The Validation Launch: Benchmarking Phase 12's Channel Capacity Fix

On its surface, message [msg 3150] appears to be nothing more than a routine bash command: start a background daemon process with a specific configuration, redirect its output to a log file, and capture its process ID. But in the context of an intensive optimization campaign spanning dozens of engineering iterations, this single command marks a pivotal transition — the moment when a carefully reasoned theoretical fix meets the unforgiving reality of empirical measurement.

The Message

The assistant executes:

FIL_PROOFS_PARAMETER_CACHE=/data/zk/params nohup /home/theuser/curio/extern/cuzk/target/release/cuzk-daemon --config /tmp/cuzk-p11-int12.toml > /home/theuser/cuzk-p12-chanfix-pw10.log 2>&1 & echo "PID=$!"
PID=496436

The command launches the cuzk-daemon — a high-performance Groth16 proof generation server for Filecoin's Proof-of-Replication (PoRep) protocol — using a configuration file from Phase 11 (cuzk-p11-int12.toml) but with a freshly compiled binary containing a Phase 12 code change. The output is redirected to a log file whose name encodes the experiment: cuzk-p12-chanfix-pw10.log — Phase 12, channel fix, partition_workers=10.

The Context: A Memory Crisis

To understand why this message matters, one must trace back through the optimization history. The SUPRASEAL_C2 Groth16 proof generation pipeline for Filecoin PoRep is a memory-intensive beast, with a peak footprint of approximately 200 GiB per proof in its original form. The engineering team had been methodically working through optimization phases:

Phase 9 optimized PCIe transfer patterns, achieving 14.2% throughput improvement in single-worker mode before hitting DDR5 bandwidth contention.
Phase 10 attempted a two-lock GPU interlock architecture but was abandoned after discovering fundamental CUDA device-global synchronization conflicts that caused OOM and performance regression.
Phase 11 implemented memory-bandwidth-aware pipeline scheduling with three targeted interventions to reduce DDR5 memory bandwidth contention.
Phase 12 introduced a split GPU proving API that decoupled the GPU worker's critical path from CPU post-processing by allowing b_g2_msm to run asynchronously in the background while the GPU worker immediately looped back for the next job. Phase 12's split API was a breakthrough — it achieved 37.1 seconds per proof — but it came with a hidden memory trap. The pipeline architecture used a bounded channel between CPU synthesis tasks and GPU workers, with a default capacity of 1 (synthesis_lookahead = 1). This channel was supposed to provide backpressure: if the GPU was busy, the synthesis task would block on send(), preventing unbounded memory growth from pre-synthesized proofs. But the channel capacity of 1 was designed for the non-partitioned pipeline. When partition_workers (pw) was set to 10 or 12, up to that many partitions could synthesize concurrently. The semaphore controlling concurrent syntheses would release a permit when synthesis completed — before the send() to the channel. With channel capacity of 1, nine out of ten completed syntheses would block on send(), each holding approximately 16 GiB of synthesis output (or ~4 GiB after the early a/b/c free optimization) in memory while waiting. At pw=12, this caused the process to OOM at 668 GiB peak RSS.

The Fix: Channel Capacity Auto-Scaling

The assistant's reasoning in the preceding messages ([msg 3142], [msg 3144]) reveals a clear analytical process. The initial attempt to fix the memory problem — holding the semaphore permit until after the channel send — capped memory but killed throughput (regression to 40.5 seconds per proof). The assistant recognized this as the wrong approach:

The right fix: Increase the channel capacity so completed syntheses flow into the channel buffer instead of blocking while holding their full memory allocations.

The insight was elegant: the channel capacity should auto-scale to match partition_workers when partition mode is active. With capacity set to pw, all completed syntheses can enqueue without blocking. When the channel is full, the (pw+1)th send blocks — which is exactly the backpressure desired. Memory becomes bounded at approximately 2 × pw × per_partition_size, a predictable and manageable footprint.

The assistant implemented this by modifying the channel creation code in engine.rs to compute effective_lookahead = max(synthesis_lookahead, partition_workers) when partition mode is active. The config file cuzk-p11-int12.toml doesn't set synthesis_lookahead, so it defaults to 1 — but the auto-scaling logic overrides this to 10 (matching pw=10).

Why This Specific Configuration?

The choice of pw=10 is deliberate and conservative. The assistant states in [msg 3149]: "First, pw=10 (the known-good configuration) to verify no regression." This is a standard engineering practice: before testing a fix at the edge where it previously failed (pw=12), first validate that it doesn't break what already works. The config file is reused from Phase 11 (cuzk-p11-int12.toml), ensuring that only the code change (channel capacity auto-scaling) differs from the previous baseline.

The environment variables and paths reveal the production setup: FIL_PROOFS_PARAMETER_CACHE=/data/zk/params points to a cache of Filecoin zero-knowledge parameters, and the binary path /home/theuser/curio/extern/cuzk/target/release/cuzk-daemon shows this is a development build from the curio repository's external cuzk dependency.

Assumptions and Risks

The assistant makes several assumptions in this message:

The daemon will start successfully: The freshly compiled binary could have runtime errors not caught at compile time (the build only showed pre-existing visibility warnings, not errors).
The SRS preload will complete: The configuration specifies preload = ["porep-32g"], which loads the Structured Reference String for the 32 GiB PoRep circuit — a multi-gigabyte data structure that takes ~16 seconds to load.
The log file will capture all output: Using nohup and redirecting both stdout and stderr ensures the daemon survives shell exit and all output is captured.
The benchmark will produce comparable results: The assistant assumes that the only difference between this run and the Phase 12 baseline is the channel capacity fix, and that any throughput difference will be attributable to this change. A subtle risk is that the larger channel capacity could worsen memory usage rather than improve it — if the GPU workers cannot keep up with synthesis, the channel buffer could fill with completed jobs, each holding significant memory. The assistant's reasoning acknowledges this: "Memory is bounded at ~2×pw × per_partition_size." The factor of 2 comes from the fact that pw syntheses can be in-flight (holding memory during computation) while pw completed jobs sit in the channel buffer.

Knowledge Flow

Input knowledge required to understand this message includes: the architecture of the cuzk pipeline (synthesis tasks feeding GPU workers via a bounded channel), the partition mode that splits proofs into independently processable chunks, the semaphore-based concurrency control for partition synthesis, the memory characteristics of each partition (~16 GiB of evaluation vectors before the early a/b/c free optimization, ~4 GiB after), and the history of Phase 12's split API and its OOM failures at pw=12.

Output knowledge created by this message includes: the first empirical data point for the channel capacity fix at pw=10 (later revealed in [msg 3151] as achieving effective_lookahead=10 with configured_lookahead=1 and partition_workers=10), and the foundation for subsequent benchmarks at higher pw values. The log file cuzk-p12-chanfix-pw10.log becomes a permanent record of the daemon's behavior under this configuration.

The Broader Significance

This message exemplifies a critical pattern in systems optimization: the transition from design to validation. The assistant had spent the preceding messages ([msg 3140] through [msg 3148]) deeply analyzing the memory backpressure problem, reasoning about channel capacity dynamics, implementing the auto-scaling fix, and building the binaries. Message [msg 3150] is the first concrete action that tests whether that reasoning holds in practice.

The log filename itself — cuzk-p12-chanfix-pw10.log — encodes the experimental methodology: a clear label identifying the phase (p12), the intervention (chanfix), and the parameter under test (pw10). This systematic naming convention enables the assistant to later compare results across runs and attribute performance changes to specific variables.

The subsequent messages ([msg 3151] through [msg 3159]) reveal the outcome: the fix works at pw=10 (RSS peaks at ~390 GiB, within budget), but throughput shows a slight regression (38.8 seconds per proof vs 37.1 seconds baseline). This leads to further investigation — is the regression noise, or does the larger channel somehow alter scheduling dynamics? The assistant dives into buffer counters, GPU timing logs, and inter-completion intervals to diagnose the cause.

In the broader narrative of the optimization campaign, this message represents the moment when a carefully designed memory backpressure mechanism is put to the test — a test that will ultimately determine whether Phase 12 can deliver on its promise of high throughput without OOM.