The Warmup That Didn't Crash: Validating an OOM Fix in Real-Time

In the middle of a marathon debugging session spanning Docker builds, lifecycle management, and hardware-aware configuration, message [msg 1113] stands as a quiet moment of validation. The assistant, having spent multiple rounds tracing and fixing an Out of Memory (OOM) crash that plagued low-RAM GPU instances during cuzk PoRep proving benchmarks, finally observes the fix working in the wild. The message is deceptively simple—a status report and a follow-up command—but it carries the weight of hours of debugging, a strategic pivot from hardcoded thresholds to dynamic configuration, and the satisfaction of a hypothesis confirmed by real hardware.

The Message in Full

The assistant writes:

PCE file was created (26GB) and the warmup proof is running. The daemon is processing partitions on the GPU. Memory usage is stable at 92GB (down from 107GB earlier — the PCE extraction peak passed). The OOM fix is working perfectly — with partition_workers=2, the machine handled PCE extraction fine.

Let me wait for the warmup proof to complete and the daemon to restart with full workers:

[bash] sleep 120 && ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -p 33080 root@ssh1.vast.ai 'tail -30 /tmp/benchmark-full.log 2>/dev/null' 2>/dev/null

GPU workers/device: 2 GPU threads: 32 Pipeline: true ================================================================

PCE file not found — warmup will trigger extraction. Daemon is running with reduced partition_workers to avoid OOM. This takes 2-5 minutes on first run... Warmup attempt 1... [2m2026-03-12T02:05:25.086432Z[0m [32m INFO[0m [2mcuzk_bench[0m[2m:[0m loading C1 output [3mpath[0m[2m=[0m/data/32gbench/c1.json [2m2026-03-12T02:05:25.135443Z[0m [...

Context: The OOM Crisis

To understand the significance of this message, one must appreciate what preceded it. The assistant had deployed cuzk proving instances on Vast.ai, a GPU rental marketplace, to benchmark PoRep (Proof of Replication) performance. The system worked well on high-RAM machines like the Norway instance (500GB RAM, 1x RTX 4090), but a BC Canada instance with only 125GB RAM was being killed by the kernel's OOM killer during the very first proof attempt.

The root cause was a memory explosion during PCE (Pre-Compiled Constraint Evaluator) extraction. PCE is a critical optimization in the cuzk proving engine: it pre-compiles the circuit constraints into a GPU-friendly format, avoiding repeated synthesis overhead. But generating the PCE file requires loading the entire circuit, synthesizing all constraints, and writing a multi-gigabyte cache file (the final PCE was 26GB). If the daemon was configured with too many partition workers—the threads that synthesize constraint partitions in parallel—the memory spike during PCE extraction would exceed available RAM and trigger an OOM kill.

The assistant's fix, implemented across two files (benchmark.sh and entrypoint.sh), was elegantly simple: detect whether the PCE cache file already exists before starting the daemon. If it does not exist, start the daemon with partition_workers=2—a minimal configuration that keeps memory pressure low during the expensive extraction phase. After the PCE file is generated and the daemon has finished its warmup proof, restart the daemon with the full partition worker count for the actual benchmark. This "two-phase startup" decouples the memory-intensive extraction from the throughput-oriented benchmarking.

What the Message Reveals

The assistant's observations in [msg 1113] confirm that this strategy works. Three data points stand out:

The PCE file was created (26GB). This is the first validation gate. The extraction completed successfully without crashing the instance. On a machine with 376GB total RAM, the daemon peaked at 107GB during extraction (as noted in the comparison to earlier readings) and then settled to 92GB after the peak passed. The 26GB PCE file on disk represents the permanent artifact of this extraction—future runs on this instance will skip the extraction entirely and jump straight to proving.

Memory usage is stable at 92GB. This is well within the 376GB available, leaving 284GB for the operating system, the benchmark client, and GPU driver overhead. The "down from 107GB" detail is particularly telling: it shows the memory spike during PCE extraction was real but bounded. With partition_workers=2, the spike reached 107GB; with the old configuration of partition_workers=10 or partition_workers=16, it would have been proportionally larger—likely exceeding the 125GB that killed the BC Canada instance.

The warmup proof is running. The daemon is processing partitions on the GPU, as shown by the daemon log entries about GPU prove completion. The benchmark log confirms the warmup sequence: "PCE file not found — warmup will trigger extraction. Daemon is running with reduced partition_workers to avoid OOM. This takes 2-5 minutes on first run..." This is exactly the flow the assistant designed.

The Reasoning Behind the Follow-Up Command

The assistant doesn't stop at observation. It issues a sleep 120 command followed by a tail of the benchmark log, intending to check whether the warmup proof completes and the daemon restarts with full workers. This reveals the assistant's mental model of the fix's lifecycle:

Phase 1 (just observed): Daemon starts with partition_workers=2, PCE extraction runs, warmup proof begins.
Phase 2 (about to check): Warmup proof completes, benchmark.sh detects the PCE file now exists, kills the daemon, and restarts it with the full partition worker count (10, in this case, auto-configured based on the machine's 376GB RAM).
Phase 3 (future): The actual 12-proof benchmark runs at full speed, measuring proofs/hour. The assistant is verifying that the transition from Phase 1 to Phase 2 works correctly. This is the critical handoff: if the daemon fails to restart, or if the restart causes a crash, the fix is incomplete. The 120-second sleep is calibrated to the expected 2-5 minute warmup duration mentioned in the benchmark log.

Assumptions and Their Validity

The message rests on several assumptions, most of which are well-founded:

The PCE file is durable. The assistant assumes that once written, the 26GB PCE file will persist across daemon restarts. This is reasonable—the file is written to a persistent volume (/var/tmp/filecoin-proof-parameters/) and is not tied to the daemon process. However, there is an implicit assumption that no other process or cleanup mechanism will delete it between the warmup and the benchmark restart.

Memory usage will decrease after PCE extraction. The assistant notes memory dropping from 107GB to 92GB, attributing this to the PCE extraction peak passing. This is correct: PCE extraction requires holding the entire circuit representation in memory while writing the cache file, after which those structures can be freed. The remaining 92GB represents the daemon's steady-state memory for SRS loading, GPU buffers, and partition worker threads.

The daemon will restart cleanly. The benchmark script's logic for killing and restarting the daemon assumes that kill $DPID followed by a fresh invocation of cuzk will work without port conflicts, stale state, or race conditions. This is a reasonable assumption for a controlled benchmark environment, but in production, a rolling restart with health checks would be more robust.

The machine has sufficient RAM for full workers. The assistant earlier noted that the vast API reported cpu_ram: 76800 (75GB), but the actual system has 376GB. The auto-configuration logic in entrypoint.sh correctly used the real free -g value, not the API value, so partition_workers=10 was chosen. This assumption—that the system's actual RAM is the correct basis for configuration—is validated by the stable memory readings.

Input Knowledge Required

To fully understand this message, one needs familiarity with several domains:

The cuzk proving engine. cuzk is a GPU-accelerated zk-SNARK prover for Filecoin's proof types (PoRep, WinningPoSt, WindowPoSt, SnapDeals). It uses a partitioned synthesis approach where constraint partitions are processed in parallel by worker threads. The PCE (Pre-Compiled Constraint Evaluator) is a cache file that stores the compiled circuit structure, avoiding repeated synthesis.

Vast.ai and GPU instance management. Vast.ai is a decentralized GPU rental marketplace. Instances are identified by numeric IDs, accessed via SSH tunnels through a proxy, and managed through a custom vast-manager service that tracks lifecycle states (registered, benchmarking, killed).

Linux memory management and OOM behavior. The OOM killer activates when the system's available memory plus swap is exhausted. In the BC Canada failure, the daemon's memory allocation during PCE extraction exceeded the 125GB physical RAM, triggering an OOM kill. The fix works by reducing parallelism during the memory-intensive phase.

The benchmark workflow. benchmark.sh orchestrates the entire proving benchmark: downloading C1 input data, starting the cuzk daemon, submitting proofs via cuzk-bench, and recording the proofs/hour rate. The two-phase startup is a modification to this script.

Output Knowledge Created

This message produces several valuable outputs:

Empirical validation of the OOM fix. The most important output is proof that the two-phase startup strategy works on real hardware. The PCE extraction completed, memory stayed within bounds, and the warmup proof began executing. This is not theoretical—it's a live observation from a production-like environment.

A concrete data point for memory profiling. The assistant records that PCE extraction on a 32GiB PoRep proof with partition_workers=2 peaks at ~107GB and settles to ~92GB. This is useful for capacity planning: it tells operators that a machine needs at least ~120GB of RAM for the extraction phase, and ~100GB for steady-state proving, per GPU.

Confirmation of the auto-configuration logic. The instance correctly configured itself with partition_workers=10 and concurrency=5 based on its 376GB RAM and 2 GPUs. This validates the hardware-aware formulas in entrypoint.sh.

A baseline for the benchmark run. The assistant is about to check whether the warmup completes and the full benchmark proceeds. The outcome of that check will determine whether the fix is complete or needs further refinement.

Mistakes and Subtle Issues

While the message is largely correct, there are subtle points worth examining:

The "down from 107GB" comparison is slightly misleading. The 107GB reading was taken at a different point in the daemon's lifecycle than the 92GB reading. The 107GB reading (from [msg 1110]) showed used: 107Gi with available: 268Gi, while the 92GB reading in this message shows used: 92Gi with available: 284Gi. The 15GB difference could be due to PCE structures being freed, but it could also be due to kernel buffer cache fluctuation or the daemon's SRS loading completing and some mappings being released. The assistant attributes it specifically to "the PCE extraction peak passed," which is a reasonable but not definitively proven explanation.

The warmup proof is still running. The assistant hasn't yet confirmed that the warmup proof completes successfully. The daemon could still crash during the GPU prove phase, or the benchmark script could fail to detect the PCE file and restart correctly. The message is a mid-flight status report, not a final validation. The assistant's follow-up command acknowledges this uncertainty.

The 26GB PCE file size is for a 32GiB PoRep proof. Different proof types (WinningPoSt, WindowPoSt, SnapDeals) will produce PCE files of different sizes. The assistant's fix is generic—it applies to any proof type—but the memory requirements will scale with the circuit size. A larger proof type could still OOM even with partition_workers=2 if the circuit itself is too large for the available RAM.

The instance has 376GB RAM, not 75GB as the vast API reported. This discrepancy between the API's cpu_ram field and the actual free -g output is a known quirk of Vast.ai's reporting. The assistant's auto-configuration logic correctly uses the real value, but the API discrepancy could cause issues if other parts of the system rely on the API value for decision-making.

The Thinking Process Visible in the Message

The assistant's reasoning is visible in the structure of the message itself. It begins with a summary of observations ("PCE file was created... memory usage is stable... the OOM fix is working perfectly"), then transitions to a verification step ("Let me wait for the warmup proof to complete and the daemon to restart with full workers"). This pattern—observe, conclude, then verify the next phase—is characteristic of systematic debugging.

The assistant is also implicitly comparing against a counterfactual: the BC Canada instance that OOM-killed. The subtext of "the machine handled PCE extraction fine" is "unlike the previous instance that crashed." The assistant is running a mental diff between the failed configuration (full partition workers during PCE extraction on 125GB RAM) and the working configuration (reduced partition workers during PCE extraction on 376GB RAM).

The choice of a 120-second sleep is also revealing. The assistant knows from the benchmark log's message ("This takes 2-5 minutes on first run...") that the warmup is expected to take 2-5 minutes. By sleeping 120 seconds (2 minutes), the assistant is checking at the earliest plausible completion time. If the warmup finishes faster than expected, the log will show the restart; if it's still running, the assistant will see progress and can wait longer. This is a pragmatic trade-off between responsiveness and patience.

Conclusion

Message [msg 1113] is a milestone in a larger debugging journey. It captures the moment when a carefully designed fix meets real hardware and passes the first test. The assistant doesn't declare victory—there are still verification steps ahead—but the evidence is compelling: the PCE file exists, memory is stable, and the warmup proof is running. The two-phase startup strategy, born from the ashes of a crashed BC Canada instance, is working.

What makes this message interesting is not just the technical content but the mindset it reveals. The assistant is simultaneously an engineer (designing the fix), an operator (deploying and monitoring it), and a scientist (observing the results and forming hypotheses). The message is a status report, yes, but it's also a reasoning trace—a window into how a complex distributed system is debugged, one observation at a time.