The Memory Ceiling: When Early Deallocation Isn't Enough

Introduction

In the high-stakes world of GPU-accelerated proof generation for Filecoin's Proof-of-Replication (PoRep), every millisecond counts and every gigabyte matters. The Phase 12 optimization effort for the SUPRASEAL_C2 Groth16 proving pipeline had just achieved a promising 2.4% throughput improvement by splitting the GPU proving API into asynchronous start/finalize phases. But a critical question remained: could the system handle more parallelism? Message 3075 captures the moment when an elegant optimization hypothesis collides with the messy reality of memory fragmentation, forcing a strategic retreat and a deeper understanding of where the true bottlenecks lie.

Context: The Phase 12 Split API and the Memory Investigation

The Phase 12 redesign had decoupled the b_g2_msm CPU computation from the GPU worker loop, allowing GPU workers to immediately pick up the next synthesized partition instead of waiting ~1.7s for the CPU post-processing to complete. This split API introduced a PendingProofHandle that held all Rust-side data until finalization. The benchmark results were promising: 37.1 seconds per proof at pw=10 (partition workers=10), a ~2.4% improvement over the Phase 11 baseline of 38.0 seconds.

However, attempts to increase synthesis parallelism to pw=12 or pw=15 resulted in Out-of-Memory (OOM) errors, with RSS peaking at 668 GiB on a 755 GiB system. The user asked a pointed question in message 3055: "the pending proof handle has nothing that can be freed early?" This sparked a detailed investigation into what the PendingProofHandle actually held and whether the b_g2_msm background thread still needed all of it.

The assistant traced through the C++ CUDA code and discovered a critical insight: the prep_msm_thread (which runs b_g2_msm) did not need the massive a, b, c NTT evaluation vectors — each ~4 GiB per circuit, totaling ~12 GiB per partition. These vectors were only used by the GPU kernel region, which completed before generate_groth16_proofs_start_c returned. The assistant implemented an early deallocation in prove_start, clearing these vectors immediately after the C++ call returned, before storing the handle for finalization.

The Message: Confronting the Data

Message 3075 is the assistant's response after running the pw=12 benchmark with the early deallocation fix. The RSS monitor told a sobering story:

Huge spikes — 305 GiB already at the first burst. These are the synthesis allocations overwhelming the system. The 2 extra workers don't add 26 GiB linearly — they add more because of overlapping allocation patterns and fragmentation.

>

This isn't a Phase 12 problem per se. Let me kill the daemon and try something different — let me see if pw=10 is actually stable now with the early free, and focus on whether there's a throughput gain rather than chasing pw=12:

The peak RSS was still ~650 GiB — barely different from the 668 GiB before the early deallocation. The ~18 GiB saved by freeing a/b/c vectors was a rounding error compared to the ~300 GiB memory spike from the two additional synthesis workers.

WHY This Message Was Written

This message represents a moment of re-evaluation. The assistant had pursued a specific hypothesis: that the PendingProofHandle was holding memory unnecessarily, and that freeing the a, b, c vectors early would reduce memory pressure enough to enable pw=12. The hypothesis was logical — those vectors were ~12 GiB per partition, and with multiple partitions in-flight, the savings should have been substantial.

But the data disproved the hypothesis. The RSS trace showed that memory was spiking to 305 GiB at the first burst — before the pending handles could even accumulate. This meant the bottleneck wasn't in the finalization pipeline at all. It was in the synthesis stage itself. The two extra synthesis workers (from pw=10 to pw=12) weren't adding 26 GiB linearly; they were creating overlapping allocation patterns that caused glibc's memory allocator to fragment and hold onto far more memory than the sum of individual allocations.

The message is written to acknowledge this reality and pivot. The assistant explicitly says "This isn't a Phase 12 problem per se" — a crucial admission that the memory ceiling is architectural, not something that can be fixed by tweaking the pending proof handle. The decision to "kill the daemon and try something different" reflects a strategic choice: stop fighting a losing battle against memory fragmentation and instead consolidate the gains already made at pw=10.

How Decisions Were Made

The decision-making process in this message is visible in its structure. First comes observation: "Huge spikes — 305 GiB already at the first burst." Then analysis: "These are the synthesis allocations overwhelming the system." Then a refined mental model: "The 2 extra workers don't add 26 GiB linearly — they add more because of overlapping allocation patterns and fragmentation."

The key insight is the recognition that the problem is systemic, not local. The assistant had been looking at the wrong part of the pipeline. The synthesis stage — where multiple CPU threads concurrently generate the constraint system evaluations — creates allocation patterns that the glibc allocator cannot efficiently manage. Each thread allocates and deallocates large vectors of field elements (~130 million elements × 32 bytes = ~4 GiB per vector), and when 12 threads do this simultaneously, the allocator's arena-based strategy fragments the heap.

The decision to abandon pw=12 and focus on pw=10 throughput is a pragmatic one. The assistant recognizes that the 755 GiB system has a hard ceiling, and that pushing past it would require architectural changes to the synthesis stage — a much larger undertaking than the Phase 12 split API. Instead, the message pivots to validating that pw=10 is stable with the early free and measuring whether there's any throughput gain from the fix.

Assumptions Made

The primary assumption was that the PendingProofHandle's a, b, c vectors were a significant contributor to the OOM at pw=12. This assumption was reasonable — each partition's vectors are ~12 GiB, and with multiple partitions in various stages of completion, the savings from early deallocation should have been measurable. The assistant correctly traced the C++ code to confirm that the prep_msm_thread didn't need these vectors, and the fix was correctly implemented.

However, the assumption that this would be sufficient to enable pw=12 proved incorrect. The RSS trace showed that the memory spike happened too early in the pipeline — during synthesis, not during finalization. The early deallocation fix was a correct optimization, but it was targeting the wrong bottleneck.

A secondary assumption was that the memory increase from pw=10 to pw=12 would be roughly linear — ~26 GiB more for synthesis (2 workers × ~13 GiB each). The actual increase was ~300 GiB, revealing that fragmentation and allocation pattern interference dominate at higher concurrency levels.

Mistakes and Incorrect Assumptions

The most significant mistake was the framing itself: looking at the PendingProofHandle as the cause of the OOM. The assistant had correctly identified that the handle held ~12 GiB of unnecessary vectors, but this was a small fraction of the total memory pressure. The real culprit was the synthesis stage's allocation patterns under high concurrency.

This isn't a coding mistake — the early deallocation is still a valid improvement. It's a diagnostic mistake: the assistant was looking at the wrong part of the pipeline because the Phase 12 changes had made the pending handle a visible new component. It's a natural cognitive bias to suspect the thing you just changed.

The assistant also initially assumed that RSS would decrease proportionally to the freed memory. But glibc doesn't return freed memory to the OS immediately — it holds onto it in arenas for reuse. The malloc_trim function can release it, but the Rust allocator doesn't call it automatically. This means that even if the a/b/c vectors are freed, the RSS doesn't drop until the allocator decides to release the pages.

Input Knowledge Required

To fully understand this message, one needs:

  1. The Phase 12 architecture: Understanding that the split API introduced a PendingProofHandle that holds synthesis data until b_g2_msm completes, and that this extends the lifetime of those allocations by ~1.7 seconds.
  2. The memory profile of Groth16 proof generation: Each partition's ProvingAssignment contains a, b, c vectors of ~130 million field elements each (~4 GiB per vector, ~12 GiB total), plus input_assignment and aux_assignment vectors of similar size, plus density bitmaps and temporary buffers.
  3. The GPU pipeline stages: The GPU kernel region (which reads a/b/c vectors) completes before generate_groth16_proofs_start_c returns, while the prep_msm_thread (which runs b_g2_msm) runs in the background and only reads input_assignment, aux_assignment, and density bitmaps.
  4. The concurrency model: The pw parameter controls how many partition synthesis tasks run concurrently. Each synthesis task allocates ~13 GiB of temporary data. The j parameter controls how many proof jobs are queued from the scheduler. The gw parameter controls GPU workers.
  5. glibc memory allocator behavior: The arena-based allocation strategy can cause RSS to far exceed the sum of live allocations when multiple threads allocate and deallocate large blocks concurrently, because each thread gets its own arena and freed memory isn't returned to the OS.
  6. The RSS monitoring setup: The assistant had set up a background process polling RSS every 5 seconds and logging to a file, which provided the trace showing the 305 GiB spike at the first burst.

Output Knowledge Created

This message creates several important pieces of knowledge:

  1. Confirmation that pw=12 is not achievable on a 755 GiB system with the current architecture: The memory ceiling is hard and cannot be overcome by optimizing the pending proof handle alone.
  2. Identification of synthesis-stage memory allocation as the primary bottleneck: The problem isn't in the Phase 12 changes but in the fundamental memory pattern of concurrent synthesis.
  3. Validation that the early deallocation fix is correct but insufficient: The ~18 GiB saved is real but dwarfed by the ~300 GiB fragmentation overhead.
  4. A strategic direction: Rather than chasing higher pw values, the focus should be on optimizing pw=10 throughput and considering architectural changes to reduce synthesis memory (e.g., Sequential Partition Synthesis as proposed in earlier optimization documents).
  5. A diagnostic methodology: The message demonstrates how to use RSS monitoring to distinguish between different memory pressure sources — early spikes indicate synthesis pressure, while late-stage accumulation would indicate pending handle pressure.

The Thinking Process

The thinking visible in this message is a model of disciplined engineering reasoning. The assistant:

  1. Formulates a hypothesis: The PendingProofHandle holds unnecessary memory (a/b/c vectors) that can be freed early.
  2. Verifies the hypothesis through code analysis: Traces through C++ CUDA code to confirm the prep_msm_thread doesn't need these vectors.
  3. Implements the fix: Adds early deallocation in prove_start.
  4. Tests the fix: Runs pw=12 benchmark with RSS monitoring.
  5. Evaluates the results: The RSS trace shows 305 GiB at the first burst — the fix didn't move the needle.
  6. Refines the mental model: Realizes the problem is synthesis allocation patterns and fragmentation, not the pending handle.
  7. Makes a strategic decision: Abandons pw=12, focuses on consolidating pw=10 gains. The most impressive aspect is step 6 — the willingness to discard a perfectly logical hypothesis when the data contradicts it. The assistant doesn't try to rationalize the discrepancy or look for other variables. It accepts that the hypothesis was wrong and updates its understanding accordingly. The phrase "This isn't a Phase 12 problem per se" is particularly important. It shows the assistant distinguishing between the problem it was trying to solve (Phase 12 memory pressure) and the actual problem (synthesis memory fragmentation). This kind of reframing is essential in complex systems debugging.

Conclusion

Message 3075 is a small but revealing moment in a much larger optimization journey. It captures the moment when a well-reasoned hypothesis meets empirical reality and is found insufficient. The early deallocation of a, b, c vectors was a correct optimization, but it was targeting the wrong bottleneck. The real memory ceiling was in the synthesis stage, where concurrent allocation patterns create fragmentation that dwarfs the savings from any single-component optimization.

The strategic pivot — from chasing higher partition worker counts to consolidating gains at the proven configuration — reflects a mature engineering approach. Not every problem can be solved with a quick fix. Some require fundamental architectural changes, and knowing when to consolidate rather than push further is a skill that separates effective optimization from wasteful effort.

The message also demonstrates the value of instrumentation. Without the RSS monitoring trace showing the early spike, the assistant might have continued chasing the pending handle hypothesis, adding complexity without addressing the root cause. The data forced a re-evaluation that saved time and pointed toward more productive directions.

In the end, the Phase 12 split API stands as a ~2.4% improvement at pw=10, and the memory ceiling at pw=12 becomes a problem for another day — perhaps addressed by the Sequential Partition Synthesis proposal that would fundamentally change how synthesis memory is managed.