ChrisHegarty commented on code in PR #14426:
URL: https://github.com/apache/lucene/pull/14426#discussion_r2027333343


##########
lucene/core/src/java/org/apache/lucene/codecs/KnnVectorsReader.java:
##########
@@ -130,4 +134,56 @@ public KnnVectorsReader getMergeInstance() {
    * <p>The default implementation is empty
    */
   public void finishMerge() throws IOException {}
+
+  /** A string representing the off-heap category for quantized vectors. */
+  public static final String QUANTIZED = "QUANTIZED";
+
+  /** A string representing the off-heap category for the HNSW graph. */
+  public static final String HNSW_GRAPH = "HNSW_GRAPH";
+
+  /** A string representing the off-heap category for raw vectors. */
+  public static final String RAW = "RAW";
+
+  /**
+   * Returns the desired size of off-heap memory the given field. This size 
can be used to help
+   * determine the memory requirements for optimal search performance, which 
can be greatly affected
+   * by page faults when not enough memory is available.
+   *
+   * <p>For reporting purposes, the backing off-heap index structures are 
broken into three
+   * categories: 1. {@link #RAW}, 2. {@link #HNSW_GRAPH}, and 3. {@link 
#QUANTIZED}. The returned
+   * map will have zero or one entry for each of these categories.
+   *
+   * <p>The long value is the size in bytes of the off-heap space needed if 
the associated index
+   * structure were to be fully loaded in memory. While somewhat analogous to 
{@link
+   * Accountable#ramBytesUsed()} (which reports actual on-heap memory usage), 
the metrics reported
+   * by this method are not actual usage but rather the amount of available 
memory needed to fully
+   * load the index into memory, rather than an actual RAM usage requirement.
+   *
+   * <p>To determine the total desired off-heap memory size for the given 
field:
+   *
+   * <pre>{@code
+   * 
getOffHeapByteSize(field).values().stream().mapToLong(Long::longValue).sum();
+   * }</pre>
+   *
+   * @param fieldInfo the fieldInfo
+   * @return a map of the desired off-heap memory requirements by category
+   * @lucene.experimental
+   */
+  public abstract Map<String, Long> getOffHeapByteSize(FieldInfo fieldInfo);

Review Comment:
   @mayya-sharipova the expected usage is that the caller would get all the 
fieldInfos from the reader ( `LeafReader::getFieldInfos` ), and then iterate 
over them checking for vector info. Which should be straightforward to do 
without an additional API point.



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