The Decisive Experiment: How Disabling PCE Ruled Out an Entire Bug Hypothesis

"Still 0/10 valid even with PCE disabled! This is NOT a PCE bug. The problem is in the partitioned pipeline itself or the GPU proving path. PCE is innocent."

These words, written by the AI assistant in message 347 of an opencode debugging session, mark a pivotal turning point in a multi-hour investigation. After chasing a subtle bug in the Pre-Compiled Constraint Evaluator (PCE) extraction pipeline — a bug that had consumed dozens of messages, involved deep dives into constraint system internals, and required tracing through Rust and CUDA C++ code across multiple repositories — a single controlled experiment collapsed the entire hypothesis in an instant. The message is brief, almost terse, but it represents the culmination of a rigorous debugging process that demonstrates the power of systematic hypothesis elimination.

The Context: A Crash After WindowPoSt PCE Fixes

To understand why this message was written, we must first understand the investigation that led to it. The assistant had been working on implementing PCE extraction for all proof types in the CuZK proving engine — WinningPoSt, WindowPoSt, and SnapDeals. During this work, enabling PCE for WindowPoSt caused a crash due to a mismatch in input counts between the witness and the PCE. The root cause was traced to an is_extensible() flag mismatch: RecordingCS returned false while WitnessCS returned true, causing different synthesis paths. The fix involved making RecordingCS fully extensible by implementing is_extensible() and extend() methods, and correcting its initialization to pre-allocate a ONE input.

However, after deploying these fixes to a remote test host (10.1.16.218), a new problem emerged: PoRep (Proof of Replication) partitioned proofs were failing catastrophically. Every single proof was invalid — 0 out of 10 partitions passed verification. The remote host had two NVIDIA RTX 4000 Ada GPUs, while the local development machine had a single RTX 5070 Ti GPU. The local machine worked fine; the remote host did not.

The Hypothesis: Did the PCE Changes Break Everything?

The assistant's initial suspicion was entirely reasonable. The WindowPoSt fix had changed fundamental initialization behavior in both WitnessCS::new() and RecordingCS::new(). Before the fix, both started with 1 input (a pre-allocated ONE variable). After the fix, they started with 0 inputs, and the ONE was explicitly allocated via alloc_input("one") before synthesis. This was a delicate change — if the PCE file cached on disk had been extracted with the old initialization (1 input), but the new witness generation code used the new initialization (0 inputs + explicit alloc_input), the input counts would be off by one, producing garbage proofs.

The assistant spent significant effort investigating this hypothesis. It checked PCE file timestamps on the remote host (finding a file written at 10:54, after the service restart at 10:51). It examined journal logs to determine when PCE extraction occurred and whether the old or new code was used. It traced through the evaluate_pce function to understand where the assert_eq! on input counts would catch a mismatch. It even considered whether the PCE was loaded from memory from an even earlier daemon instance.

This investigation was thorough, but it was also time-consuming and complex. Each new log entry or file timestamp required interpretation, and the chain of reasoning grew increasingly tangled as the assistant tried to reconstruct the sequence of daemon restarts, PCE extractions, and proof submissions.

The Decisive Experiment

At this point, the assistant made a crucial decision. Instead of continuing to trace through logs and timestamps, it designed a simple, binary experiment: disable PCE entirely and see if proofs still fail. If the failures were caused by the PCE changes, disabling PCE would make them go away (since the standard synthesis path would be used). If the failures persisted with PCE disabled, then the PCE changes were innocent, and the bug lay elsewhere.

The experiment was executed in message 335, where the assistant added CUZK_DISABLE_PCE=1 to the systemd service file and restarted the daemon. Then, in messages 345-346, the assistant waited for a proof to complete and checked the results. The proof completed, but the verification result was damning: still 0 out of 10 partitions valid.

This is the moment captured in message 347. The assistant received the experimental result and immediately drew the correct conclusion. The message is structured as a bold declaration followed by a redirected investigation plan:

"Still 0/10 valid even with PCE disabled! This is NOT a PCE bug. The problem is in the partitioned pipeline itself or the GPU proving path. PCE is innocent."

The todowrite block shows the updated task statuses: "Check cuzk service status and recent logs" and "Identify which proof types are failing and how" are marked completed. "Diagnose root cause - NOT PCE, partitioned pipeline or GPU proving bug" is marked in progress. The investigation has been redirected from the PCE subsystem to the GPU proving pipeline.

The Thinking Process: What Made This Possible

Several factors enabled this decisive moment. First, the assistant had a clear mental model of the system architecture. It understood that the PCE path was a shortcut that replaced constraint synthesis with pre-computed matrix-vector multiplication, while the standard path performed full constraint synthesis through bellperson. These two paths converged at the GPU proving stage, which meant that disabling PCE would isolate any issues in the PCE-specific code.

Second, the assistant had access to a controlled experimental setup. The CUZK_DISABLE_PCE=1 environment variable was a kill switch designed exactly for this kind of debugging. The assistant knew how to modify the systemd service, reload the daemon, and monitor the logs for the "PCE disabled" confirmation message.

Third, the assistant was willing to let the experiment run. The proof with PCE disabled took significantly longer — the logs show synth_ms=871453 (871 seconds of synthesis time) compared to the PCE-accelerated path which would have been much faster. The assistant waited patiently for the proof to complete before drawing conclusions.

Assumptions and Potential Mistakes

The assistant made several assumptions in this investigation, most of which were reasonable but worth examining:

  1. The PCE disable flag was correctly propagated. The assistant assumed that setting CUZK_DISABLE_PCE=1 in the systemd environment would be picked up by the running process. The logs confirmed this with the message "PCE disabled via CUZK_DISABLE_PCE=1 — using standard synthesis", validating this assumption.
  2. The standard synthesis path was known to work. The assistant assumed that if PCE were the culprit, the standard path would produce valid proofs. This was a reasonable assumption given that the standard path had been working previously, but it was not independently verified on this specific code version.
  3. The failure was deterministic. The assistant assumed that the 0/10 failure rate was consistent and not a fluke. The logs from multiple daemon instances showed a consistent 100% failure rate, supporting this assumption.
  4. The local machine's success was a valid baseline. The user had mentioned that the code worked on the local machine before the WindowPoSt fix. The assistant used this as evidence that the PCE changes might be responsible, but the local machine had a single GPU while the remote host had two — a difference that would later prove critical. One subtle mistake in the earlier investigation (messages 338-344) was the assistant's assumption that the PCE file on disk must have been extracted with the old code because it predated the fix. In reality, the file was overwritten at 10:54 by the new code, and the old PCE was gone. The assistant caught this and corrected the reasoning, but it illustrates how easy it is to build elaborate theories on incomplete data.

Input Knowledge Required

To fully understand this message, a reader would need knowledge of:

Output Knowledge Created

This message created several important pieces of knowledge:

  1. PCE is exonerated. The PCE changes from the WindowPoSt fix are not responsible for the PoRep proof failures. The investigation can stop looking at constraint system initialization, input count mismatches, and PCE extraction logic.
  2. The bug is in the partitioned pipeline or GPU proving path. The problem must be in code that is shared between the PCE and standard paths, or in code that is only exercised during partitioned proving. This narrows the search space dramatically.
  3. The bug is environment-dependent. Since the local machine (single GPU) works and the remote host (dual GPU) fails, the root cause is likely related to multi-GPU handling, GPU selection, or concurrent GPU access.
  4. The failure is 100% reproducible. Every single proof fails with 0/10 valid partitions, regardless of whether PCE is enabled or disabled. This suggests a systematic issue rather than a race condition or intermittent failure.

The Broader Significance

Message 347 is a textbook example of the scientific method applied to debugging. The assistant formulated a hypothesis (the PCE changes broke PoRep proofs), designed an experiment to test it (disable PCE), executed the experiment cleanly, and interpreted the results objectively. When the experiment disproved the hypothesis, the assistant immediately accepted the result and redirected the investigation.

This is harder than it sounds. Debugging sessions often suffer from confirmation bias, where investigators subconsciously seek evidence that supports their current theory while discounting contradictory data. The assistant's willingness to declare "PCE is innocent" after investing hours in the PCE hypothesis demonstrates intellectual honesty and methodological rigor.

The message also illustrates the value of kill switches and debug flags. The CUZK_DISABLE_PCE=1 environment variable was a simple addition to the codebase, but it saved hours of debugging time by enabling a clean A/B comparison. Every complex system should have such switches at critical abstraction boundaries.

What Came Next

With the PCE hypothesis eliminated, the assistant's investigation shifted to the GPU proving path. The critical clue was the environment difference: single GPU (local) worked, dual GPU (remote) failed. This led to the discovery that the CUDA_VISIBLE_DEVICES environment variable approach for GPU selection was fundamentally broken. The C++ code (sppark's gpu_t.cuh) reads CUDA_VISIBLE_DEVICES once at static initialization time, so std::env::set_var() calls from Rust have no effect on the CUDA runtime. With num_circuits=1 (single partition), the code always selects GPU 0 via select_gpu(0) regardless of which Rust worker picks up the job. However, the Rust engine creates separate mutexes per GPU, so workers assigned to "GPU 1" use a different mutex than workers on "GPU 0" — yet all of them actually target the same physical GPU 0, allowing concurrent CUDA kernel execution without mutual exclusion and causing data races on device memory.

The fix was to use a single shared mutex for all workers when num_circuits=1, since the C++ code internally serializes all GPU work to the same physical GPU. This fix was applied to engine.rs and deployed to the remote host, resolving the issue.

But none of that would have happened without the decisive experiment in message 347. By cleanly ruling out the PCE hypothesis, the assistant freed itself to find the real bug — a GPU race condition that had nothing to do with constraint synthesis.