CAGRA max_queries auto configuration (#1613)

This PR changes the behavior of max_queries in CAGRA.

In the search operation of CAGRA, a batch of queries is divided into several `max_queries` queries sub-batches and operated; by increasing `max_queries,` higher throughput can be obtained. However, if one forgets to set `max_queries,` the default value, 1, will be used, resulting in lower throughput. This is not user-friendly.


This PR modifies CAGRA to automatically set `max_queries` as the batch size by default.

An alternative way is to remove `max_queries`.

Authors:
  - tsuki (https://github.com/enp1s0)

Approvers:
  - Tamas Bela Feher (https://github.com/tfeher)

URL: #1613

cpp/include/raft/neighbors/cagra_types.hpp

@@ -55,8 +55,8 @@ enum class search_algo {
 enum class hash_mode { HASH, SMALL, AUTO };
 
 struct search_params : ann::search_params {
-  /** Maximum number of queries to search at the same time (batch size). */
-  size_t max_queries = 1;
+  /** Maximum number of queries to search at the same time (batch size). Auto select when 0.*/
+  size_t max_queries = 0;
 
   /** Number of intermediate search results retained during the search.
    *

cpp/include/raft/neighbors/detail/cagra/cagra_search.cuh

@@ -65,7 +65,9 @@ void search_main(raft::resources const& res,
                  static_cast<size_t>(queries.extent(0)),
                  static_cast<size_t>(queries.extent(1)));
   RAFT_EXPECTS(queries.extent(1) == index.dim(), "Querise and index dim must match");
-  uint32_t topk = neighbors.extent(1);
+  const uint32_t topk = neighbors.extent(1);
+
+  if (params.max_queries == 0) { params.max_queries = queries.extent(0); }
 
   std::unique_ptr<search_plan_impl<T, internal_IdxT, DistanceT>> plan =
     factory<T, internal_IdxT, DistanceT>::create(
@@ -74,8 +76,8 @@ void search_main(raft::resources const& res,
   plan->check(neighbors.extent(1));
 
   RAFT_LOG_DEBUG("Cagra search");
-  uint32_t max_queries = plan->max_queries;
-  uint32_t query_dim   = queries.extent(1);
+  const uint32_t max_queries = plan->max_queries;
+  const uint32_t query_dim   = queries.extent(1);
 
   for (unsigned qid = 0; qid < queries.extent(0); qid += max_queries) {
     const uint32_t n_queries = std::min<std::size_t>(max_queries, queries.extent(0) - qid);

cpp/test/neighbors/ann_cagra.cuh

@@ -166,7 +166,7 @@ class AnnCagraTest : public ::testing::TestWithParam<AnnCagraInputs> {
  protected:
   void testCagra()
   {
-    if (ps.algo == search_algo::MULTI_CTA && ps.max_queries > 1) {
+    if (ps.algo == search_algo::MULTI_CTA && ps.max_queries != 1) {
       GTEST_SKIP() << "Skipping test due to issue #1575";
     }
     size_t queries_size = ps.n_queries * ps.k;
@@ -377,9 +377,9 @@ inline std::vector<AnnCagraInputs> generate_inputs()
     {100},
     {1000},
     {1, 8, 17},
-    {1, 16},       // k
+    {1, 16},          // k
     {search_algo::SINGLE_CTA, search_algo::MULTI_CTA, search_algo::MULTI_KERNEL},
-    {1, 10, 100},  // query size
+    {0, 1, 10, 100},  // query size
     {0},
     {256},
     {1},