RFC: integrate PiPNN as an alternative graph-index build algorithm

[SeliMeli]

Summary

This RFC proposes adding PiPNN (Pick-in-Partitions Nearest Neighbors, arXiv:2602.21247) as a second graph-construction algorithm for DiskANN's disk index, alongside the existing Vamana builder.

PiPNN produces a graph byte-compatible with Vamana's disk format and search API, at up to 6.3× lower build time on our measured workloads (Enron 10M, BigANN 10M). Vamana remains the default and the only algorithm supported for incremental inserts; PiPNN is the proposed faster path for full rebuilds.

Two-stage integration

• Stage 1 (this RFC): Land PiPNN behind a BuildAlgorithm selector with Vamana as the default. PiPNN is opt-in via a pipnn Cargo feature. Existing build sites see no behavior change. Stage 1 defines explicit milestones (M0–M7) gating Stage 2 readiness.
• Stage 2 (separate proposal, conditional on Stage 1 milestones): Retire Vamana's full-rebuild path. Vamana remains the implementation for incremental inserts via the hybrid update model — PiPNN for bulk/full rebuilds, Vamana for online inserts, full PiPNN rebuild triggered by recall decay.

Highlights

• Disk format and search API are unchanged — the on-disk graph layout, PQ codes, search pipeline, and public Rust types remain backward-compatible. PiPNN configuration is added under a new tagged-enum variant.
• Algorithm description: 4-phase batch builder (Partition / Local k-NN per leaf via GEMM / HashPrune merge / optional final RobustPrune). Detailed in the RFC.
• Honest gap accounting: explicit milestones for checkpoint/resume, scalar-quantization parity, label-filtered indexes, three-tier memory mitigation, production validation, and operational readiness — each required before Stage 2 is credible.
• Memory mitigation: documented three-tier build path (one-shot / disk-edges / merged-shards) selected by the existing build_ram_limit_gb knob, bringing PiPNN's peak RSS to or below Vamana's at a configurable build-time cost.
• Batch-only is a paper-confirmed algorithmic property of PiPNN, not just our implementation. The hybrid update model is the workaround.

Reviewers

Please read the full RFC for trade-offs, benchmark tables, and milestone definitions.

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds an RFC proposing PiPNN as an opt-in, feature-gated alternative to Vamana for DiskANN disk-index graph construction, keeping disk format and search API unchanged.

Changes:

• Introduces RFC 01049 detailing PiPNN’s algorithm, integration plan, and two-stage rollout
• Specifies a BuildAlgorithm selector design and feature-gating strategy (pipnn)
• Documents benchmark results and Stage-1 milestones gating potential Stage-2 deprecation of Vamana full rebuilds

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.51%. Comparing base (3d3ed4c) to head (4de515c).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

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Flag	Coverage Δ
miri	89.51% <ø> (-1.09%)	⬇️
unittests	89.14% <ø> (-1.43%)	⬇️

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[hildebrandmw]

Most of this PR is dedicated to how PiPNN will integrate into diskann-disk, and I do not have the review capacity nor much stake in that side of the integration.

What I am curious about, though, is how we plan to integrate PiPNN into the project as a whole. #856 integrates the algorithm as a separate crate with kind of its own set of abstractions, its own quantization. I have a very strong preference to keep diskann homogeneous, even if it means working on lower levels of the stack to build what we need in a more reuseable way (e.g., quantized matrix multiply, which is currently being worked on for multi-vectors). Basically, investing in shared capability where everyone can benefit.

I'm not necessarily asking that this blocks forward progress, but I would like to see milestones that commit to converging on shared abstractions/implementations somewhere in the roll out of late stage 1 or early stage 2. I would love for the PiPNN integration to leave whole project stronger and not just be a faster disk build.