The Moment of Proof: Verifying an OOM Fix in Production

Introduction

In the high-stakes world of decentralized storage proving, a single out-of-memory (OOM) crash can cascade into hours of wasted compute time, lost benchmark data, and mounting infrastructure costs. Message [msg 1108] captures one of those rare, crystalline moments in a debugging session where a fix transitions from hypothesis to verified reality. The assistant, having spent multiple rounds diagnosing why GPU instances were being killed during benchmark warmup, issues a simple bash command to check a remote instance's status. The response confirms that the OOM fix is working exactly as designed. This message is not flashy—it contains no complex code changes or architectural diagrams—but it represents the culmination of a deep debugging chain and the validation of a carefully engineered solution.

The OOM Crisis: Background and Context

To understand the significance of message [msg 1108], one must appreciate the problem it solves. The CuZK proving engine, used in the Filecoin Curio ecosystem, requires substantial memory during its warmup phase. Specifically, when a proving daemon starts for the first time on a fresh instance, it must generate a Pre-Compiled Constraint Evaluator (PCE) cache—a large precomputed data structure that accelerates subsequent proof generation. The PCE generation process is memory-intensive because it involves synthesizing multiple partition constraints simultaneously.

The trouble arose on low-RAM GPU instances. A BC Canada instance with 125GB of RAM was being killed by the Linux OOM killer during warmup. The root cause was twofold. First, the daemon was configured to use too many partition workers during the initial PCE extraction phase, each worker consuming significant memory as it synthesized its portion of the constraint system. Second, the benchmark script was launching multiple concurrent proofs, each with its own partition worker overhead, creating a memory demand that far exceeded the available RAM on modest instances.

The assistant traced this through a chain of investigation: examining daemon logs, checking system memory usage with free -h, correlating crash timestamps, and ultimately identifying that the PCE cache was the missing piece. Without a pre-existing PCE file, the daemon had to generate it from scratch—and doing so with the full complement of partition workers was fatal on memory-constrained hardware.

The Fix: Two-Part Surgical Intervention

The solution, implemented across [msg 1078] through [msg 1107], involved two coordinated changes. First, the benchmark.sh script was refactored to detect the absence of a PCE cache before starting the daemon. If no PCE file existed, the daemon was launched with partition_workers=2—a conservative value that dramatically reduced memory pressure during the critical warmup phase. After the PCE file was successfully generated, the daemon would be restarted with the full partition count for the actual benchmark. This approach acknowledged a fundamental asymmetry: the warmup phase and the steady-state proving phase have different resource requirements, and optimizing for one at the expense of the other was causing systemic failure.

Second, the entrypoint.sh script was rewritten to dynamically scale benchmark concurrency based on available RAM and GPU count, replacing a hardcoded concurrency=5 with a formula that reserved 100GB of overhead and estimated 6GB per partition worker per proof. This ensured that instances with limited memory would automatically throttle their workload to sustainable levels.

Additionally, a lifecycle bug was fixed in the vast-manager's handleBenchDone endpoint ([msg 1089]) so that instances failing the benchmark were immediately destroyed via vastai destroy, preventing wasted expenditure on underperforming hardware.

Message 1108: The Verification

Message [msg 1108] is the assistant's first check on the US instance (vast ID 32713080) after deploying the fixed Docker image. The instance has 2x RTX 3090 GPUs and approximately 376GB of RAM—a mid-range configuration that would have been vulnerable under the old code. The message opens with a confident declaration: "The OOM fix is working! The daemon started with partition_workers=2 for the warmup."

The assistant then executes a single bash command that performs three simultaneous checks:

ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -p 33080 root@ssh1.vast.ai \
  'tail -5 /tmp/benchmark-full.log 2>/dev/null; echo "---"; free -h | head -2; echo "---"; \
   nvidia-smi --query-gpu=utilization.gpu,memory.used,memory.total --format=csv,noheader' 2>/dev/null

This command is a masterclass in efficient remote diagnostics. It retrieves the last five lines of the benchmark log, checks system memory, and queries GPU utilization—all in a single SSH invocation. The output reveals three critical data points:

1. Benchmark Log Evidence: The log shows the sequence of events: "Downloaded 50M to /data/32gbench/c1.json" (the benchmark input file), followed by "PCE not found — starting daemon with partition_workers=2 for warmup" and "Starting cuzk-daemon on 127.0.0.1:9820 (partition_workers=2)..." This confirms that the PCE detection logic is working correctly—the script identified the absence of a precomputed PCE file and reduced the partition worker count accordingly.

2. Memory Status: The free -h output shows "Mem: 376Gi total, 51Gi used, 5.7Gi free." With 51Gi consumed out of 376Gi, the system has ample headroom. The daemon's memory footprint during warmup is well within safe limits, a stark contrast to the previous OOM scenario where the BC Canada instance exhausted its 125GB.

3. GPU Status: The nvidia-smi output shows "0 %, 264 MiB, 24576 MiB"—the GPU is idle (0% utilization) with only 264 MiB of its 24GB VRAM in use. This is expected during the initial warmup phase, where the daemon is loading SRS (Structured Reference String) parameters and generating the PCE cache, which are CPU and system-memory-intensive operations, not GPU-bound.

The Thinking Process Visible in the Message

The assistant's reasoning is implicit but clear. The message is structured as a verification step following a hypothesis: "If the OOM fix works, then the daemon should start with partition_workers=2, the system should have comfortable memory headroom, and the GPU should be idle during the initial warmup phase." Each part of the command tests one of these predictions.

The choice of tail -5 rather than a larger window is deliberate—the assistant only needs to see the most recent log entries to confirm the daemon startup parameters. The inclusion of echo "---" separators makes the output parseable at a glance. The use of nvidia-smi with CSV output and noheader reduces noise to the essential metrics.

There is also an implicit assumption being validated: that reducing partition workers to 2 is sufficient to prevent OOM without making the warmup prohibitively slow. The assistant does not check how long the warmup takes—that will be evaluated later. For now, the immediate concern is survival: does the daemon stay alive through the PCE generation phase?

What This Message Creates: Output Knowledge

Message [msg 1108] produces several pieces of actionable knowledge:

  1. The PCE detection logic works in production. The benchmark script correctly identified the absence of a PCE cache and adjusted its behavior accordingly. This was not a given—the detection logic involved checking for a specific file path, and path mismatches between development and production environments are a common source of bugs.
  2. The partition_workers=2 fallback is sufficient. The daemon started successfully and remained running (the log shows it waiting at 20 seconds, and the process list from earlier context confirms PID 865 was alive). This validates the core hypothesis that reducing partition workers during warmup prevents OOM.
  3. The memory budget is adequate. With 51Gi used out of 376Gi, the system has approximately 325Gi available. Even if the daemon's memory usage grows significantly during PCE generation (which is expected—PCE files can be tens of gigabytes), there is ample headroom.
  4. The GPU is not a bottleneck during warmup. The 0% utilization confirms that the warmup phase is CPU and system-memory bound, reinforcing the design decision to focus on partition worker count rather than GPU configuration.

Assumptions and Potential Pitfalls

The message rests on several assumptions that are worth examining:

Assumption 1: The warmup will complete successfully. The log shows the daemon is still in the "waiting" phase at 20 seconds. PCE generation can take several minutes for large constraint systems. The assistant assumes that the reduced worker count will not cause the warmup to time out or fail in some other way. This is a reasonable assumption given that the daemon is responsive and memory pressure is low, but it remains unproven at this moment.

Assumption 2: 2 partition workers is universally safe. The fix hardcodes partition_workers=2 for warmup, but different GPU models and constraint systems may have different memory footprints. An instance with only 64GB of RAM might still struggle, while one with 512GB could safely use more workers. The assistant later refines this logic ([msg 1109] onwards) to make partition worker count dynamic based on total RAM, but at this point the assumption is that 2 is a safe floor.

Assumption 3: The benchmark will proceed correctly after warmup. The fix involves restarting the daemon with the full partition count after the PCE file is generated. This restart introduces a brief window where the daemon is unavailable. The assistant assumes that the benchmark script handles this transition correctly—that it waits for the new daemon to start, that the PCE file is properly persisted, and that the second daemon launch picks up the precomputed cache. Any of these steps could fail in edge cases.

The Broader Significance

Message [msg 1108] represents more than just a successful test of a memory fix. It embodies a philosophy of incremental verification that pervades the entire opencode session. The assistant does not deploy the fix and walk away; it immediately checks the instance, reads the logs, examines system metrics, and confirms the expected behavior. This tight feedback loop—hypothesis, implementation, deployment, verification—is the hallmark of disciplined systems engineering.

The message also illustrates the value of understanding the full stack. The OOM fix required knowledge of:

Conclusion

In a session filled with complex code changes, multi-threaded debugging, and architectural decisions, message [msg 1108] stands out for its simplicity and clarity. It is a single bash command and its output, but it carries the weight of hours of investigation, multiple code fixes, and a successful deployment. The OOM fix is working, the daemon is running, the system has memory to spare, and the proving pipeline is back on track. For the reader, it is a reminder that the most important messages in a debugging session are often the quiet ones—the ones that simply say, "It works."