Added tunable parameter useHNSW to enable/disable HNSW

lucene/sandbox/src/java/org/apache/lucene/sandbox/vectorsearch/CuVSCodec.java

@@ -36,7 +36,10 @@ public CuVSCodec(String name, Codec delegate) {
     super(name, delegate);
     KnnVectorsFormat format;
     try {
-      format = new CuVSVectorsFormat(1, 128, 64, MergeStrategy.NON_TRIVIAL_MERGE, IndexType.CAGRA);
+      boolean useHNSW = Boolean.parseBoolean(System.getProperty("lucene.cuvs.hnsw", "true"));
+      format =
+          new CuVSVectorsFormat(
+              1, 128, 64, MergeStrategy.NON_TRIVIAL_MERGE, IndexType.CAGRA, useHNSW);
       setKnnFormat(format);
     } catch (LibraryException ex) {
       Logger log = Logger.getLogger(CuVSCodec.class.getName());

lucene/sandbox/src/java/org/apache/lucene/sandbox/vectorsearch/CuVSVectorsFormat.java

@@ -48,6 +48,7 @@ public class CuVSVectorsFormat extends KnnVectorsFormat {
   public static final int DEFAULT_GRAPH_DEGREE = 64;
   public static final MergeStrategy DEFAULT_MERGE_STRATEGY = MergeStrategy.NON_TRIVIAL_MERGE;
   public static final IndexType DEFAULT_INDEX_TYPE = IndexType.CAGRA;
+  public static final boolean DEFAULT_USE_HNSW = true;
 
   static CuVSResources resources = cuVSResourcesOrNull();
 
@@ -61,6 +62,7 @@ public class CuVSVectorsFormat extends KnnVectorsFormat {
   final int graphDegree;
   final MergeStrategy mergeStrategy;
   final CuVSVectorsWriter.IndexType indexType; // the index type to build, when writing
+  final boolean useHNSW;
 
   /**
    * Creates a CuVSVectorsFormat, with default values.
@@ -73,7 +75,8 @@ public CuVSVectorsFormat() {
         DEFAULT_INTERMEDIATE_GRAPH_DEGREE,
         DEFAULT_GRAPH_DEGREE,
         DEFAULT_MERGE_STRATEGY,
-        DEFAULT_INDEX_TYPE);
+        DEFAULT_INDEX_TYPE,
+        DEFAULT_USE_HNSW);
   }
 
   /**
@@ -86,15 +89,22 @@ public CuVSVectorsFormat(
       int intGraphDegree,
       int graphDegree,
       MergeStrategy mergeStrategy,
-      IndexType indexType) {
+      IndexType indexType,
+      boolean useHNSW)
+      throws LibraryException {
     super("CuVSVectorsFormat");
     this.mergeStrategy = mergeStrategy;
     this.cuvsWriterThreads = cuvsWriterThreads;
     this.intGraphDegree = intGraphDegree;
     this.graphDegree = graphDegree;
     this.indexType = indexType;
+    this.useHNSW = useHNSW;
   }
 
+  public boolean isHNSWEnabled() {
+    return useHNSW;
+}
+
   private static CuVSResources cuVSResourcesOrNull() {
     try {
       resources = CuVSResources.create();
@@ -133,14 +143,15 @@ public CuVSVectorsWriter fieldsWriter(SegmentWriteState state) throws IOExceptio
         mergeStrategy,
         indexType,
         resources,
-        flatWriter);
+        flatWriter,
+        useHNSW);
   }
 
   @Override
   public CuVSVectorsReader fieldsReader(SegmentReadState state) throws IOException {
     checkSupported();
     var flatReader = flatVectorsFormat.fieldsReader(state);
-    return new CuVSVectorsReader(state, resources, flatReader);
+    return new CuVSVectorsReader(state, resources, flatReader, useHNSW);
   }
 
   @Override
@@ -156,6 +167,7 @@ public String toString() {
     sb.append("graphDegree=").append(graphDegree);
     sb.append("mergeStrategy=").append(mergeStrategy);
     sb.append("resources=").append(resources);
+    sb.append("useHNSW=").append(useHNSW);
     sb.append(")");
     return sb.toString();
   }

lucene/sandbox/src/java/org/apache/lucene/sandbox/vectorsearch/CuVSVectorsReader.java

@@ -69,18 +69,21 @@ public class CuVSVectorsReader extends KnnVectorsReader {
   private static final Logger log = Logger.getLogger(CuVSVectorsReader.class.getName());
 
   private final CuVSResources resources;
+  private final boolean useHNSW;
   private final FlatVectorsReader flatVectorsReader; // for reading the raw vectors
   private final FieldInfos fieldInfos;
   private final IntObjectHashMap<FieldEntry> fields;
   private final IntObjectHashMap<CuVSIndex> cuvsIndices;
   private final IndexInput cuvsIndexInput;
 
   public CuVSVectorsReader(
-      SegmentReadState state, CuVSResources resources, FlatVectorsReader flatReader)
+      SegmentReadState state, CuVSResources resources, FlatVectorsReader flatReader, boolean useHNSW)
       throws IOException {
     this.resources = resources;
     this.flatVectorsReader = flatReader;
     this.fieldInfos = state.fieldInfos;
+    this.useHNSW = Boolean.getBoolean("lucene.cuvs.hnsw");
+    log.info("CuVSVectorsReader initialized. useHNSW=" + this.useHNSW);
     this.fields = new IntObjectHashMap<>();
 
     String metaFileName =
@@ -239,6 +242,8 @@ private IntObjectHashMap<CuVSIndex> loadCuVSIndices() throws IOException {
   }
 
   private CuVSIndex loadCuVSIndex(FieldEntry fieldEntry) throws IOException {
+    log.info("Loading CuVS index for field: ");
+
     CagraIndex cagraIndex = null;
     BruteForceIndex bruteForceIndex = null;
     HnswIndex hnswIndex = null;
@@ -263,7 +268,8 @@ private CuVSIndex loadCuVSIndex(FieldEntry fieldEntry) throws IOException {
       }
 
       len = fieldEntry.hnswIndexLength();
-      if (len > 0) {
+      if (useHNSW && len > 0) {
+        log.info("Attempting to load HNSW index.");
         long off = fieldEntry.hnswIndexOffset();
         try (var slice = cuvsIndexInput.slice("hnsw index", off, len);
             var in = new IndexInputInputStream(slice)) {
@@ -350,7 +356,23 @@ public void search(String field, float[] target, KnnCollector knnCollector, Bits
     assert topK > 0 : "Expected topK > 0, got:" + topK;
 
     Map<Integer, Float> result;
-    if (knnCollector.k() <= 1024 && cuvsIndex.getCagraIndex() != null) {
+    if(useHNSW && cuvsIndex.getHNSWIndex() != null) {
+      log.info("Searching with HNSW index");
+      var hnswQuery = new com.nvidia.cuvs.HnswQuery.Builder()
+                          .withQueryVectors(new float[][] { target })
+                          .withTopK(knnCollector.k())
+                          .build();
+      List<Map<Integer, Float>> searchResult = null;
+      try {
+        searchResult = cuvsIndex.getHNSWIndex().search(hnswQuery).getResults();
+      }catch (Throwable t) {
+         handleThrowable(t);
+      }
+
+      assert searchResult.size() == 1;
+      result = searchResult.getFirst();
+    }
+    else if (knnCollector.k() <= 1024 && cuvsIndex.getCagraIndex() != null) {
       // log.info("searching cagra index");
       CagraSearchParams searchParams =
           new CagraSearchParams.Builder(resources)

lucene/sandbox/src/java/org/apache/lucene/sandbox/vectorsearch/CuVSVectorsWriter.java

@@ -129,6 +129,8 @@ public boolean hnsw() {
       return hnsw;
     }
   }
+
+  private final boolean useHNSW;
 
   public CuVSVectorsWriter(
       SegmentWriteState state,
@@ -138,7 +140,8 @@ public CuVSVectorsWriter(
       MergeStrategy mergeStrategy,
       IndexType indexType,
       CuVSResources resources,
-      FlatVectorsWriter flatVectorsWriter)
+      FlatVectorsWriter flatVectorsWriter,
+      boolean useHNSW)
       throws IOException {
     super();
     this.mergeStrategy = mergeStrategy;
@@ -148,6 +151,8 @@ public CuVSVectorsWriter(
     this.graphDegree = graphDegree;
     this.resources = resources;
     this.flatVectorsWriter = flatVectorsWriter;
+    this.useHNSW = Boolean.getBoolean("lucene.cuvs.hnsw");
+    log.info("CuVSVectorsWriter initialized. useHNSW=" + this.useHNSW);
     this.infoStream = state.infoStream;
 
     String metaFileName =
@@ -261,6 +266,14 @@ private void writeBruteForceIndex(OutputStream os, float[][] vectors) throws Thr
   }
 
   private void writeHNSWIndex(OutputStream os, float[][] vectors) throws Throwable {
+    if (!useHNSW) { // Skip HNSW writing if disabled
+      log.warning("Skipping HNSW indexing because useHNSW is false.");
+      return;
+    }
+
+    if (vectors.length == 0) {
+      log.warning("HNSW indexing failed because no vectors were provided.");
+  }
     if (vectors.length < 2) {
       throw new IllegalArgumentException(vectors.length + " vectors, less than min [2] required");
     }
@@ -347,7 +360,7 @@ private void writeFieldInternal(FieldInfo fieldInfo, float[][] vectors) throws I
       }
 
       hnswIndexOffset = cuvsIndex.getFilePointer();
-      if (indexType.hnsw()) {
+      if (useHNSW && indexType.hnsw()) {
         var hnswIndexOutputStream = new IndexOutputOutputStream(cuvsIndex);
         if (vectors.length > MIN_CAGRA_INDEX_SIZE) {
           try {