The Moment the Daemon Starts: Launching Phase 8 of the Dual-Worker GPU Interlock

The Message

cd /home/theuser/curio/extern/cuzk && nohup ./target/release/cuzk-daemon --config /tmp/cuzk-phase8.toml > /tmp/cuzk-phase8-daemon.log 2>&1 &
echo "PID: $!"
sleep 3
tail -20 /tmp/cuzk-phase8-daemon.log
PID: 2926812
[2026-02-19T00:55:44.927126Z]  INFO cuzk_daemon: cuzk-daemon starting
[2026-02-19T00:55:44.927148Z]  INFO cuzk_daemon: configuration loaded listen=0.0.0.0:9820
[2026-02-19T00:55:44.935307Z]  INFO cuzk_daemon: rayon global thread pool configured rayon_threads=192
[2026-02-19T00:55:44.935332Z]  INFO cuzk_core::engine: starting cuzk...

At first glance, this is an unremarkable operational message: a developer launches a daemon in the background, waits three seconds, and checks the log. The daemon starts, prints a few informational lines, and begins its initialization. But in the context of a months-long optimization campaign to eliminate GPU idle gaps in a Filecoin proof-of-replication (PoRep) proving pipeline, this message represents a critical inflection point. It is the first live execution of Phase 8: Dual-Worker GPU Interlock, an architectural change that would ultimately deliver a 13–17% throughput improvement by allowing two GPU workers to interlock their access to CUDA kernels, keeping the GPU saturated while one worker does CPU preprocessing.

Context: The Long Road to Phase 8

To understand why this message was written, one must appreciate the optimization journey that preceded it. The cuzk SNARK proving engine, part of the Curio Filecoin mining stack, generates Groth16 proofs for the Supraseal C2 pipeline. Earlier phases had already transformed the architecture: Phase 6 introduced a slotted partition pipeline that reduced peak memory from 228 GiB to 71 GiB; Phase 7 implemented per-partition dispatch with cross-sector pipelining. Yet each improvement revealed new bottlenecks.

The critical insight driving Phase 8 emerged from Phase 7 benchmarks. GPU utilization traces showed a recurring pattern: the GPU would go idle between partition proofs because a single GPU worker was blocked on CPU-side work—specifically, on a coarse-grained C++ static std::mutex in the generate_groth16_proofs_c function that locked the entire GPU operation, including CPU preprocessing steps that did not need GPU access. The mutex was originally placed to serialize access to CUDA kernels (NTT/MSM operations), but its scope was too broad: it held the lock during CPU-side batching, argument preparation, and result copying, not just during the actual GPU kernel launches. This meant that while one proof's CPU preprocessing was running under the lock, the GPU sat idle, and no other proof could use it.

Phase 8's design was elegant in its simplicity: narrow the C++ mutex to cover only the CUDA kernel region (NTT+MSM, batch additions, tail MSMs), and move CPU preprocessing and the b_g2_msm computation outside the lock. Then spawn two GPU workers per device instead of one. While worker A holds the mutex and runs CUDA kernels, worker B does CPU preprocessing for the next partition. When A finishes and releases the mutex, B can immediately acquire it and launch kernels, while A moves on to CPU work for the subsequent partition. The GPU never idles.

This design required changes across seven files and approximately 195 lines of code: the C++ CUDA kernel was refactored to accept a passed-in mutex pointer with narrowed scope; FFI plumbing threaded the mutex through supraseal-c2 → bellperson → pipeline.rs; and the engine spawned gpu_workers_per_device workers (default 2) sharing a per-GPU C++ mutex allocated via new create_gpu_mutex/destroy_gpu_mutex helpers. The build had succeeded after resolving a Send trait issue with raw pointer capture in async closures—a subtle Rust concurrency problem where a *mut c_void inside a SendableGpuMutex wrapper prevented the async closure from being Send, requiring conversion to a usize before entering the async block.

Why This Message Was Written: The Reasoning and Motivation

This message is the bridge between implementation and validation. The assistant had just completed all code changes for Phase 8, successfully compiled both the daemon and the benchmark tool, and updated the todo list to reflect completion of the C++ mutex refactor, FFI plumbing, bellperson integration, and engine changes. The next logical step was to test whether the implementation actually works—whether the daemon starts, accepts connections, and can prove sectors with the new dual-worker architecture.

The motivation was twofold. First, the assistant needed to verify that the Phase 8 configuration (gpu_workers_per_device = 2) was correctly parsed and that the daemon could initialize without crashes or assertion failures. Second, and more importantly, the assistant needed to gather benchmark data to quantify the throughput improvement. The Phase 7 baseline had been established at 50.7 seconds per proof with c=5 j=3 and 59.8 seconds per proof with c=5 j=2. The Phase 8 design promised to eliminate the GPU idle gaps entirely, and the only way to confirm this was to run the daemon, submit proof jobs, and measure.

The choice of nohup and background execution reflects a pragmatic operational pattern: the daemon is a long-lived server process that must persist beyond the terminal session. The three-second sleep before tailing the log gives the daemon time to initialize and print its startup messages. The config file /tmp/cuzk-phase8.toml was freshly written in the preceding message (msg 2218) and includes the critical gpu_workers_per_device = 2 setting alongside the Phase 7 parameters (partition_workers = 20, synthesis_lookahead = 3, synthesis_concurrency = 1).

Assumptions Embedded in This Message

Every operational command carries implicit assumptions, and this one is no exception. The assistant assumes that:

  1. The binary is correct: The release build at ./target/release/cuzk-daemon is the freshly compiled artifact from the Phase 8 code changes, not a stale binary from a previous build. The assistant had run cargo build --release -p cuzk-daemon in msg 2214 and confirmed it compiled successfully (with only dead-code warnings), so this assumption is reasonable.
  2. The config file is valid: The Phase 8 TOML config at /tmp/cuzk-phase8.toml contains all required fields and no syntax errors. The assistant wrote this file in msg 2218, copying the Phase 7 config structure and adding the gpu_workers_per_device field. The log output confirms this assumption held—the daemon printed "configuration loaded" without error.
  3. The environment is ready: The SRS parameter cache at /data/zk/params exists and contains the preload data for porep-32g. The GPU devices are available (the config uses devices = [] which means auto-detect). The /tmp directory is writable for log output. The daemon's PID file management and signal handling work correctly.
  4. The daemon will start quickly: The three-second sleep before tailing the log assumes the daemon will complete its initialization within that window. This is a reasonable heuristic—the startup messages appear within milliseconds of launch—but it does not account for longer initialization tasks like SRS preloading, which could take minutes and would not appear in the first 20 lines of the log.
  5. No port conflicts: The daemon listens on 0.0.0.0:9820. The assistant had killed any existing daemon process in msg 2219 (pkill -f cuzk-daemon), so the port should be free. But if another process had grabbed the port in the intervening second, the daemon would fail silently in the background.
  6. The nohup output redirection works: The 2>&1 redirects stderr to stdout, and both are written to the log file. This is standard shell practice and unlikely to fail, but it does mean any startup errors would only be visible in the log file, not on the terminal.

Mistakes and Incorrect Assumptions

While the daemon starts successfully in this message, a retrospective analysis reveals several potential issues that the assistant did not account for:

The SRS preload assumption: The config specifies preload = ["porep-32g"], which means the daemon will attempt to load the structured reference string (SRS) for the 32 GiB PoRep circuit at startup. This is a multi-gigabyte file that can take several minutes to load from disk. The three-second window in this message would not capture the SRS preload completion—the daemon might appear to start successfully but then stall during preload, or crash if the file is missing or corrupted. The log output shows only "starting cuzk..." which is the engine initialization, not the SRS preload. A subsequent message (msg 2221, not shown here) would reveal whether the preload succeeded.

The partition_workers = 20 setting: This value was carried over from the Phase 7 config, where it was identified as the sweet spot for the 96-core machine. But Phase 8's dual-worker architecture changes the CPU utilization pattern—now two workers per GPU are doing CPU preprocessing simultaneously. The optimal partition_workers might shift, and the assistant would later discover (in the sweep of chunk 1) that pw=10–12 actually performs better under Phase 8. At the time of this message, however, the assistant was still operating under the Phase 7 assumption that pw=20 was optimal.

The rayon_threads=192 configuration: The daemon configures 192 rayon threads, which is double the number of physical cores (96) on this machine. This is a deliberate choice for CPU-bound work, but with two GPU workers per device doing CPU preprocessing, the thread count might contribute to contention. The assistant would later observe that partition_workers=30 caused CPU contention that starved GPU preprocessing threads, confirming that thread management is a delicate balance.

Input Knowledge Required to Understand This Message

A reader encountering this message in isolation would need substantial context to grasp its significance. The essential background includes:

Output Knowledge Created by This Message

This message produces several concrete artifacts:

  1. A running daemon process (PID 2926812) listening on port 9820, ready to accept proof generation requests. This is the first live instance of the Phase 8 architecture.
  2. A log file at /tmp/cuzk-phase8-daemon.log containing the daemon's startup sequence. The first four lines confirm: the daemon binary executes, the configuration is parsed correctly, the rayon thread pool is initialized with 192 threads, and the engine begins its initialization.
  3. Evidence of successful compilation and deployment: The daemon starts without crashes, segfaults, or assertion failures. This validates that the C++ CUDA kernel changes, FFI plumbing, bellperson integration, and engine modifications are syntactically correct and link properly.
  4. A timestamped record: The startup at 2026-02-19T00:55:44 provides a reference point for subsequent benchmark runs. The daemon's uptime can be correlated with benchmark results to account for warm-up effects.
  5. Confirmation of the configuration: The log shows listen=0.0.0.0:9820 and rayon_threads=192, confirming that the Phase 8 TOML file was parsed correctly and the settings were applied.

The Thinking Process Visible in This Message

While the message itself is purely operational—a bash command followed by its output—the reasoning behind it is visible in the surrounding messages. The assistant had just completed a complex multi-file refactoring (messages 2181–2219) and was following a systematic validation protocol: build, then test, then benchmark. This message is the "test" step.

The choice to use nohup and background the process reveals a deliberate strategy: the daemon is a server that will be used for multiple benchmark runs, so it should persist across commands. The three-second sleep is a heuristic—long enough for startup messages to appear, short enough to not waste time if the daemon fails to start. The tail -20 captures the first 20 lines of the log, which should include the startup banner and any immediate errors.

The assistant's thinking also reflects an understanding of the daemon's initialization sequence: first the binary starts, then it loads configuration, then it initializes the rayon thread pool, then it begins engine initialization. Each log line corresponds to a stage in this sequence, and the assistant is watching for the next expected line ("starting cuzk...") to confirm progress.

Notably, the assistant does not immediately check whether the daemon is actually serving requests. The next step (visible in subsequent messages) would be to run the benchmark tool against this daemon. The assistant is building confidence incrementally: first confirm the daemon starts, then confirm it responds to requests, then measure performance.

Conclusion

This message, for all its apparent simplicity, is the culmination of a deep optimization effort. The daemon that starts here embodies seven files of changes, a narrowed C++ mutex, dual-worker interleaving, and the promise of 13–17% better throughput. The log lines are brief—four informational messages in under nine milliseconds—but they represent the successful deployment of an architectural change that took weeks to design, implement, and debug. The assistant's methodical approach—build, launch, verify, benchmark—reflects a disciplined engineering practice that prioritizes empirical validation over theoretical reasoning. The daemon starts, the engine initializes, and the GPU workers await their first proof jobs. The real test is yet to come.