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

src/search/hnsw_indexer.cc

@@ -24,7 +24,6 @@
 
 #include <algorithm>
 #include <cmath>
-#include <memory>
 #include <queue>
 #include <random>
 #include <unordered_set>
@@ -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() {

src/search/hnsw_indexer.h

@@ -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,

tests/cppunit/hnsw_index_test.cc

@@ -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(); }