The Phantom Cache: Debugging a Silent Build Failure in a Remote GPU Proving System

In the middle of a complex debugging session spanning code changes, Docker builds, and remote deployments, a single message captures a moment of diagnostic tension. The assistant, having just discovered that a carefully implemented code change failed to take effect in a deployed binary, hypothesizes that Docker's layer caching is the culprit and initiates a clean rebuild. This message — <msg id=2746> — is a snapshot of the moment a developer suspects their build pipeline is lying to them.

The Message

[assistant] Binary is from 17:25 — the build was recent. But the Docker cache might have not picked up the status.rs change. Let me check:
[bash] DOCKER_BUILDKIT=1 docker build --no-cache -f Dockerfile.cuzk-rebuild -t cuzk-rebuild:synthfix2 . 2>&1 | tail -15
#15 99.97    Compiling cuzk-server v0.1.0 (/build/extern/cuzk/cuzk-server)
#15 101.3 warning: `cuzk-core` (lib) generated 3 warnings (run `cargo fix --lib -p cuzk-core` to apply 1 suggestion)
#15 101.3    Compiling cuzk-daemon v0.1.0 (/build/extern/cuzk/cuzk-daemon)
#15 106.9     Finished `release` profile [optimized] target(s) in 1m 45s
#15 DONE 107.2s

#16 [stage-1 1/1] COPY --from=builder /build/extern/cuzk/target/release/cuzk-daemon /cuzk
#16 DONE 0.1s

#17 exporting to image
#17 exporting l...

At first glance, this looks like a routine rebuild command. But the context transforms it into something far more interesting: a diagnostic probe aimed at a silent failure that could have been caused by any of a half-dozen different bugs, only one of which was the build cache.

The Puzzle: A Display That Wouldn't Update

The story begins with a seemingly simple UI bug. The assistant had built a comprehensive status monitoring panel for the CuZK GPU proving engine — a live dashboard showing synthesis concurrency, GPU worker states, memory usage, and pipeline progress. One field, synth_max, displayed the maximum number of concurrent synthesis operations the system could support. It was showing 4.

But the system had 400 GiB of memory budget, and each SnapDeals partition required approximately 9 GiB. Simple arithmetic: 400 / 9 ≈ 44. The display should have shown synth: 34/44 (34 active out of 44 possible), but instead it showed synth: 34/4.

The assistant had already diagnosed the root cause of this discrepancy in the preceding messages. The synth_max value was being sourced from the synthesis_concurrency configuration parameter — a static value that limited batch-level dispatch concurrency, not per-partition synthesis. The real concurrency limiter was the memory budget, which the assistant had recently implemented as part of a unified memory manager. The fix was straightforward: compute synth_max dynamically from the budget in the StatusTracker::snapshot() method, using the partition size constant to derive the effective maximum.

The assistant made the edits ([msg 2729], [msg 2730], [msg 2731]): removing the static synth_max field from the Inner struct and replacing it with a budget-derived computation in the snapshot function. The code compiled cleanly ([msg 2733]). The Docker build succeeded ([msg 2734]). The binary was extracted, uploaded via SCP to the remote test machine at 141.0.85.211, and deployed ([msg 2735], [msg 2736], [msg 2737]).

And then the status endpoint returned synth: 0/4.

The Hypothesis: Docker Layer Cache

The assistant's first instinct was correct: check whether the new binary was actually running. The initial deployment attempt failed because the old process was still dying ([msg 2738], [msg 2739]). After a clean restart, the status showed synth: 0/4 — still the old value ([msg 2740]). The user confirmed the daemon had been dying and was just starting up ([msg 2742]). After waiting, the status updated to synth: 34/4 ([msg 2743]). The active count was now reflecting real synthesis work, but the max was stubbornly stuck at 4.

The assistant's reasoning in [msg 2744] is explicit: "Still showing /4. That's wrong — I see the issue. The old binary was cached by Docker and the new change didn't make it in." The assistant checks the Docker image creation timestamp and the binary's modification time, both showing 17:25 — the time of the first build. The conclusion: the Docker build cache didn't pick up the status.rs change.

This is the moment captured in the subject message. The assistant runs docker build --no-cache to force a complete rebuild from scratch, bypassing all layer caching. The build output shows that cuzk-core (which contains status.rs) is indeed being recompiled, along with cuzk-daemon and cuzk-server. The --no-cache flag ensures that every Docker layer is rebuilt from zero, eliminating any possibility of stale cache artifacts.

The Deeper Problem: What the Hypothesis Missed

But here's the critical question: was Docker cache actually the problem? The build output in the subject message shows cuzk-core being compiled — and the warning about cuzk-core generating 3 warnings is the same warning message that appeared in the first build ([msg 2733]). This means the compiler did process the status.rs file in both builds. If the code change was present in the source tree, the compiler would have picked it up regardless of Docker layer caching, because Docker's Rust compilation happens inside the container using the source files copied into the build context.

The real issue might have been something entirely different. Several possibilities exist:

  1. The code change was incomplete or incorrect. The assistant made edits to status.rs but we never see the actual diff. The snapshot() function might have had a bug — perhaps it was computing synth_max but the field was still being read from the old Inner struct somewhere else, or the budget's total_bytes() method returned an unexpected value.
  2. The config file was overriding the computed value. The daemon was started with --config /tmp/cuzk-memtest-config.toml. If this config file explicitly set synthesis_concurrency = 4, and the code still had a fallback path that used the config value when the budget-derived value was zero or invalid, the config would win.
  3. The overlay filesystem was serving a stale binary. This turned out to be a real problem later in the session ([chunk 20.1]). The container's overlay filesystem cached the old binary in a lower layer, and even cp to /usr/local/bin would silently serve the stale version because the path existed in a lower layer. The workaround was deploying to a path not present in any lower layer, like /data/cuzk-ordered.
  4. The snapshot() function was not being called. The status API might have been reading from a cached snapshot or using a different code path that bypassed the new computation. The assistant never definitively resolved which of these was the true cause in this message. The --no-cache build produced a new binary, but whether that binary actually fixed the display issue remained an open question at the chunk's end.

The Thinking Process: A Window into Debugging Under Pressure

What makes this message valuable is the visible thinking process. The assistant is operating under real-world constraints: a remote machine with limited access (SSH via non-standard port 40612), a Docker-based build pipeline, and a live proving system processing real proofs. Each debug cycle takes minutes — the Docker build alone takes nearly two minutes, plus SCP transfer time, plus process restart time.

The assistant's reasoning follows a classic debugging pattern:

  1. Observe the symptom: synth: 34/4 instead of synth: 34/44
  2. Form a hypothesis: The code change didn't make it into the binary
  3. Identify the suspected mechanism: Docker layer caching
  4. Test the hypothesis: Force a clean rebuild with --no-cache
  5. Observe the result: The build succeeds, producing a new binary But notice what's missing from this chain: verification that the source code change was actually correct. The assistant never re-read the edited status.rs to confirm the edit was applied properly. It never added debug logging to verify the budget-derived value at runtime. It never checked whether the config file was overriding the value. The hypothesis about Docker cache was convenient — it was easy to test and the fix was straightforward — but it may not have been the right hypothesis. This is a common cognitive trap in debugging: when a change doesn't take effect, the first instinct is to blame the deployment pipeline rather than the change itself. The build system, the network, the filesystem — these are all plausible failure points that shift blame away from the developer's own code. The assistant fell into this trap, and the message captures that moment perfectly.

The Broader Context: A System Under Construction

To fully understand this message, it's essential to know what the assistant was building. The CuZK proving engine is a GPU-accelerated system for generating zero-knowledge proofs for Filecoin storage proofs. The assistant had been implementing a unified memory manager that replaced static concurrency limits with a dynamic, budget-based system. This was a major architectural change touching multiple modules: memory.rs for the budget implementation, engine.rs for the dispatch logic, status.rs for the monitoring display, and pipeline.rs for the PCE cache.

The synth_max display bug was a symptom of a deeper architectural mismatch. The old system used synthesis_concurrency as a static cap on concurrent synthesis operations. The new system replaced that with a memory budget that dynamically determined how many partitions could be synthesized simultaneously based on available RAM. But the status display was still wired to the old config value, creating a misleading UI that showed 34/4 — more active syntheses than the supposed maximum.

The fix required not just changing a number, but rethinking what "maximum concurrency" meant in a budget-gated system. The maximum isn't a fixed parameter anymore — it's a function of the budget and the partition size, which can change as SRS and PCE allocations fluctuate. Computing it dynamically in snapshot() was the right approach, but the implementation had to be correct.

Output Knowledge and Unresolved Questions

This message creates new knowledge: the --no-cache build completed successfully, producing a binary at /tmp/cuzk-synthfix2 that was uploaded to the remote machine. But it leaves the fundamental question unanswered: would this new binary finally show the correct synth_max value?

The message also reveals an important insight about the build pipeline: Docker's layer caching can indeed cause confusion when files are modified between builds, especially when the Dockerfile uses COPY instructions that may not invalidate layers based on file content changes in the expected way. The --no-cache flag is a blunt but effective tool for eliminating this variable.

For the reader, this message illustrates the messy reality of debugging distributed systems. The clean narrative of "identify bug, fix code, deploy, verify" rarely holds in practice. Instead, each fix passes through multiple layers of indirection — source tree, compiler, Docker build, container extraction, SCP transfer, process restart — and any of these layers can introduce silent failures. The developer's job is to systematically eliminate each layer until the true cause is found.

The assistant would go on to discover another issue entirely — the partition scheduling order problem — and pivot to fix that before returning to the synth_max display. The overlay filesystem issue would be discovered later, and the synth_max fix might have been working all along, masked by the stale binary problem. The story of this message is not one of a solved mystery, but of the patience required to chase down silent failures in complex deployment pipelines.