Improve parallelism of refine host

cpp/include/raft/neighbors/detail/refine_host-inl.hpp

@@ -43,6 +43,58 @@ template <typename DC, typename IdxT, typename DataT, typename DistanceT, typena
     "neighbors::refine_host(%zu, %zu -> %zu)", n_queries, orig_k, refined_k);
 
   auto suggested_n_threads = std::max(1, std::min(omp_get_num_procs(), omp_get_max_threads()));
+
+  // If the number of queries is small, separete the distance calculation and
+  // the top-k calculation into separete loops, and apply finer-grained thread
+  // parallelism to the distance calculation loop.
+  if (n_queries < size_t(suggested_n_threads)) {
+    std::vector<std::vector<std::tuple<DistanceT, IdxT>>> refined_pairs(
+      n_queries, std::vector<std::tuple<DistanceT, IdxT>>(orig_k));
+
+    // For efficiency, each thread should read a certain amount of array
+    // elements. The number of threads for distance computation is determined
+    // taking this into account.
+    auto n_elements    = std::max(size_t(512), dim);
+    auto max_n_threads = raft::div_rounding_up_safe<size_t>(n_queries * orig_k * dim, n_elements);
+    auto suggested_n_threads_for_distance = std::min(size_t(suggested_n_threads), max_n_threads);
+
+    // The max number of threads for topk computation is the number of queries.
+    auto suggested_n_threads_for_topk = std::min(size_t(suggested_n_threads), n_queries);
+
+    // Compute the refined distance using original dataset vectors
+#pragma omp parallel for collapse(2) num_threads(suggested_n_threads_for_distance)
+    for (size_t i = 0; i < n_queries; i++) {
+      for (size_t j = 0; j < orig_k; j++) {
+        const DataT* query = queries.data_handle() + dim * i;
+        IdxT id            = neighbor_candidates(i, j);
+        DistanceT distance = 0.0;
+        if (static_cast<size_t>(id) >= n_rows) {
+          distance = std::numeric_limits<DistanceT>::max();
+        } else {
+          const DataT* row = dataset.data_handle() + dim * id;
+          for (size_t k = 0; k < dim; k++) {
+            distance += DC::template eval<DistanceT>(query[k], row[k]);
+          }
+        }
+        refined_pairs[i][j] = std::make_tuple(distance, id);
+      }
+    }
+
+    // Sort the query neighbors by their refined distances
+#pragma omp parallel for num_threads(suggested_n_threads_for_topk)
+    for (size_t i = 0; i < n_queries; i++) {
+      std::sort(refined_pairs[i].begin(), refined_pairs[i].end());
+      // Store first refined_k neighbors
+      for (size_t j = 0; j < refined_k; j++) {
+        indices(i, j) = std::get<1>(refined_pairs[i][j]);
+        if (distances.data_handle() != nullptr) {
+          distances(i, j) = DC::template postprocess(std::get<0>(refined_pairs[i][j]));
+        }
+      }
+    }
+    return;
+  }
+
   if (size_t(suggested_n_threads) > n_queries) { suggested_n_threads = n_queries; }
 
 #pragma omp parallel num_threads(suggested_n_threads)