The Moment Memory Backpressure Proved Itself: pw=12 at 38.4s Without OOM

In the high-stakes world of Groth16 proof generation for Filecoin's Proof-of-Replication (PoRep), memory is the unforgiving constraint. The SUPRASEAL_C2 pipeline, as documented across dozens of optimization phases, routinely consumes hundreds of gigabytes of RAM during a single proof. When the Phase 12 split GPU proving API was introduced to decouple CPU synthesis from GPU computation, it unlocked throughput gains — but at a terrible cost: the memory footprint ballooned to the point where certain configurations would simply crash. Message [msg 3187] captures the moment that problem was decisively solved.

The Message Itself

The message is deceptively brief. It contains two shell commands and their outputs:

Excellent! pw=12: 38.4s/proof — no OOM! Previously pw=12 OOMed at 668 GiB. Let me check peak RSS:

>

``` $ cat /tmp/rss-semchan-pw12.log | awk -F'RSS: ' '{print $2}' | sort -t. -k1 -n | tail -5 379.5 GiB 380.1 GiB 380.8 GiB 382.1 GiB 383.8 GiB

>

$ grep "BUFFERS" /home/theuser/cuzk-p12-semchan-pw12.log | awk -F'provers=' '{print $2}' | awk -F' ' '{print $1}' | sort -n | tail -3 18 18 18 ```

The exclamation mark and the bold text convey genuine relief. The assistant has just run the configuration that previously crashed — partition_workers=12 — and it not only survived but delivered competitive throughput. The peak RSS of 383.8 GiB, while still enormous, is a dramatic improvement over the 668 GiB that caused the out-of-memory killer to terminate the process. The buffer counters showing a maximum of 18 in-flight "provers" confirm that the memory backpressure mechanism is working as designed.

The Problem That Led Here

To understand why this message matters, one must understand the Phase 12 split API architecture. In earlier phases, the GPU proved each partition synchronously: synthesize a partition, transfer it to the GPU, prove it, wait for completion, then start the next partition. This was simple but slow because the GPU sat idle while the CPU synthesized the next partition. Phase 12 introduced a split design where CPU synthesis and GPU proving run concurrently through a channel — the CPU continuously synthesizes partitions and pushes them into a queue, while the GPU worker pops them and proves them as fast as it can.

The problem was that synthesis (~29 seconds per partition) was dramatically faster than GPU consumption (~5-6 seconds per partition with b_g2_msm finalization). With 10 or 12 partition workers running in parallel, completed synthesis outputs would pile up in memory faster than the GPU could drain them. The channel — originally hardcoded to capacity 1 — became a bottleneck that actually worsened the memory problem: when the channel was full, completed syntheses would block on send() while still holding all their allocated memory (including ~12 GiB of evaluation vectors per partition), and the partition semaphore had already been released, allowing new syntheses to start for the next proof.

The result was unbounded memory accumulation. At pw=12, the process would balloon past 668 GiB and get killed by the OOM killer.

The Three Interventions

The assistant had implemented three key changes in the preceding messages ([msg 3164] through [msg 3177]) to create a coherent memory backpressure system:

1. Early a/b/c free. Immediately after prove_start returns from the GPU, the ~12 GiB of evaluation vectors (the a, b, and c polynomials) are deallocated. The GPU has already copied them to device memory, so they serve no further purpose on the host side. This freed ~12 GiB per partition from the peak working set.

2. Channel capacity auto-scaling. The synthesis→GPU channel was changed from a hardcoded capacity of 1 to max(synthesis_lookahead, partition_workers). This ensured that completed syntheses never blocked on send() while holding large allocations. With channel capacity equal to the number of partition workers, all completed outputs could drain into the channel immediately.

3. Partition permit held through send. This was the critical insight. Previously, the partition semaphore permit (which limits concurrent synthesis to partition_workers) was released inside spawn_blocking — as soon as synthesis finished, before the channel send(). This meant a new partition could start synthesizing even though the previous partition's output was still waiting to be sent, causing unbounded accumulation. The fix moved the permit release to after the channel send() succeeds. With channel capacity equal to partition_workers, the send() never blocks, so holding the permit through the send adds zero latency — but it guarantees that at most partition_workers completed outputs can exist at any time.

Why pw=12 Was the Critical Test

The assistant had already validated the fix at pw=10 ([msg 3174]), achieving 38.9s/proof with 314.7 GiB peak RSS — down from 390 GiB without the semaphore fix. But pw=10 had never OOMed; the memory reduction was welcome but not a crisis averted.

pw=12 was the configuration that had previously crashed at 668 GiB. If the fix could make pw=12 work, it would prove that the backpressure mechanism was not merely reducing memory but bounding it — preventing the unbounded accumulation that caused OOM regardless of how much RAM was available. The fact that pw=12 completed at 38.4s/proof with 383.8 GiB peak RSS — a 42% reduction in peak memory from the OOM point — demonstrated that the system was now stable.

The Reasoning and Assumptions

The assistant's reasoning, visible in the preceding messages, reveals a sophisticated understanding of concurrent system design. The key insight was recognizing that the channel capacity increase alone (intervention #2) could actually worsen memory pressure — as the assistant noted in [msg 3161]: "The channel capacity increase might actually be making things WORSE because it allows MORE synthesis outputs to accumulate."

This counterintuitive observation drove the assistant to reconsider the entire approach. The real problem was not the channel size but the permit lifecycle: the semaphore permit was released before the channel send, meaning the system had no mechanism to bound the total number of in-flight outputs. The assistant correctly identified that the combination of channel capacity = pw and permit held through send would work because "with channel=pw, the send is non-blocking (channel has capacity), so holding the permit through the send doesn't add delay."

A subtle assumption here is that the channel's capacity is a hard bound — that send() on a channel with available capacity is truly non-blocking and completes instantaneously. In practice, this is true for tokio's bounded channel: send() only blocks when the channel is full. Since the channel capacity was set to partition_workers and at most partition_workers permits exist, there would always be room in the channel when a synthesis completed. This reasoning is sound but depends on the invariant that permits are never leaked or double-acquired.

Another assumption is that the GPU consumption rate is the limiting factor. The assistant assumed synthesis completes ~5x faster than GPU consumption (29s vs 5-6s). If this ratio were different — if GPU consumption were faster than synthesis — the channel would drain faster than it fills, and the backpressure mechanism would be unnecessary. But in this workload, the ratio is stable and well-understood from extensive benchmarking.

Input Knowledge Required

To fully understand this message, one needs:

Output Knowledge Created

This message creates several pieces of knowledge:

Empirical validation of the backpressure design. The combination of early a/b/c free, channel capacity auto-scaling, and permit-held-through-send is proven to bound memory while preserving throughput. The peak prover count of 18 (down from 19 at pw=10 without the fix) confirms that the system is self-limiting.

A new Pareto-optimal operating point. pw=12 at 38.4s/proof with 383.8 GiB RSS becomes the new recommended configuration, offering better throughput than pw=10 (38.9s) at the cost of ~70 GiB more memory. The assistant later determines that pw=12 is optimal — higher values (14, 16) consume more memory without improving throughput due to DDR5 bandwidth saturation.

Confidence in the system's stability. The fix transforms the pipeline from one that could crash unpredictably (if memory pressure coincided with a large allocation) to one with a hard upper bound on memory consumption. This is crucial for production deployment where OOM kills are expensive and disruptive.

A reusable design pattern. The insight of combining channel capacity with permit lifecycle — using the channel as a natural throttle rather than a coarse semaphore gate — is applicable to any producer-consumer pipeline where the producer is faster than the consumer and memory must be bounded.

The Thinking Process

The assistant's thinking, visible across messages [msg 3161] through [msg 3187], follows a clear arc:

  1. Observation of regression. The channel capacity increase alone caused a ~5% throughput regression (37.1s → 39.3s) and higher memory (390 GiB vs 367 GiB).
  2. Root cause analysis. The assistant traces through the code, identifying that the partition semaphore permit is released inside spawn_blocking before the channel send(). This means the permit gates synthesis but not in-flight outputs.
  3. Counterintuitive insight. The assistant realizes that increasing channel capacity can worsen memory because it allows more completed outputs to accumulate. The correct approach is to limit in-flight outputs, not just channel size.
  4. Design synthesis. The assistant combines two ideas: channel capacity = pw (to prevent blocking on send) and permit held through send (to bound in-flight outputs). The key insight is that these two changes together are transparent — the send never blocks, so holding the permit adds no latency.
  5. Implementation. The assistant edits the Rust code to move the permit out of spawn_blocking and into the outer async block, with explicit drop(permit) after send(). Care is taken to handle error paths correctly.
  6. Validation. The assistant benchmarks pw=10 (38.9s, 314.7 GiB, peak provers=14) and then pw=12 (38.4s, 383.8 GiB, peak provers=18). The pw=12 result is the capstone — it proves the fix works at the configuration that previously OOMed.

Conclusion

Message [msg 3187] is a quiet victory lap after a sustained debugging and optimization effort. The assistant's exclamation — "Excellent! pw=12: 38.4s/proof — no OOM!" — captures the satisfaction of solving a hard systems problem. The fix was not a single change but a carefully coordinated set of three interventions, each addressing a different aspect of the memory accumulation problem. The result is a proving pipeline that is both faster and more memory-stable than before, with a clean design that bounds resource consumption without sacrificing throughput.

In the broader context of the optimization journey spanning Phases 9 through 12, this message represents the point where the split API design finally matured from a promising prototype into a production-ready system. The memory backpressure mechanism — using channel capacity as a natural throttle with permit lifecycle as the enforcement mechanism — is a elegant solution to a problem that had previously required crude OOM-kill recovery.