The Clean Slate: Why a Single Bash Command Marks a Turning Point in PCE Optimization

The Message

All three build clean. Let me remove the old v1 file and run the benchmark with the raw format: ``bash rm -f /tmp/pce-porep-32g.bin /data/zk/params/pce-porep-32g.bin ``

At first glance, this appears to be the most mundane possible action in a coding session: deleting two temporary files before running a benchmark. Yet within the context of the SUPRASEAL_C2 Groth16 proof generation pipeline for Filecoin PoRep, this rm command represents a pivotal transition — the moment when an optimization hypothesis meets empirical validation. The assistant is not merely cleaning up disk space; it is clearing the way for a benchmark that will determine whether weeks of architectural work have succeeded or fallen short.

The Weight of 25.7 Gigabytes

To understand why this message matters, one must first grasp what these files represent. The two paths — /tmp/pce-porep-32g.bin and /data/zk/params/pce-porep-32g.bin — are serialized instances of a PreCompiledCircuit<Scalar>, a data structure that encodes the complete constraint system for Filecoin's Proof-of-Replication (PoRep) circuit. At 25.7 GiB, this is not a configuration file or a lightweight cache; it is a massive binary artifact containing 722 million scalar field elements, three compressed sparse row (CSR) matrices (A, B, C), and density bitmaps that together define the circuit topology for the Groth16 prover.

The first path, /tmp/pce-porep-32g.bin, resides on a tmpfs (RAM-backed filesystem), offering the fastest possible I/O. The second, /data/zk/params/pce-porep-32g.bin, sits on an NVMe drive, representing the persistent storage location where the daemon would load PCE data at startup. Both files were produced by the previous serialization format — bincode — and both are now obsolete. The assistant deletes them because the format has fundamentally changed.

The Discovery That Made This Necessary

The decision to delete these files was not arbitrary; it was the inevitable consequence of a discovery made just moments earlier in the conversation. In message [msg 1637], the assistant ran the first PCE disk serialization benchmark and received alarming results:

| Operation | Time | Effective Throughput | |---|---|---| | Save (tmpfs) | 16.8s | 1.6 GB/s | | Save (NVMe) | 30.6s | 0.9 GB/s | | Load (tmpfs) | 49.9s | 0.6 GB/s |

The load time of 49.9 seconds was the critical finding. This was not merely slow — it was slower than extracting the PCE from scratch (47 seconds). The entire premise of disk persistence was that loading a pre-computed circuit would be faster than recomputing it. If loading took longer than extraction, disk persistence had no value. The bottleneck was not I/O bandwidth (tmpfs delivers 5+ GB/s), but the per-element deserialization overhead of bincode. For each of the 722 million scalar values, bincode was parsing individual byte sequences, applying type-level deserialization logic, and constructing Rust objects one at a time. At 14.6 million scalars per second, this CPU-bound parsing was the culprit.

The assistant's analysis in [msg 1638] was swift and quantitative. Using a Python calculation, it determined that a raw binary format — which would treat &[Scalar] as &[u8] and perform a single bulk read — could achieve load times of 3–5 seconds, a 10–16× improvement. The key insight was that blstrs::Scalar is #[repr(transparent)] over blst_fr, which is a fixed [u64; 4] array. The memory layout is known, stable, and safe to reinterpret as raw bytes.

The Implementation Decision

Message [msg 1642] shows the assistant rewriting the entire disk.rs module to replace bincode with a raw binary format. The new format retains the 32-byte header (magic bytes, version, circuit dimensions) for validation, but replaces the bincode payload with length-prefixed raw byte dumps: each vector (row_ptrs: Vec<u32>, cols: Vec<u32>, vals: Vec<Scalar>) is written as a u64 byte count followed by the raw memory contents. This is essentially write(fd, &buf) at the OS level — the fastest possible serialization.

The decision to use unsafe byte reinterpretation was a deliberate engineering tradeoff. The alternative — using a library like serde_bytes or implementing a safe zero-copy deserializer — would add complexity and potentially reintroduce per-element overhead. The assistant judged that the memory layout guarantees of Vec<u32> (contiguous, aligned, no padding) and Vec<Scalar> (repr(transparent) over a fixed-size array) made the unsafe cast safe in practice. This is a common pattern in high-performance Rust: accept localized unsafety for dramatic performance gains, with the understanding that the invariants are well-understood and unlikely to change.

Why Delete the Old Files?

With the new raw format implemented and all three build targets (cuzk-pce, cuzk-bench, cuzk-daemon) compiling cleanly in <msg id=1643-1645>, the assistant arrives at the subject message. The rm -f command serves multiple purposes:

  1. Format incompatibility: The old bincode-format files cannot be loaded by the new raw-format reader. If they remain on disk, the benchmark tool might attempt to load them (depending on path precedence) and fail with a deserialization error, wasting time and obscuring results.
  2. Clean measurement: Benchmarking the new format requires a cold start — loading the file from scratch, not from the kernel's page cache. By deleting the old files, the assistant ensures that the subsequent benchmark measures the raw format's true load performance without interference from cached pages of the old file.
  3. Eliminating confusion: The two paths represent different storage tiers (tmpfs vs NVMe). Having stale files in either location could lead to confusing benchmark results where the wrong file is loaded, or where a mix of old and new formats produces misleading timings.
  4. Psychological closure: Deleting the old format files is a ritual of sorts — a visible signal that the old approach has been superseded and the team is moving forward with the new design.

Assumptions and Risks

The assistant makes several assumptions in this message, some explicit and some implicit:

The raw format will actually be faster. The 10–16× improvement is a theoretical estimate based on I/O bandwidth calculations. Real-world performance depends on many factors: the kernel's page cache behavior, the overhead of Vec::reserve and Vec::set_len during loading, the cost of the single unsafe transmutation, and whether the CPU memory bandwidth is sufficient to sustain the copy. The benchmark will reveal whether theory matches practice.

The unsafe byte reinterpretation is sound. This is the most significant risk. If blstrs::Scalar changes its internal representation (e.g., adding padding, changing alignment, or switching to a different field implementation), the raw binary format would produce silently wrong results. The assistant mitigates this by using a version field in the 32-byte header, but the fundamental assumption is that [u64; 4] is and will remain the canonical representation.

No other process depends on the old files. The daemon might have been configured to load from /data/zk/params/pce-porep-32g.bin at startup. Deleting the file while the daemon is not running is safe, but if the daemon were restarted before the new format file is written, it would fail to find the PCE cache and fall back to extraction — a performance regression but not a correctness bug.

The benchmark will produce actionable results. There is always a risk that the benchmark reveals the raw format is only marginally faster, or that some other bottleneck (e.g., the 32-byte header validation, the allocation of 25.7 GiB of memory) dominates the load time. The assistant is proceeding with confidence based on the analysis, but the data will tell the true story.

The Broader Architectural Context

This message does not exist in isolation. It is the culmination of a multi-session investigation into the SUPRASEAL_C2 pipeline's ~200 GiB peak memory footprint, documented across four major documents and nine identified bottlenecks. The PCE disk persistence work is part of a larger strategy: the "Persistent Prover Daemon" proposal aims to eliminate SRS (Structured Reference String) loading overhead by keeping the prover alive across multiple proof requests. PCE preloading is a critical component — if the daemon can load the pre-computed circuit from disk in 3–5 seconds at startup, the first proof request suffers no penalty. If loading takes 50 seconds (as bincode did), the first-proof latency is worse than not using PCE at all.

The assistant's work on the raw binary format is therefore not merely a serialization optimization; it is an enabler for the entire daemon architecture. Without fast PCE loading, the Persistent Prover Daemon proposal would be dead on arrival — the first proof would always pay a heavy startup cost, negating the benefits of persistence.

What Comes Next

The rm command is followed by a benchmark run (visible in the next message, [msg 1647]). The results will determine whether the raw format delivers on its promise. If successful, the assistant will integrate PCE preloading into the daemon startup path, update the project documentation, and commit the changes to git. If unsuccessful, the assistant will need to investigate further — perhaps the unsafe cast has hidden costs, or the allocation of 25.7 GiB dominates the load time, or some other factor intervenes.

Either way, this single bash command marks the boundary between design and validation. The old format is gone. The new format is about to be tested. The assistant has committed to the raw binary approach and is now taking the irrevocable step of destroying the evidence of the old approach. It is a small act, but in the context of a 200 GiB proving pipeline where every second counts, it is a meaningful one.