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perf(search): share a thread-local RNG for HNSW and drop seed (#2398) #3562

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perf(search): share a thread-local RNG for HNSW and drop seed (#2398) #3562

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3e3b1aa
perf(search): lazily initialize HNSW random generator
advisedy Jul 20, 2026
5cfd455
fix(search): only call random_device when seed is not set
advisedy Jul 20, 2026
6c3ad7d
Merge branch 'unstable' into #2398
advisedy Jul 20, 2026
d53751b
Merge branch 'unstable' into #2398
jihuayu Jul 21, 2026
3b7a688
perf(search): use a static thread-local RNG for HNSW indexes
advisedy Jul 23, 2026
ff60ae2
perf(search): drop seed, make the RNG a class member
advisedy Jul 23, 2026
859bbaf
Merge branch 'unstable' into #2398
advisedy Jul 23, 2026
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7 changes: 3 additions & 4 deletions src/search/hnsw_indexer.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -24,7 +24,6 @@

#include <algorithm>
#include <cmath>
#include <memory>
#include <queue>
#include <random>
#include <unordered_set>
Expand Down Expand Up @@ -172,12 +171,12 @@ StatusOr<double> ComputeSimilarity(const VectorItem& left, const VectorItem& rig
}
}

HnswIndex::HnswIndex(const SearchKey& search_key, HnswVectorFieldMetadata* vector, engine::Storage* storage,
std::random_device::result_type seed)
thread_local std::mt19937 HnswIndex::generator{std::random_device()()};

HnswIndex::HnswIndex(const SearchKey& search_key, HnswVectorFieldMetadata* vector, engine::Storage* storage)
: search_key(search_key),
metadata(vector),
storage(storage),
generator(std::mt19937(seed)),
m_level_normalization_factor(1.0 / std::log(metadata->m)) {}

uint16_t HnswIndex::RandomizeLayer() {
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6 changes: 3 additions & 3 deletions src/search/hnsw_indexer.h
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Expand Up @@ -89,11 +89,11 @@ struct HnswIndex {
HnswVectorFieldMetadata* metadata;
engine::Storage* storage = nullptr;

std::mt19937 generator;
double m_level_normalization_factor;

HnswIndex(const SearchKey& search_key, HnswVectorFieldMetadata* vector, engine::Storage* storage,
std::random_device::result_type seed = std::random_device()());
static thread_local std::mt19937 generator;

HnswIndex(const SearchKey& search_key, HnswVectorFieldMetadata* vector, engine::Storage* storage);

static StatusOr<std::vector<VectorItem>> DecodeNodesToVectorItems(engine::Context& ctx,
const std::vector<NodeKey>& node_key,
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3 changes: 1 addition & 2 deletions tests/cppunit/hnsw_index_test.cc
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Expand Up @@ -66,15 +66,14 @@ struct HnswIndexTest : TestBase {
std::string idx_name = "hnsw_test_idx";
std::string key = "vector";
std::unique_ptr<redis::HnswIndex> hnsw_index;
const std::random_device::result_type seed = 14863; // fixed seed for reproducibility

HnswIndexTest() {
metadata.vector_type = redis::VectorType::FLOAT64;
metadata.dim = 3;
metadata.m = 3;
metadata.distance_metric = redis::DistanceMetric::L2;
auto search_key = redis::SearchKey(ns, idx_name, key);
hnsw_index = std::make_unique<redis::HnswIndex>(search_key, &metadata, storage_.get(), seed);
hnsw_index = std::make_unique<redis::HnswIndex>(search_key, &metadata, storage_.get());
}

void TearDown() override { hnsw_index.reset(); }
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