The Moment of Truth: Running the Phase 6 Slotted Pipeline Benchmark

In the lifecycle of any performance optimization project, there comes a pivotal moment when months of design, implementation, and refactoring are finally put to the test against real hardware. Message [msg 1726] captures exactly such a moment for the cuzk SNARK proving engine — the first execution of the Phase 6 slotted partition pipeline benchmark. This single bash command, dispatched by the assistant after a lengthy implementation session, would determine whether a carefully architected redesign of the Filecoin PoRep proof generation pipeline delivered on its promises, or whether hidden hardware realities would upend the theoretical predictions.

The Context: Why This Message Was Written

The message exists because the assistant had just completed implementing the Phase 6 slotted partition pipeline — a substantial architectural change to how the cuzk engine generates Groth16 proofs for Filecoin's Proof-of-Replication (PoRep). The core idea, documented in c2-optimization-proposal-6.md, was to replace the existing "batch-all-then-prove" model with a finer-grained approach: instead of synthesizing all 10 partition circuits at once (consuming ~228 GiB of memory) and then proving them on the GPU in a single batch, the slotted pipeline would break the work into smaller "slots." Each slot would be synthesized independently and immediately handed off to the GPU for proving, allowing synthesis and GPU work to overlap in time while dramatically reducing peak memory.

The implementation touched multiple files across the codebase. The assistant had refactored C1 JSON deserialization out of the per-partition synthesis function into a shared ParsedC1Output struct, avoiding redundant 51 MB parses per slot. It had added a ProofAssembler struct for collecting per-slot proof bytes, implemented the main prove_porep_c2_slotted() function using std::thread::scope with a bounded sync_channel(1) for synthesis/GPU overlap, added slot_size configuration to PipelineConfig, wired the slotted pipeline into the engine's process_batch method, and added a SlottedBench subcommand to the benchmarking binary with GPU utilization tracking. All of this compiled cleanly — the three build targets (cuzk-core, cuzk-daemon, cuzk-bench) all passed without errors or warnings.

But compilation is not validation. The message [msg 1726] is the moment where the assistant transitions from "does it compile?" to "does it work, and does it perform as predicted?"

What the Message Actually Does

The message executes a single bash command:

cd /home/theuser/curio/extern/cuzk && CUDA_VISIBLE_DEVICES=0 \
  FIL_PROOFS_PARAMETER_CACHE=/data/zk/params \
  ./target/release/cuzk-bench slotted-bench \
  --c1 /data/32gbench/c1.json \
  --slot-sizes 10,5,2,1 2>&1

This launches the cuzk-bench binary with the newly added slotted-bench subcommand. The arguments specify:

The Assumptions Baked Into This Benchmark

Every benchmark encodes a set of assumptions, and this one is no exception. The most critical assumption was that GPU proving costs would scale roughly linearly with circuit count. The design document had predicted that slot_size=2 would be the sweet spot, achieving ~38.9 seconds total time with good synthesis/GPU overlap. This prediction was based on measurements taken with 10-circuit batches, extrapolated linearly downward.

A second assumption was that the overhead of multiple GPU invocations would be negligible. The slotted pipeline calls the GPU proving function once per slot, so slot_size=1 would require 10 separate GPU calls instead of 1. The design assumed each call would have a small fixed overhead, making the total GPU time approximately equal to the batch-all case.

A third, more subtle assumption was that the benchmark environment — a single proof on an idle machine — would be representative of steady-state performance. The assistant was aware that GPU utilization metrics might be lower in single-proof mode, but the design doc had predicted 80–88% GPU utilization even for single-proof runs.

The Input Knowledge Required to Understand This Message

To fully grasp what this message represents, one needs substantial context about the cuzk proving engine and its pipeline. The C1 JSON file is the output of the first phase of PoRep proof generation, containing 10 "vanilla proofs" (one per partition) along with the replica ID, seed, and commitments (comm_r, comm_d). The PCE is a pre-compiled circuit evaluator that replaces the original bellperson constraint system, reducing synthesis time by avoiding per-constraint allocation overhead. The SRS files are multi-gigabyte parameter files needed for the Groth16 prover, loaded into GPU memory at proof time.

The slot_size parameter controls how many partitions are grouped into each slot. With 10 total partitions, slot_size=10 means all partitions are synthesized together and proven in one GPU call. Slot_size=5 creates two slots of 5 partitions each. Slot_size=2 creates five slots. Slot_size=1 creates ten individual slots.

The hardware context is also essential: a 96-core AMD Threadripper PRO 7995WX CPU (Zen4 architecture) paired with an RTX 5070 Ti GPU (Blackwell architecture, 16 GB VRAM). The machine has ample system memory (hundreds of GiB), but the goal of the slotted pipeline is to reduce the ~228 GiB peak memory of the batch-all approach to something more manageable.

The Output Knowledge Created

This message produces the raw benchmark data that would drive the next phase of analysis. The output — captured in the tool result file at /home/theuser/.local/share/opencode/tool-output/tool_c6f2db8df001T1RhBh3N7CQMHK — contains per-slot timing breakdowns (synthesis time, GPU time, GPU active percentage), RSS memory measurements at key points (after C1 load, after PCE extraction, after SRS load, before proof, peak during proof, after drop), and proof size information.

This data would reveal a critical finding: the GPU's b_g2_msm operation has a ~22-23 second fixed cost that does not scale with circuit count for batch sizes of 2 or more. This single discovery would invalidate the design doc's predictions for slot_size=2 and slot_size=5, showing them to be significantly slower than the batch-all baseline. Only slot_size=1 would prove viable, achieving the predicted ~39 second total time with dramatically reduced memory (~27 GiB peak), but at the cost of 10 separate GPU invocations totaling ~31 seconds of GPU time.

The Thinking Process Visible in This Message

While the message itself is just a bash command, the reasoning behind it is visible in the surrounding context. The assistant had just completed the implementation and verified that all three build targets compiled cleanly. The todo list shows the progression: read existing code, refactor C1 deserialization, implement ProofAssembler, implement the slotted pipeline function, add slot_size config, wire into engine, add bench subcommand, build, fix type errors, build again, fix warnings — and finally, run the benchmark.

The choice of slot sizes (10, 5, 2, 1) is itself a deliberate experimental design. Slot_size=10 establishes the baseline (no slotted behavior). Slot_size=5 tests a moderate granularity. Slot_size=2 tests the predicted sweet spot. Slot_size=1 tests the extreme case of maximum overlap. This is a systematic sweep, not a random selection.

The assistant also chose to run all four configurations in a single invocation of the benchmark binary rather than as separate commands, ensuring consistent system state (same PCE extraction, same SRS load) across all measurements.

The Discovery That Followed

The subsequent message ([msg 1729]) would contain the assistant's analysis of the results, revealing that the slotted pipeline's performance was radically different from predictions. The b_g2_msm fixed cost meant that slot_size=2 and slot_size=5 were actually worse than the batch-all baseline — 177.8 seconds and 98.3 seconds respectively, compared to 63.4 seconds for slot_size=10. Only slot_size=1 showed promise at ~39 seconds total, but with the caveat that GPU time per slot (3.1 seconds) was much smaller than synthesis time per slot (~29 seconds), limiting the benefits of overlap.

This is the kind of discovery that only comes from actually running code on real hardware. The design doc's linear extrapolation from 10-circuit measurements failed to account for the fixed-cost structure of the GPU's MSM operations. The message [msg 1726] is the moment that made this discovery possible — the bridge between theory and reality, between "it compiles" and "it works (or doesn't) as expected."