Chasing the Phantom: Debugging Non-Deterministic Proof Failures in CuZK's PCE Pipeline

Introduction

In the course of a complex debugging session spanning a zero-knowledge proving engine called CuZK, a single message stands out as a microcosm of the investigative process. Message 243 in the conversation captures a moment where the assistant, having already resolved a structural crash in WindowPoSt proving, confronts a far more elusive adversary: a non-deterministic proof failure that appears to strike at random. The message is deceptively simple — a single bash command issued over SSH to a remote calibnet host — but it represents a critical pivot in the investigation, a moment where the assistant shifts from analyzing the symptom (randomly invalid partitions) to hunting for its root cause in the system's initialization state.

The Message in Full

The assistant writes:

[assistant] I don't see the startup/PCE loading logs. The service probably started earlier. Let me check when it started:
[bash] ssh 10.1.16.218 'sudo journalctl -u cuzk --no-pager -n 5000 2>&1 | grep -E "cuzk.*starting|daemon.*starting|PCE.*loaded|PCE.*disk|engine.*started|num_inputs" | head -20'
Mar 02 10:38:17 cs-calib cuzk[442807]: 2026-03-02T10:38:17.475300Z  INFO verify_porep_partitions{job_id="porep-181521-32"}: cuzk_core::prover: starting per-partition verification job_id="porep-181521-32" num_partitions=10
Mar 02 10:39:04 cs-calib cuzk[442807]: 2026-03-02T10:39:04.852127Z  INFO verify_porep_partitions{job_id="porep-181521-8"}: cuzk_core::prover: starting per-partition verification job_id="porep-181521-8" num_partitions=10
Mar 02 10:40:48 cs-calib cuzk[442807]: 2026-03-02T10:40:48...

The command searches the last 5000 lines of the cuzk service journal for any mention of startup events, PCE loading from disk, or input dimensions. The results are telling: none of the expected startup logs appear. Only verification activity logs are visible, confirming that the service had been running long before the captured journal window.

Why This Message Was Written

To understand why the assistant issued this particular command, one must trace the chain of reasoning that led to this point. The user had deployed the CuZK proving engine to a remote calibnet host and reported a baffling problem: PoRep (Proof-of-Replication) proofs were failing with a pattern of random partition invalidity. On one run, 7 out of 10 partitions would validate; on a retry with the same proof data, only 1 out of 10 would pass. This non-deterministic behavior was the smoking gun — it ruled out a pure synthesis bug, which would produce deterministic failures, and pointed instead to something in the runtime state.

The assistant's investigation in the preceding messages (233–242) had already established several critical facts:

  1. The host was running a "slightly stale build" — the user's own words — meaning the binary on the remote machine might not include the latest fixes to WitnessCS::new() and RecordingCS::new() that had been developed to harmonize constraint system types.
  2. The PCE (Pre-Compiled Constraint Evaluator) fast path was active — every partition synthesis log showed "using PCE fast path for synthesis circuit_id=porep-32g," confirming that the PCE extraction and caching mechanism was operational.
  3. The failure pattern was non-deterministic — partitions 0, 8, and 9 failed in one run, while a different set failed on retry. This randomness strongly suggested a data race, a stale or corrupted PCE cache, or a GPU memory contention issue.
  4. The PoRep pipeline uses partitioned GPU proving — with 6 partition workers and 2 GPU workers per device, there was ample opportunity for concurrent access issues. The assistant had already hypothesized that the PCE on disk might have been generated with the old RecordingCS::new() code, which pre-allocated a ONE input. If the PCE was generated with one set of dimensions but the runtime synthesis expected different dimensions, the mismatch could produce subtly corrupted proofs. However, the assistant also recognized that PoRep doesn't use synthesize_extendable (the function that triggered the WindowPoSt crash), so the WitnessCS::new() fix might not directly affect PoRep. This ambiguity needed resolution. The message at index 243 was therefore written to answer a specific question: What PCE data is actually loaded on this host, and was it generated with the old or new code? The assistant needed to see the startup logs to determine: - Whether the PCE was loaded from disk (pre-existing, possibly stale) or generated fresh at startup - The number of inputs (num_inputs) the PCE expects - Whether there were any warnings or errors during PCE loading

The Reasoning and Decision-Making Process

The assistant's thinking, visible in the preceding messages, reveals a systematic diagnostic approach. After observing the non-deterministic failures, the assistant considered several hypotheses:

Hypothesis 1: Stale PCE data. The PCE file on disk might have been generated with the old RecordingCS initialization that pre-allocated a ONE input. If the runtime code now expects zero pre-allocated inputs (after the harmonization fix), the PCE evaluation would produce incorrect witness assignments. This would explain why some partitions pass and others fail — the mismatch might only manifest for certain circuit configurations.

Hypothesis 2: Data race in GPU proving. The partitioned pipeline spawns multiple GPU workers concurrently. If two workers share GPU state without proper synchronization, they could corrupt each other's proofs. The pattern of only the first few partitions succeeding (1, 2, 3, 4, 5, 6, 7 in one run) while later ones failed suggested a progressive corruption — exactly the signature of a race condition.

Hypothesis 3: Randomness issue in r_s/s_s generation. Each partition generates its own random r_s and s_s values for the Groth16 randomization. If the random number generator was improperly seeded or shared across threads, partitions could end up with identical or correlated randomness, producing invalid proofs.

The assistant had already ruled out Hypothesis 3 by examining the code and confirming that each partition independently generates its own randomness. Hypothesis 2 remained viable but would require deeper investigation of the GPU worker synchronization code. Hypothesis 1 was the most immediately testable — and that's what message 243 aimed to test.

The decision to run journalctl with -n 5000 (the last 5000 lines) rather than searching the full journal was a practical one. The service had been running for some time, and the full journal could be enormous. By taking the last 5000 lines and filtering for startup-related keywords, the assistant hoped to catch any initialization messages that fell within the journal's recent buffer. The head -20 further limited output to avoid flooding the terminal.

Assumptions Embedded in the Message

Every diagnostic step rests on assumptions, and this message is no exception. The assistant assumed that:

  1. The PCE loading would produce log messages. This assumption was reasonable — the codebase uses structured logging extensively, and the PCE loading path in pipeline.rs includes info!-level log statements. However, if the logging level was set higher (e.g., warn or error only) on the deployed binary, the messages might not appear.
  2. The journal would contain startup logs. The journalctl command captured the last 5000 lines, but if the service had been running for hours or days, the startup logs would have rotated out. The assistant acknowledged this possibility with the preamble "The service probably started earlier."
  3. The stale build hypothesis was worth pursuing. The assistant had already done significant work to fix the WitnessCS::new() initialization, and it was natural to suspect that a pre-fix build might be the culprit. However, as the assistant noted, PoRep doesn't use synthesize_extendable, so the fix might not be relevant to this particular failure mode.
  4. The non-deterministic pattern was the key diagnostic signal. The assistant correctly identified that random valid/invalid patterns point away from deterministic bugs (like incorrect circuit synthesis) and toward runtime state issues. This assumption guided the entire investigation.

Mistakes and Incorrect Assumptions

The message itself doesn't contain explicit mistakes — it's a diagnostic command that returns useful negative information. However, the broader investigation reveals a subtle incorrect assumption that becomes apparent only in hindsight: the assistant assumed that the PCE loading logs would be visible in the journal at all. The service had apparently been running for an extended period, and the startup logs had rotated out of the 5000-line buffer. This meant the command returned only verification activity logs, not the initialization data the assistant sought.

This is not a mistake per se — it's a natural limitation of working with remote logs. But it forced the assistant to pivot to alternative investigative strategies in subsequent messages (checking the PCE files directly on disk, examining the build timestamp, etc.).

Another assumption that proved partially incorrect was that the stale build was the most likely cause. While the stale build did contribute to some issues (as later chunks reveal), the root cause of the random partition failures turned out to be a pre-existing GPU pipeline race condition unrelated to the PCE changes. The assistant's focus on the PCE data, while reasonable, temporarily diverted attention from the GPU worker synchronization code.

Input Knowledge Required to Understand This Message

To fully grasp what the assistant is doing here, a reader needs to understand several layers of context:

The CuZK proving engine architecture. CuZK is a GPU-accelerated zero-knowledge proving engine that supports multiple proof types (WinningPoSt, WindowPoSt, SnapDeals, PoRep). It uses a Pre-Compiled Constraint Evaluator (PCE) to cache the constraint system structure, avoiding redundant synthesis work across proofs of the same type. The partitioned pipeline splits proof work across multiple GPU workers for parallelism.

The PCE extraction and caching mechanism. When a proof type is first encountered, CuZK "extracts" the circuit structure into a PCE file on disk. Subsequent proofs of the same type can skip full synthesis and instead use the PCE fast path, which replays the constraint evaluation using cached structure data. This is where the RecordingCS and WitnessCS types come in — they record and replay constraint structures respectively.

The WitnessCS/RecordingCS harmonization fix. In the preceding segment, the assistant discovered that RecordingCS and WitnessCS had different initialization behaviors: RecordingCS::new() started with zero inputs while WitnessCS::new() pre-allocated a ONE input. This mismatch caused the WindowPoSt crash when synthesize_extendable created child constraint systems. The fix made both types start with zero inputs, with the ONE input allocated explicitly by the caller.

The remote deployment context. The host at 10.1.16.218 is a calibnet (calibration network) node running the CuZK daemon as a systemd service. It has passwordless sudo access and uses a specific configuration (/etc/cuzk.toml) that preloads the porep-32g PCE and configures GPU workers.

The journalctl tool and systemd logging. The assistant uses journalctl -u cuzk to access the systemd journal for the cuzk service. The --no-pager flag prevents interactive pagination, and -n 5000 limits output to the last 5000 lines. The 2>&1 redirects stderr to stdout to capture all output.

Output Knowledge Created by This Message

The primary output of this message is negative knowledge: the confirmation that the PCE loading logs are not present in the recent journal. This is valuable information that shapes the subsequent investigation:

  1. The service started much earlier than the journal window. The absence of startup logs means the service has been running for a long time, or the journal has been rotated. This rules out the simple approach of reading PCE dimensions from startup logs.
  2. Alternative investigation paths are needed. Since the journal doesn't contain the desired information, the assistant must either restart the service (to generate fresh startup logs), examine the PCE files directly on disk, or check the build timestamp of the running binary.
  3. The PCE is actively being used. The logs show "using PCE fast path" for every partition, confirming that the PCE cache is populated and functional. The issue is not that the PCE is missing or failing to load — it's that the loaded PCE might produce incorrect results.
  4. The stale build hypothesis remains viable but unconfirmed. Without seeing the PCE dimensions, the assistant cannot determine whether the PCE was generated with old or new code. This ambiguity keeps the hypothesis alive but forces the investigation to consider other possibilities as well.

The Thinking Process in the Reasoning

The assistant's reasoning, visible in the preceding messages and the brief preamble to the bash command, reveals a structured diagnostic methodology. The assistant:

  1. Observes the symptom: Random partition invalidity with a non-deterministic pattern.
  2. Identifies what the symptom rules out: Pure synthesis bugs are deterministic, so the randomness must come from runtime state.
  3. Generates hypotheses: Stale PCE data, GPU race condition, randomness issue in r_s/s_s.
  4. Tests the most tractable hypothesis first: Check the PCE loading state by examining startup logs.
  5. Adapts when the test returns inconclusive results: The absence of startup logs doesn't disprove the PCE hypothesis, but it forces a pivot to alternative investigation methods. This is textbook debugging methodology — start with the most testable hypothesis, gather data, and let the results guide the next step. The assistant doesn't commit to any single theory but systematically collects evidence. The preamble "I don't see the startup/PCE loading logs. The service probably started earlier" is particularly telling. It shows the assistant is already processing the negative result before the command finishes — the absence of expected log messages is itself a signal. The assistant then reframes the question: instead of "what does the startup log say?" it becomes "when did the service start?" The command is adjusted accordingly, searching for any evidence of the service's start time.

Conclusion

Message 243 is a small but pivotal moment in a complex debugging session. It represents the transition from broad hypothesis generation to targeted data collection, and it demonstrates the assistant's ability to adapt its investigative strategy when initial assumptions prove unfruitful. The message also illustrates a fundamental truth about debugging distributed systems: the most valuable information is often what you cannot find. The absence of PCE loading logs in the journal didn't solve the mystery, but it narrowed the search space and set the stage for the deeper investigation that would eventually isolate the GPU pipeline race condition as the true culprit.

In the broader narrative of the CuZK development session, this message marks the point where the assistant recognizes that the easy answers (stale build, PCE mismatch) don't fit the evidence, and begins the harder work of tracing through the GPU proving pipeline. It's a reminder that in systems debugging, the path to the root cause is rarely straight — it's a spiral of hypotheses, tests, and refinements, each turn bringing you closer to the truth.