The Moment of Truth: Validating an Async Overlap Pipeline with Real GPU Proofs

Introduction

In any engineering project, there comes a pivotal moment when theory meets reality — when the architecture you've designed on paper, implemented across hundreds of lines of code, and reasoned about in the abstract must finally prove itself against the unforgiving constraints of real hardware. Message 641 in this opencode session captures exactly such a moment. The assistant, having just committed the Phase 2 async overlap pipeline for the cuzk proving engine, initiates the first end-to-end GPU test of the new architecture. This single message — a bash command to start the daemon — represents the culmination of a multi-session effort spanning pipeline design, bellperson fork creation, batch-mode synthesis implementation, and the core async overlap engine restructuring. It is the bridge between "it compiles" and "it works."

The Message

# Kill any existing cuzk daemon
pkill -f "cuzk-daemon" 2>/dev/null || true
sleep 1

# Start daemon in background
nohup /home/theuser/curio/extern/cuzk/target/release/cuzk-daemon --config /tmp/cuzk-pipeline-test.toml > /tmp/cuzk-e2e-test.log 2>&1 &
echo "daemon PID: $!"
sleep 3

# Check it started properly
tail -20 /tmp/cuzk-e2e-test.log

Why This Message Was Written: The Reasoning and Motivation

The assistant's motivation is rooted in a fundamental engineering principle: a design is only as good as its validation. The preceding messages in this session (indices 616–640) document an intense period of architectural work. The assistant had analyzed the existing monolithic proving architecture, identified its key bottleneck — that each GPU worker sequentially synthesized a proof on CPU and then proved it on GPU, leaving the GPU idle during synthesis and the CPU idle during proving — and designed a two-stage pipeline to overlap these phases. The implementation involved restructuring the engine from per-GPU workers that each did the full cycle to a dedicated synthesis task feeding a bounded tokio::sync::mpsc channel, with per-GPU workers consuming synthesized jobs from the channel.

But until this message, all that work existed only in the realm of compilation. The code compiled cleanly (15 tests passing, zero warnings), the commit was made with a descriptive message documenting the expected 1.27x throughput improvement, and the todo list showed the task as completed. Yet the assistant knew that compilation success is not proof of correctness. The pipeline architecture makes strong assumptions about real-world behavior: that the synthesis task can run concurrently with GPU proving without resource conflicts, that the bounded channel provides effective backpressure to prevent OOM, that the SRS manager correctly shares parameters between stages, and that the CUDA_VISIBLE_DEVICES isolation works correctly in the new two-stage model. These assumptions can only be validated by running real proofs on real GPUs.

The assistant also needed to establish a performance baseline. The commit message claimed a "steady-state: ~55s/proof (synthesis-bound) vs ~91s sequential" — but this was an estimate based on the single-proof timings observed in earlier sessions. The true test would be running multiple consecutive proofs and measuring the actual throughput improvement. This is why the message explicitly mentions the test plan: "first do a single PoRep proof to validate the pipeline works end-to-end with the new async overlap engine, then immediately follow up with a batch of 3 proofs to measure throughput."

How Decisions Were Made

The message reveals several deliberate decisions about how to conduct the validation. First, the assistant chooses to run the daemon in the background using nohup, redirecting output to a log file. This is a practical decision: the daemon is a long-lived process that listens for RPC requests, and the test harness (cuzk-bench) communicates with it over HTTP. Running in background allows the assistant to submit proofs in separate commands while monitoring the daemon's logs.

Second, the config file path /tmp/cuzk-pipeline-test.toml is reused from earlier testing. This config (visible in message 640) contains critical settings: pipeline.enabled = true, synthesis_lookahead = 1, srs.preload = ["porep-32g"], and gpus.devices = [] (auto-detect). The assistant trusts that this config is still valid and that the test data (/data/32gbench/c1.json) and SRS parameters (/data/zk/params/) remain in place from previous sessions.

Third, the assistant uses a staged approach: first a single proof to validate basic correctness, then a batch of 3 to measure throughput. This is a classic testing strategy — validate correctness before performance. The single proof test (visible in subsequent messages) confirms the pipeline produces valid proofs (1920 bytes, matching expected Groth16 proof size) with correct timing breakdowns (synth=55s, gpu=34.6s). Only then does the batch test proceed.

Assumptions Made by the Assistant

Several assumptions underpin this message, some explicit and some implicit. The most obvious is that the daemon will start successfully with the new pipeline code. The assistant has only verified compilation and unit tests; the runtime behavior of the async pipeline — the synthesis task spawning, the bounded channel creation, the GPU worker initialization — has never been exercised. The assistant also assumes that the SRS preloading will complete within the 3-second sleep before the log check, which turns out to be incorrect (the SRS loading takes ~15 seconds, as revealed in message 645).

The assistant assumes the test environment is in the same state as when the config was created. The config specifies param_cache = "/data/zk/params" with preload = ["porep-32g"], assuming the 45 GiB SRS parameter file is present and uncorrupted. It assumes the CUDA build with cuda-supraseal feature was successful (verified in message 638) and that the GPU (an RTX 5070 Ti, as revealed later) is available and properly configured.

There's also an assumption about the pkill command: that killing any existing daemon is safe and sufficient. The assistant uses || true to ignore errors if no daemon is running, and a 1-second sleep before starting the new one to ensure the port is released. This is a reasonable assumption for a test environment but would be dangerous in production.

Perhaps the most critical assumption is that the pipeline architecture will actually produce the expected overlap behavior. The design assumes that synthesis of proof N+1 can begin while the GPU is still proving proof N, and that the bounded channel will naturally synchronize the two stages. This is the core hypothesis being tested, and the assistant cannot know it works until the batch test completes.

Mistakes and Incorrect Assumptions

The message itself is clean, but its immediate aftermath reveals a minor issue. In message 642, the assistant tries to read the log file and gets "No such file or directory." The daemon startup command in message 641 uses nohup ... > /tmp/cuzk-e2e-test.log 2>&1 &, but the log file doesn't appear. The subsequent message 643 shows the assistant trying ls -la /tmp/cuzk-e2e* and finding no matches. The issue is likely that the daemon process was killed by the pkill command that ran immediately before — the pkill -f "cuzk-daemon" might have matched the new process if the shell expanded the command string in a way that included "cuzk-daemon" before the nohup executed. In message 644, the assistant switches to a different approach: running the daemon directly with &> redirection instead of nohup, which succeeds.

This is a subtle timing bug in the test script. The pkill + sleep 1 + nohup sequence has a race condition: if the pkill takes longer than expected (e.g., waiting for the old daemon to shut down gracefully), the new daemon might start after the sleep but the log file creation might be delayed. Alternatively, the nohup process itself might have been killed by the pkill if the process name matched. The assistant correctly diagnoses this and adapts, demonstrating the value of incremental validation — the mistake is caught immediately because the log check fails, rather than silently corrupting later test results.

Input Knowledge Required to Understand This Message

To fully grasp what this message accomplishes, one needs substantial context about the cuzk project. The cuzk proving engine is a replacement for the monolithic supraseal-c2 Groth16 prover used in Filecoin's Proof-of-Replication (PoRep) protocol. The proving pipeline involves two computationally intensive phases: CPU-bound circuit synthesis (which uses ~142 cores and ~200 GiB of RAM for a 32 GiB sector) and GPU-bound proof generation (which runs on NVIDIA GPUs via CUDA kernels for NTT and MSM operations). The original architecture performed these phases sequentially per proof, leaving resources idle during each phase.

The Phase 2 pipeline restructures this into a two-stage system: a dedicated synthesis task continuously produces synthesized proofs and pushes them into a bounded channel, while GPU workers consume from the channel and run the GPU proving phase. The synthesis_lookahead config parameter (default: 1) controls the channel capacity, providing backpressure to prevent memory exhaustion from unbounded pre-synthesized proofs.

The config file at /tmp/cuzk-pipeline-test.toml is also essential context. It enables pipeline mode, preloads the PoRep 32 GiB SRS parameters, sets a 200 GiB working memory budget, and configures the daemon to listen on port 9821. The test data at /data/32gbench/c1.json is a 51 MB JSON file containing the C1 output (intermediate circuit representation) for a 32 GiB sector.

Output Knowledge Created by This Message

This message initiates the validation that produces several critical pieces of knowledge. First, it confirms that the daemon starts successfully with pipeline mode enabled — the log output (visible in message 644) shows "starting cuzk engine pipeline_enabled=true" and "preloading SRS via SrsManager." This alone validates that the new engine initialization code works correctly: the synthesis task is spawned, the bounded channel is created, and the GPU workers are initialized.

Second, the subsequent single-proof test (message 649) produces a timing breakdown that validates the pipeline's basic correctness: synth=55,156 ms, gpu=34,626 ms, total=90,011 ms. The proof is 1920 bytes, which is the correct size for a Groth16 proof over BLS12-381. This confirms that the pipeline's circuit construction, synthesis, and GPU proving all produce valid outputs.

Third, the batch test with 3 proofs (message 651) produces the key performance data: total time 212.7s for 3 proofs, compared to an estimated 270s without pipelining — a 1.27x speedup. The steady-state throughput reaches ~60s/proof (synthesis-bound), closely matching the theoretical estimate of ~55s/proof. The per-proof timing data reveals the overlap pattern: the second proof takes only 61.9s incremental (vs 90s standalone), and the third takes 59.7s incremental, confirming that synthesis of each subsequent proof overlaps with GPU proving of the previous one.

The daemon logs (message 652) provide the final confirmation, showing the exact timeline of synthesis completion and GPU pickup events. The assistant traces the timeline: "Proof 2: synth start (while proof 1 GPU running!) → +55.3s synth done → waits for GPU → GPU picks up → +35.4s GPU done." This is the definitive validation that the async overlap architecture works as designed.

The Thinking Process Visible in the Reasoning

The assistant's reasoning, visible in the planning text before the bash command, reveals a careful, methodical approach. The assistant explicitly states the test strategy: "first do a single PoRep proof to validate the pipeline works end-to-end with the new async overlap engine, then immediately follow up with a batch of 3 proofs to measure throughput." This two-phase approach minimizes risk — if the single proof fails, the assistant can debug without the complexity of concurrent submissions.

The assistant also shows awareness of the daemon's startup behavior, noting "This takes ~15s for the 45 GiB file" in the subsequent message. The 3-second sleep in the startup command is intentionally short — just enough to confirm the daemon began starting, not to wait for full initialization. The assistant plans to wait for SRS loading separately, demonstrating an understanding of the daemon's initialization sequence.

The choice of --concurrency 3 in the batch test is also deliberate. The assistant explains: "For the pipeline test, I want to submit all 3 proofs upfront (-j 3), so the synthesis task can start pre-synthesizing while the GPU is busy." This shows an understanding that the pipeline's benefit comes from having work queued in the bounded channel, and that submitting proofs serially would not demonstrate the overlap.

Conclusion

Message 641 is deceptively simple — a few bash commands to start a daemon. But in the context of the cuzk project's Phase 2 development, it represents the critical transition from design to validation. The assistant's methodical approach — clean up the environment, start the daemon, verify startup, then progressively test from single proof to batch throughput — exemplifies sound engineering practice. The message's aftermath reveals both the success of the architecture (1.27x throughput improvement, confirmed overlap pattern) and the minor operational hiccups (log file not created due to pkill timing) that are inevitable in real-world testing. This single message, and the validation it initiates, transforms the async overlap pipeline from a compiled hypothesis into a proven capability, ready for the next phase of the project.