The 125-Second Proof: Validating Cross-Sector Batching in the cuzk SNARK Pipeline

Message in Focus

The following message, sent by the AI assistant at index 738 of the conversation, represents a pivotal moment in the development of the cuzk proving engine — the end-to-end GPU validation of cross-sector batching for Filecoin PoRep (Proof of Replication) Groth16 proofs. The message is deceptively simple: a single bash command invocation and its output. But beneath that surface lies the culmination of weeks of architectural design, pipeline engineering, and optimization work spanning multiple phases of the project.

[assistant] [bash] echo "=== Batch=2 test: 2 concurrent proofs ===" && echo "Start: $(date +%T)" && /home/theuser/curio/extern/cuzk/target/release/cuzk-bench --addr http://127.0.0.1:9821 batch -t porep --c1 /data/32gbench/c1.json --miner-id 1000 --count 2 -j 2 2>&1 && echo "End: $(date +%T)"
=== Batch=2 test: 2 concurrent proofs ===
Start: 22:36:01
[2026-02-17T21:36:01.822839Z INFO cuzk_bench] loading C1 output path=/data/32gbench/c1.json
=== Batch Benchmark ===
proof type:  porep
count:       2
concurrency: 2

  [1/2] COMPLETED — 125.3s (prove=69382 ms, queue=519 ms)
  [2/2] COMPLETED — 125.3s (prove=69382 ms, queue=498 ms)

=== Batch Summary ===
total time:    125.4s
completed:     2
failed:        0
wall time:     avg=125.3s min=125.3s max=125.3s

Why This Message Was Written: The Motivation and Context

This message was written as the second test in a four-test validation campaign for Phase 3 of the cuzk proving engine — a feature called "cross-sector batching." The overarching goal of Phase 3 was to improve proof generation throughput by synthesizing multiple sectors' circuits together in a single batch, amortizing the fixed CPU synthesis cost across multiple proofs.

The context leading up to this message is essential. In the preceding messages (msg 717–737), the assistant had been systematically executing a test plan. It first established a baseline by running a single PoRep proof through the daemon configured without batching (max_batch_size=1), measuring a peak memory footprint of ~202.9 GiB RSS during synthesis and a total proof time of ~89 seconds (55.6s synthesis + 34.4s GPU). It then stopped the baseline daemon, started a new daemon with max_batch_size=2 and max_batch_wait_ms=30000, and executed Test 1: Timeout Flush — submitting a single proof to verify the BatchCollector correctly flushed it after the 30-second timeout. That test passed with a queue time of 30,258ms, confirming the timeout mechanism worked.

Now, in message 738, the assistant was executing Test 2: Batched Proofs — the core validation of the cross-sector batching feature. This was the moment where the entire Phase 3 architecture would either prove itself or fail. The assistant's todo list (visible in msg 737) shows this test was marked as "in_progress" with high priority, and the preceding message ended with "Now let's run Test 2: Batched proofs — submit 2 concurrent PoRep proofs."

The motivation was clear: validate that the BatchCollector, multi-sector synthesis path (synthesize_porep_c2_multi), batched GPU proving, and proof splitting pipeline all worked correctly end-to-end with real 32 GiB PoRep data on an RTX 5070 Ti GPU.## How Decisions Were Made: The Test Design

The assistant made several deliberate decisions in how this test was constructed. First, it chose to use the batch subcommand of cuzk-bench with --count 2 -j 2, meaning two proofs submitted with concurrency 2 (both submitted simultaneously). This was critical: the BatchCollector needed to see both proofs arrive at nearly the same time to trigger the synth_batch_full path rather than the timeout flush. If the proofs were submitted sequentially with a delay, the first might have timed out before the second arrived, defeating the purpose of the test.

Second, the assistant used the same --c1 /data/32gbench/c1.json input for both proofs. This was a practical choice — the C1 output (the result of Phase 1 of the PoRep proving process) is the same for both sectors being proven. In a real deployment, each sector would have its own C1 output, but for validation purposes, using identical inputs was sufficient to verify the batching machinery worked correctly.

Third, the assistant chose to run this test against the daemon that was already running from the timeout flush test (msg 735), rather than restarting it. This was a deliberate efficiency decision — the daemon already had the SRS parameters loaded and was configured with max_batch_size=2. Restarting would have incurred the ~15-second SRS loading overhead unnecessarily.

The decision to use the --addr http://127.0.0.1:9821 flag (positioned before the subcommand) rather than -a (which had failed in msg 734 due to argument ordering) shows the assistant learning from a previous mistake and adjusting its command syntax accordingly.

Assumptions Made by the Assistant

Several assumptions underlay this test:

The BatchCollector would fill immediately: The assistant assumed that submitting two proofs with concurrency 2 would cause them to arrive at the daemon within the same scheduling window, triggering synth_batch_full rather than the timeout flush. This was a reasonable assumption given the daemon's architecture, but it was not guaranteed — network latency, request queuing, or scheduling delays could have caused the proofs to arrive far enough apart that the first would time out.
Multi-sector synthesis would be as fast as single-sector synthesis: The assistant expected that synthesizing 20 circuits (2 sectors × 10 partitions each) would take approximately the same wall-clock time as synthesizing 10 circuits for a single sector. This assumption was based on the observation that CPU synthesis is embarrassingly parallel — the rayon thread pool saturates all available cores, and doubling the number of circuits simply means more work for the already-saturated cores, not more time. This assumption turned out to be correct (55.3s for 20 circuits vs 55.6s for 10 circuits).
GPU time would scale linearly: The assistant assumed that GPU proving time for 20 circuits would be approximately 2× the time for 10 circuits, since the GPU processes circuits sequentially. This was also correct (69.4s vs 34.4s).
The daemon would correctly split batched proofs: The assistant assumed that the split_batched_proofs mechanism would correctly separate the 3840-byte combined proof into two 1920-byte Groth16 proofs, one per sector. This required the pipeline to track per-sector circuit boundaries during synthesis and pass them to the GPU prover.
The daemon was still running with the correct configuration: The assistant assumed that the daemon started in msg 726 was still alive and hadn't crashed during the timeout flush test. This was verified implicitly by the fact that the cuzk-bench command succeeded in connecting to it.

Mistakes and Incorrect Assumptions

There were no outright mistakes in this message, but there were limitations in what the output revealed. The batch benchmark output shows only the client-side perspective: it reports that both proofs completed in 125.3s with queue times of ~500ms and prove times of ~69.4s. However, this output does not show the critical internal details:

It does not show that the synthesis phase took 55.3s for both proofs combined (the amortization win).
It does not show that the BatchCollector triggered synth_batch_full (immediate flush, not timeout).
It does not show that synthesize_porep_c2_multi{num_sectors=2} was invoked.
It does not show the total_circuits=20 log message confirming 20 circuits were synthesized.
It does not show the proof splitting and per-sector delivery. The assistant recognized this limitation and immediately followed up in msg 739 by examining the daemon logs to verify these internal details. The batch benchmark output alone was insufficient to confirm the batching behavior — it only confirmed that both proofs completed successfully. The real validation required correlating the client-side timings with server-side log messages. Another subtle issue: the queue time reported by the client (~500ms) is misleadingly low. This is the time the proof spent in the daemon's request queue before being assigned to the BatchCollector, not the time it spent waiting in the BatchCollector. The actual batch wait time was zero because the collector filled immediately. The 30,000ms timeout was never triggered for this test, which is exactly the desired behavior for a full batch.

Input Knowledge Required

To fully understand this message, one needs knowledge of:

Filecoin PoRep (Proof of Replication): The proving system used to prove that a storage provider is storing a unique copy of data. A 32 GiB sector requires proving 10 "partitions" (circuits), each corresponding to a layer of a Merkle tree.
Groth16 proofs: The zero-knowledge proof system used by Filecoin. Each proof is 1920 bytes for PoRep (192 bytes for WinningPoSt).
The cuzk proving engine: A custom SNARK proving engine built on top of bellperson (a Bellman fork) and supraseal-c2 (CUDA GPU acceleration). The engine implements a pipelined architecture where CPU synthesis and GPU proving are decoupled and can overlap.
Phase 3 batching architecture: The BatchCollector component that accumulates proofs of the same type until either max_batch_size is reached (immediate flush) or max_batch_wait_ms elapses (timeout flush). The multi-sector synthesis path (synthesize_porep_c2_multi) that combines multiple sectors' circuits into a single synthesis job. The split_batched_proofs mechanism that separates the combined proof output into per-sector proofs.
The baseline performance numbers: The assistant had established in earlier messages that a single PoRep proof takes ~89s total (55.6s synthesis + 34.4s GPU) with a peak memory of ~202.9 GiB RSS.
The hardware: An RTX 5070 Ti GPU with sufficient VRAM for 20-circuit proving, and a CPU with enough cores to saturate the synthesis workload.

Output Knowledge Created

This message produced several critical pieces of knowledge:

Quantitative throughput improvement: Two proofs completed in 125.3s total, yielding an amortized time of 62.7s per proof. Compared to the 89s baseline for a single proof, this represents a 1.42× throughput improvement. The synthesis cost was fully amortized across the two sectors.
Confirmation of the batching architecture: The fact that both proofs completed with identical timings (125.3s each) and no failures confirmed that the BatchCollector, multi-sector synthesis, batched GPU proving, and proof splitting all worked correctly.
GPU time scaling confirmation: The prove time of 69.4s for 2 sectors (20 circuits) vs 34.4s for 1 sector (10 circuits) confirmed that GPU time scales linearly with circuit count, as expected.
Synthesis amortization confirmation: The total wall time of 125.3s for 2 proofs vs ~89s for 1 proof confirmed that synthesis is the bottleneck that benefits most from batching.
A validated benchmark methodology: The test demonstrated that the cuzk-bench batch command with -j 2 (concurrency equal to count) is an effective way to test batching behavior, and that client-side timings must be supplemented with server-side log analysis for full validation.

The Thinking Process Visible in the Message

While the message itself is just a bash command and its output, the thinking process is visible in the choices made:

The assistant chose to use --count 2 -j 2 rather than, say, --count 2 -j 1 (sequential submission). This shows an understanding that the BatchCollector requires concurrent arrivals to trigger the full-batch path. If the proofs were submitted sequentially with concurrency 1, the first proof would likely time out after 30s before the second was submitted, resulting in two separate single-proof batches rather than one batch-of-2.

The assistant also chose to run this test immediately after the timeout flush test without restarting the daemon. This shows an understanding that the daemon's state was still valid — the SRS was loaded, the BatchCollector was empty after the timeout flush, and the configuration was unchanged. Restarting would have been wasteful.

The timing of the test (22:36:01 start, 125.3s duration) shows the assistant expected the total time to be approximately 55s synthesis + 69s GPU ≈ 124s, which is remarkably close to the actual 125.3s. This expectation was based on the assumption that synthesis time would be the same as single-sector (since CPU cores are saturated either way) and GPU time would double.

The fact that the assistant immediately followed up by examining daemon logs (msg 739) shows it understood that the client-side output alone was insufficient to validate the batching behavior. It needed to see the synth_batch_full, synthesize_porep_c2_multi, total_circuits=20, and split_batched_proofs log messages to confirm the internal pipeline worked as designed.

Conclusion

Message 738 represents a successful validation of the Phase 3 cross-sector batching feature in the cuzk proving engine. The test demonstrated a 1.42× throughput improvement with batch size 2, confirmed that synthesis cost is fully amortized across sectors, and validated the entire BatchCollector → multi-sector synthesis → batched GPU proving → proof splitting pipeline. The message is a textbook example of a well-designed benchmark test: it builds on established baselines, tests the specific behavior of interest (batching), and produces clear quantitative results that can be compared against predictions. The assistant's follow-up analysis in msg 739–745, examining daemon logs and running additional tests (3-proof overflow, WinningPoSt bypass), completed the validation campaign and provided comprehensive evidence that Phase 3 was ready for production deployment.