline scheduling, or how many CUDA cores you throw at a problem, you cannot escape the physics of the memory wall. This article examines a single message in an opencode coding session — message index 3065 — where an AI assistant, in the midst of optimizing a Groth16 proof generation pipeline for Filecoin's PoRep (Proof of Replication), catches a subtle memory ownership bug while implementing an early deallocation strategy. The message is brief, but it reveals the assistant's deep reasoning about memory lifetimes, Rust ownership semantics, and the interplay between GPU kernel execution and CPU-side data retention.

The Context: Phase 12 and the Memory Ceiling

The session had just completed Phase 12 of a multi-phase optimization campaign for the cuzk SNARK proving engine. Phase 12 introduced a "split GPU proving API": the monolithic generate_groth16_proofs_c C++ function was refactored into start (returns a pending handle after GPU lock release) and finalize (joins the background b_g2_msm computation and runs the epilogue). This allowed GPU workers to spawn a tokio finalizer task and immediately loop back to pick up the next synthesized partition, hiding approximately 1.7 seconds of CPU-side MSM computation behind GPU work.

The benchmark results were solid: with gw=2, pw=10, gt=32, j=15, throughput improved from 38.0s/proof (Phase 11 baseline) to 37.1s/proof — a ~2.4% gain. But the victory was tempered by a hard memory ceiling. Attempting to increase synthesis parallelism to pw=12 caused the daemon to be OOM-killed, with RSS peaking at 668 GiB on a 755 GiB system. The assistant initially concluded this was a "systemic memory capacity issue, not a leak" and committed the Phase 12 changes.

Then the user asked a question that changed the trajectory of the investigation (msg 3055): "the pending proof handle has nothing that can be freed early?"

This single question triggered a deep audit of what the PendingProofHandle actually holds and whether any of it could be released before finalization.

The Investigation: Tracing Data Dependencies

The assistant's response (msg 3056) began by reading the PendingProofHandle struct definition. But the real detective work happened in the subsequent messages (3057–3062), where the assistant systematically traced the C++ prep_msm_thread — the background thread that runs b_g2_msm — to determine exactly which data structures it reads after prove_start returns.

The key insight came from analyzing the C++ code in groth16_cuda.cu. By grepping for references to prover.a, prover.b, and prover.c within the prep_msm_thread lambda, the assistant discovered that these massive NTT (Number Theoretic Transform) evaluation vectors — each approximately 130 million elements of 32-byte scalars, totaling ~12 GiB per partition — were only accessed in the GPU kernel region, which completed before generate_groth16_proofs_start_c returned. The cudaHostUnregister calls at lines 1193–1195 of the CUDA code confirmed that the GPU had finished reading these vectors.

The background prep_msm_thread needed only:

• inp_assignment_data and aux_assignment_data (the variable assignments)
• The density bitvectors (a_aux_density, b_inp_density, b_aux_density)
• Size and popcount metadata already copied into the heap-allocated pending proof struct Critically, it did NOT need provers.a, provers.b, or provers.c. This meant that ~12 GiB per partition was being held alive in the PendingProofHandle for the entire ~1.7 second duration of the background b_g2_msm computation, despite being completely unnecessary. With multiple partitions in flight, this wasted memory was a primary contributor to the OOM at pw=12.

The Subject Message: Catching a Shadow Bug

Message 3065 is the assistant's follow-up after making an initial edit to clear the a, b, c vectors. The message reads:

Wait, I also need to actually drop the old r_s/s_s, not just shadow them with empty Vecs. Let me also consider: the input_assignment and aux_assignment fields of ProvingAssignment were already moved out during synthesis (std::mem::take), so those are already empty. The lc_pool is probably non-trivial though. Let me check:

>

[bash] grep -A10 "struct LcVecPool" /home/theuser/curio/extern/bellperson/src/groth16/prover/mod.rs struct LcVecPool<Scalar> { /// Recycled buffers for Indexer values. We need 6 per enforce call /// (3 LCs × 2 Indexers each: inputs + aux). bufs: Vec<Vec<(usize, Scalar)>>, }

>

impl<Scalar> LcVecPool<Scalar> { fn new() -> Self { Self { bufs: Vec::new() } }

This message is a moment of self-correction. The assistant had previously edited prove_start to clear the a, b, c vectors, but in doing so, it had used a Rust pattern that looked correct but had a subtle flaw: variable shadowing.

The assistant realized that simply writing let r_s = Vec::new(); in the function body would shadow the parameter r_s: Vec<Fr> with a new local binding, causing the original r_s to be dropped immediately. But the assistant had also moved r_s and s_s into the PendingProofHandle struct — so the shadowing would drop the original values (freeing their memory) and then store empty Vecs in the handle. While functionally correct for freeing memory, the assistant recognized this as a fragile pattern and wanted to verify it was doing the right thing.

More importantly, the assistant broadened its analysis to consider all memory held by the PendingProofHandle. It noted that input_assignment and aux_assignment were already moved out during synthesis (via std::mem::take), so those fields were empty. But what about lc_pool — a recycling pool of temporary buffers used during constraint synthesis? The assistant ran a grep to check the LcVecPool struct definition, finding it held a Vec<Vec<(usize, Scalar)>> — potentially non-trivial but not in the multi-GiB range.

The Thinking Process: Ownership and Memory Lifetimes

What makes this message interesting is what it reveals about the assistant's mental model. The assistant is reasoning at multiple levels simultaneously:

1. Rust ownership semantics: The assistant knows that shadowing a variable causes the original value to be dropped, but it's thinking carefully about whether this is the right pattern. The parameter r_s was moved into the function — shadowing it with let r_s = Vec::new() would indeed drop the original and create a new empty Vec. But the assistant's edit also needed to store something in the handle struct, and an empty Vec is fine since the C++ side already has its own copy.
2. GPU execution model: The assistant understands that cudaHostUnregister is the definitive signal that the GPU is done reading host memory. It traced the CUDA code to confirm this call happens before the function returns, establishing the safety invariant that allows early deallocation.
3. Data flow across FFI boundary: The assistant knows that r_s and s_s (random scalars used in the proof) were already copied into the C++ heap-allocated groth16_pending_proof struct (pp->r_s_owned / pp->s_s_owned). The Rust-side copies are therefore redundant after the C++ call returns.
4. Systematic memory accounting: Rather than stopping at the a/b/c vectors, the assistant methodically considers every field of ProvingAssignment and PendingProofHandle that could hold significant memory. It checks lc_pool by reading the source, confirming it's a recycling buffer that's probably modest in size.

Assumptions and Potential Mistakes

The assistant makes several assumptions in this message:

• That cudaHostUnregister implies GPU completion: This is a correct assumption — cudaHostUnregister requires that all preceding GPU operations on the registered memory have completed. The CUDA runtime ensures this synchronization.
• That the lc_pool is negligible: The assistant assumes the recycling pool isn't holding GiB-scale allocations. This is reasonable given its purpose (reusing temporary buffers per-constraint), but it doesn't verify the actual size at runtime.
• That the input_assignment and aux_assignment fields are already empty: This is confirmed — they were moved out during synthesis via std::mem::take. One potential oversight: the assistant doesn't check whether lc_pool could have accumulated significant memory across the many constraints in a partition. The LcVecPool holds Vec<Vec<(usize, Scalar)>> — each inner Vec could be sizable if not trimmed. However, the pool's purpose is recycling, so its size is bounded by the peak concurrent usage during synthesis, not the total across all constraints.

Input and Output Knowledge

Input knowledge required to understand this message includes:

• Rust's ownership model and the difference between shadowing and mutation
• CUDA memory management APIs (cudaHostRegister, cudaHostUnregister)
• The Groth16 proof structure (a/b/c polynomials, NTT evaluation vectors, MSM)
• The Phase 12 split API architecture (start/finalize, background prep_msm_thread)
• The Filecoin PoRep circuit structure and partition sizes (~130M constraints) Output knowledge created by this message includes:
• Confirmation that lc_pool is structurally a Vec<Vec<(usize, Scalar)>> — modest in size
• The realization that the shadowing approach for r_s/s_s needs careful verification
• A more complete picture of what memory is held by PendingProofHandle after prove_start

The Broader Impact

This early deallocation of the a/b/c vectors freed ~12 GiB per partition immediately after prove_start returned. However, as the subsequent investigation revealed (documented in the chunk summary), this alone was insufficient to enable pw=12. The real bottleneck turned out to be the partition semaphore: it released its permit immediately after synthesis completed, allowing synthesized partitions to pile up while blocking on the single-slot GPU channel. The buffer counters the assistant later built showed the provers counter peaking at 28 — meaning 28 synthesized partitions were queued holding their full ~16 GiB datasets.

The assistant eventually solved this by increasing the GPU channel capacity from 1 to partition_workers, allowing a natural buffer of completed jobs without blocking the semaphore. But the early deallocation fix remained an important piece of the puzzle — every 12 GiB saved per partition reduced peak memory pressure and contributed to the stability of the final configuration.

Conclusion

Message 3065 captures a moment of careful reasoning about memory ownership in a complex Rust/C++/CUDA system. The assistant catches its own potential mistake with variable shadowing, broadens its analysis to consider all memory sources, and demonstrates a systematic approach to understanding data lifetimes across the FFI boundary. It's a small message — barely a paragraph of reasoning and a grep command — but it reveals the depth of understanding required to optimize at the intersection of GPU execution models, Rust ownership semantics, and high-performance proving systems. The 12 GiB per partition that the assistant identified and freed was not the whole solution, but it was an essential step toward understanding and ultimately conquering the memory capacity ceiling.