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[diskann-inmem] Add scalar quantization - #1484
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Mark Hildebrand (hildebrandmw)
added this pull request to stack #1468
October 15, 2026 00:53
Codecov Report❌ Patch coverage is ❌ Your patch status has failed because the patch coverage (87.80%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage. Additional details and impacted files@@ Coverage Diff @@
## main #1484 +/- ##
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Harsha Simhadri (harsha-simhadri)
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🟡 Changes recommended
Inner-product query rescaling lacks distance compensation, affecting returned scores and range-search filtering.
1 open issue
What changed in this PR
Adds 1–8-bit scalar quantization to diskann-inmem, extending the quantized representation introduced for product quantization.
Changes:
- Separates insertion and query distances to support inner-product query rescaling.
- Adds scalar benchmark support and integration regression baselines.
- Simplifies distance construction and shares quantizer test utilities.
| File | Description |
|---|---|
| diskann-vector/src/distance/distance_provider.rs | Adds public distance constructor. |
| diskann-inmem/src/repr/spherical.rs | Updates distance construction in tests. |
| diskann-inmem/src/repr/quantized/scalar.rs | Implements scalar quantization and unit tests. |
| diskann-inmem/src/repr/quantized/product.rs | Reuses shared test utilities. |
| diskann-inmem/src/repr/quantized/mod.rs | Separates insertion and query distance routing. |
| diskann-inmem/src/repr/internal/quantization/rerank.rs | Simplifies reranker distance construction. |
| diskann-inmem/src/repr/full.rs | Adopts distance constructor. |
| diskann-inmem/src/neighbors.rs | Simplifies empty-list assertions. |
| diskann-inmem/integration/main.rs | Registers scalar regression tests. |
| diskann-inmem/integration/jsons/graph/scalar/two-bit-cosine.json | Configures two-bit cosine test. |
| diskann-inmem/integration/jsons/graph/scalar/two-bit-cosine-baseline.json | Records cosine regression baseline. |
| diskann-inmem/integration/jsons/graph/scalar/one-bit-l2.json | Tests one-bit L2 with optional reranking. |
| diskann-inmem/integration/jsons/graph/scalar/one-bit-l2-baseline.json | Records one-bit L2 baselines. |
| diskann-inmem/integration/jsons/graph/scalar/four-bit-ip.json | Configures four-bit inner-product test. |
| diskann-inmem/integration/jsons/graph/scalar/four-bit-ip-baseline.json | Records inner-product baseline. |
| diskann-inmem/integration/jsons/graph/scalar/eight-bit-l2.json | Configures eight-bit L2 test. |
| diskann-inmem/integration/jsons/graph/scalar/eight-bit-l2-baseline.json | Records eight-bit L2 baseline. |
| diskann-inmem/integration/index/runner.rs | Adds scalar training and index construction. |
| diskann-benchmark/src/index/inmem2.rs | Registers and implements scalar benchmarks. |
| diskann-benchmark/example/inmem2.json | Adds scalar benchmark example. |
Review effort: Balanced
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| let scale = mean_norm / norm; | ||
| let rescaled = query.iter().map(|q| q * scale).collect(); | ||
| return Calf::Owned(rescaled); |
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This is fine. This is how it's always worked. We don't really expect distances retrieved from quantized searches to be meaningful outside of ordering.
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This adds support for scalar quantization (bits 1 through 8) to
diskann-inmem, follow the pattern mostly set up in #1469. This does require theQuantizertrait to differentiate between distances for insert and distances for general queries due to the need to rescale queries for wiki-cohere embedding.I've found several areas in
diskann-quantizationthat need help:diskann-quantizationneeds better support for asymmetrical compression. It's really a shame that we need to handle the somewhat sketch query rescaling for inner-products manually.