Summary:
Add automatic per-block interpolation search selection (`kAuto` mode) for index blocks. During SST construction, each index block's key distribution is analyzed using the coefficient of variation (CV) of gaps between restart-point keys. Blocks with uniformly distributed keys are flagged via a new bit in the data block footer, and at read time, `kAuto` resolves to interpolation search for uniform blocks and binary search otherwise.
## Key changes
- **New `BlockSearchType::kAuto` enum value**: Resolves per-block at read time to either `kInterpolation` or `kBinary` based on the block's uniformity flag. Falls back to `kBinary` on older versions that don't recognize it.
- **Write-path uniformity analysis**: `BlockBuilder::ScanForUniformity()` uses Welford's online algorithm to incrementally compute the CV of key gaps at restart points. The result is stored in a new bit (bit 30) of the data block footer's packed restart count.
- **New table option `uniform_cv_threshold`** (default: -1 `disabled`): Controls how strict the uniformity check is. Set to negative to disable. Exposed in C++, Java (JNI), and `db_bench`.
- **Code reorganization**: Block entry decode helpers (`DecodeEntry`, `DecodeKey`, `DecodeKeyV4`, `ReadBe64FromKey`) moved from `block.cc` to a new shared header `block_util.h` so they can be reused by `BlockBuilder` on the write path.
- **New histogram `BLOCK_KEY_DISTRIBUTION_CV`**: Records the CV (scaled by 10000) of each index block's key distribution for observability.
- **Java bindings**: `IndexSearchType.kAuto`, `uniformCvThreshold` getter/setter, JNI portal constructor signature updated, and `HistogramType.BLOCK_KEY_DISTRIBUTION_CV` added.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/14383
Test Plan:
- `IndexBlockTest.IndexValueEncodingTest` parameterized to include `kAuto` search type alongside `kBinary` and `kInterpolation`, verifying correct seek/iteration behavior across all combinations of key distributions, restart intervals, and key lengths.
- Uniformity detection validated: blocks with uniform key distribution correctly set `is_uniform = true`, blocks with clustered/non-uniform keys set `is_uniform = false`.
- Stress test coverage
- Updated check_format_compatible to also include a "uniform" dataset. By default using uniform_cv_threshold=-1 does not result in an incompatibility issues. When manually changing the threshold (e.g. `uniform_cv_threshold=1000`), I see `bad block contents`, which is expected
## Benchmark
readrandom with `fillrandom,compact -seed=1 --statistics`:
| Benchmark | Branch | Params | avg ops/s | % change vs main | CV P50 |
|-----------|--------|--------|-----------|------------------|--------|
| readrandom | main | `binary_search, shortening=1` | 335,791 | baseline | N/A |
| readrandom | feature | `binary_search, shortening=1` (default) | 335,749 | -0.0% | 1,500 |
| readrandom | feature | `auto_search, shortening=1` (kAuto) | 366,832 | **+9.2%** | 1,500 |
| readrandom | feature | `interpolation_search, shortening=1` | 366,598 | **+9.2%** | 1,500 |
| readrandom | feature | `auto_search, shortening=2` (kAuto) | 344,631 | **+2.6%** | 1,030,000 |
| readrandom | feature | `interpolation_search, shortening=2` | 201,178 | **-40.1%** | 1,030,000 |
As seen with shortening=2, a non-uniform distribution produces a high CV, which does not use interpolation search.
## Write benchmark
There is a write overhead which scans each restart entry for a block upon Finish. In practice this is very low because currently it is only applied to index blocks.
See cpu profile (https://fburl.com/strobelight/io5hwj9h) here of `-benchmarks=fillseq,compact -compression_type=none -disable_wal=1`. Only 0.08% attributed to `ScanForUniformity`.
Reviewed By: pdillinger
Differential Revision: D94738890
Pulled By: joshkang97
fbshipit-source-id: 9661ac593c5fef89d49f3a8a027f1338a0c96766
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