The PCE Cache Hypothesis: Tracing a Subtle Off-by-One Bug in Zero-Knowledge Proof Generation

Message Overview

In the midst of a high-stakes debugging session, the assistant issues the following grep command to explore how the Pre-Compiled Constraint Evaluator (PCE) data is cached on disk:

[grep] pce.*cache|PCE.*disk|pce.*path|save_pce|load_pce|pce_dir|pce_cache
Found 27 matches
/tmp/czk/extern/cuzk/cuzk-core/src/pipeline.rs:
  Line 329: /// Map a CircuitId to the filename stem used for PCE disk persistence.
  Line 342: fn pce_disk_path(circuit_id: &CircuitId, param_cache: &std::path::Path) -> std::path::PathBuf {
  Line 346: /// Try to load a PCE from disk into the OnceLock cache.
  Line 351: pub fn load_pce_from_disk(
  Line 357:         info!(circuit_id = %circuit_id, "PCE already in memory, skipping disk load");
  Line 361:     let path = pce_disk_p...

At first glance, this appears to be a routine code search. But in the context of the unfolding investigation, this grep represents a critical pivot point—a moment when the assistant connects a user's offhand remark to a concrete, testable hypothesis about why every single PoRep proof on a remote multi-GPU machine is failing with 100% certainty.

The Debugging Context: A 100% Failure Rate

To understand why this grep matters, we must step back into the broader investigation. The assistant has been debugging a severe issue on a remote test host (10.1.16.218) equipped with two RTX 4000 Ada GPUs. The PoRep (Proof of Replication) partitioned proofs—a core component of the Filecoin storage verification system—are failing at an alarming rate. Every single proof is invalid. Not a statistical fluke, not an intermittent race condition, but a systematic, 100% failure rate.

Earlier in the session ([msg 319]), the assistant had observed this pattern and initially suspected the PCE fast path, which had recently been enabled for all proof types including PoRep. The PCE path is an optimization that bypasses full circuit synthesis by pre-computing the constraint structure (the R1CS matrices A, B, C) once per circuit topology, then reusing those pre-computed matrices across all proofs with different witness values. Instead of running expensive enforce() closures that build and evaluate millions of LinearCombination objects, the PCE path runs only the alloc() closures to capture witness values, then evaluates a = A*w, b = B*w, c = C*w via sparse matrix-vector multiplication.

The assistant had tested this hypothesis by disabling PCE entirely via CUZK_DISABLE_PCE=1 and restarting the service ([msg 335]). The result was telling: even with PCE disabled, proofs continued to fail at the same 100% rate. This conclusively ruled out the PCE path as the sole culprit, but it didn't explain why the proofs were failing.

The User's Critical Clue

Then came message [msg 337], where the user offered a seemingly casual observation:

"Of note, before fixing WindowPoSt PCE in recent commits cuzk was working on the Local machine, not sure if remote too tho"

This remark is the key that unlocks the investigation. The user is pointing out that the recent WindowPoSt PCE fix—which the assistant had implemented in the preceding segment (Segment 0)—changed the initialization behavior of two critical data structures: WitnessCS::new() and RecordingCS::new(). Before the fix, both constructors started with 1 input (pre-allocating the constant ONE variable at index 0). After the fix, they started with 0 inputs, and the caller was made responsible for explicitly allocating the ONE input via alloc_input("one").

The user's remark suggests that before this change, the system was working correctly on the local development machine. The implication is profound: the WindowPoSt fix may have inadvertently broken PoRep by introducing an inconsistency between the cached PCE data (extracted with the old initialization) and the new witness generation code.

The Hypothesis: Stale PCE Cache

This is where message [msg 339] enters the narrative. The assistant receives the user's clue and immediately pivots to investigate a specific mechanism: disk-based PCE caching. The reasoning chain is:

1. PCE data is cached on disk. The extract_precompiled_circuit function runs once per circuit topology and serializes the resulting PreCompiledCircuit (containing CSR matrices A, B, C and density bitmaps) to disk. Subsequent proofs load this cached data rather than re-extracting the circuit structure.
2. The cache was populated before the WindowPoSt fix. On the remote host, the PCE cache files were created during an earlier deployment when RecordingCS::new() started with 1 input. The cached matrices therefore have num_inputs = 1 (the pre-allocated ONE) plus whatever inputs the circuit allocates during synthesis.
3. The new code expects a different layout. After the fix, WitnessCS::new() starts with 0 inputs. The synthesize_with_pce function explicitly calls cs.alloc_input("one") before synthesis, adding ONE at index 0. The witness vector is built accordingly: w[0] = ONE, w[1..num_inputs] = input assignments, w[num_inputs..] = aux assignments.
4. The mismatch. If the cached PCE has num_inputs_old = 1 (from the old RecordingCS::new()) plus the circuit's own inputs, but the new WitnessCS produces a witness with num_inputs_new = 1 (from explicit alloc_input("one")) plus the circuit's own inputs, then num_inputs_old == num_inputs_new and the indexing should match. But if the old RecordingCS::new() started with 1 input AND the extraction code also called alloc_input("one") (which it now does explicitly), the old code might have had 2 copies of ONE—one from the constructor and one from the explicit call. This would shift all subsequent variable indices by 1, causing the CSR matrix column indices to point to the wrong witness elements. The grep command in message [msg 339] is the assistant's first step toward validating this hypothesis. By searching for all PCE cache-related code paths—pce_disk_path, load_pce_from_disk, save_pce, pce_cache—the assistant is mapping out the infrastructure that could be serving stale data.

What the Grep Reveals

The grep results show the key functions in the PCE caching layer:

• pce_disk_path (line 342): Maps a CircuitId to a filename on disk. This tells us that PCE data is stored per-circuit-type, keyed by a circuit identifier.
• load_pce_from_disk (line 351): Attempts to load a previously extracted PCE from disk into an in-memory OnceLock cache. The function first checks if the PCE is already in memory (line 357: "PCE already in memory, skipping disk load"), suggesting a lazy-loading pattern where the first proof of a given type triggers a disk load, and subsequent proofs reuse the cached data.
• The OnceLock cache: The reference to OnceLock indicates that the PCE cache is populated exactly once and then immutable—a design choice that makes the stale-cache hypothesis particularly dangerous. If the cache was populated with incorrect data during an earlier deployment, it will persist across service restarts (since it's on disk) and across all subsequent proofs. The assistant is also implicitly verifying that the PCE cache is indeed on disk (not just in memory) and that it persists across service restarts. This is critical because the assistant had already restarted the service to test the PCE-disabled hypothesis ([msg 335]). If the PCE were only in memory, the restart would have cleared it. But disk persistence means the stale data survives restarts.

The Thinking Process: Connecting the Dots

What makes this message fascinating is not the grep itself but the reasoning that motivated it. The assistant is performing a form of differential diagnosis, systematically ruling out possibilities:

1. Is the PCE path itself buggy? Ruled out by the CUZK_DISABLE_PCE=1 test—proofs still fail without PCE.
2. Is it a GPU race condition? The assistant had previously identified that CUDA_VISIBLE_DEVICES doesn't work as expected in the C++ GPU code (<msg id=330 context>), but the 100% failure rate (not intermittent) argues against a race condition.
3. Is it a data inconsistency between cached PCE and new code? This is the current hypothesis, and it's elegant because it explains both the 100% failure rate (systematic, not random) and why the local machine works (the local PCE cache was re-extracted after the fix, or the local machine never had a stale cache). The assistant is also showing awareness of the specific mechanism that could cause the inconsistency: the num_inputs shift. In the old code, RecordingCS::new() pre-allocated ONE at index 0. The extraction code then ran synthesize() which would call alloc_input("one") again, potentially creating a duplicate ONE at index 1. The CSR matrices would record column indices relative to this shifted numbering. In the new code, WitnessCS::new() starts empty, and the explicit alloc_input("one") places ONE at index 0. If the cached CSR matrices expect ONE at index 1 (because the old extraction had it there), then every column index in the matrices is off by one, causing the MatVec evaluation to read the wrong witness elements and produce garbage a/b/c values. Garbage constraint evaluations lead to invalid proofs.

Assumptions and Potential Pitfalls

The assistant's hypothesis rests on several assumptions:

1. The PCE cache on the remote host is indeed stale. The assistant hasn't verified this yet—the grep is the first step toward understanding the caching mechanism so it can check the cache contents or force a re-extraction.
2. The old code had a duplicate ONE. This assumes that RecordingCS::new() pre-allocated ONE and the extraction code also called alloc_input("one"). If the old extraction code did not call alloc_input("one") (relying on the constructor's pre-allocation), then there would be no duplicate, and the new code's explicit alloc_input("one") would produce the same layout as before. The assistant needs to verify the old code path.
3. The local machine doesn't have the same issue. The user mentioned that cuzk was working on the local machine before the fix. The assistant assumes this means the local PCE cache was either never populated (first run after the fix) or was re-extracted. But this could also mean the local machine wasn't running the PCE path at all, or was running a different code version.
4. The disk cache is the only source of stale data. The OnceLock in-memory cache could also be stale if it was populated during the same process lifetime. But since the service was restarted ([msg 335]), the in-memory cache would be cleared, forcing a reload from disk—which would re-load the stale disk cache.

Input Knowledge Required

To fully understand this message, the reader needs:

1. The concept of PCE (Pre-Compiled Constraint Evaluator): An optimization that separates circuit structure extraction (one-time) from witness generation (per-proof). The structure is captured as CSR sparse matrices A, B, C and density bitmaps.
2. The R1CS constraint system structure: Rank-1 Constraint Systems have three matrices (A, B, C) and a witness vector w. The constraint is (A*w) * (B*w) = (C*w). The witness includes input variables (public and the constant ONE) and auxiliary variables (private).
3. The WindowPoSt fix context: The assistant had recently changed WitnessCS::new() and RecordingCS::new() to start with 0 inputs instead of 1, and added explicit alloc_input("one") calls. This was to align the extensibility behavior of RecordingCS with WitnessCS (see Segment 0 summary).
4. The disk caching mechanism: PCE data is serialized to disk per circuit type and loaded lazily. The OnceLock ensures one-time initialization.
5. The debugging context: 100% proof failure rate on a remote multi-GPU host, working on local single-GPU machine, with the WindowPoSt fix being the most recent code change.

Output Knowledge Created

This message produces several forms of knowledge:

1. A map of the PCE caching infrastructure: The grep reveals the exact file and line numbers for the caching functions, enabling the assistant to read those functions next and understand the serialization format, cache key structure, and loading logic.
2. Confirmation of disk persistence: The presence of pce_disk_path and load_pce_from_disk confirms that PCE data survives service restarts, making the stale-cache hypothesis viable.
3. The OnceLock pattern: The in-memory cache is populated exactly once, meaning the first proof after a service restart triggers a disk load that persists for the lifetime of the process.
4. A prioritized next step: The assistant now knows exactly which functions to read next (pce_disk_path, load_pce_from_disk, and the save counterpart) to understand how to force a cache invalidation or verify the cache contents.

The Broader Significance

This message exemplifies a critical pattern in systems debugging: the moment when a seemingly unrelated piece of information (the user's remark about the local machine working before the fix) connects with a known code change (the WindowPoSt fix) to produce a testable hypothesis about a mysterious failure mode. The grep is not just a search—it's a reconnaissance mission to understand the terrain before launching the next debugging operation.

The assistant is also demonstrating a disciplined approach to hypothesis testing. Rather than immediately deleting the cache and re-extracting (which would be the brute-force fix), the assistant first seeks to understand the caching mechanism thoroughly. This understanding will allow for a more precise intervention—perhaps invalidating only the affected cache entries, or adding a version stamp to prevent stale cache usage, or verifying that the cache contents are indeed inconsistent before taking action.

In the larger narrative of this coding session, message [msg 339] is the turning point where the investigation shifts from "is the PCE path buggy?" to "is the PCE cache stale?"—a subtle but crucial distinction that leads to a completely different class of solution. The former would require fixing the PCE evaluation logic; the latter requires cache invalidation or versioning. This pivot, triggered by the user's casual remark and executed through a focused code search, is the kind of insight that separates efficient debugging from aimless tinkering.