irvingzhang edited a comment on issue #1295: Lucene-9004: bug fix for searching the nearest one neighbor in higher layers URL: https://github.com/apache/lucene-solr/pull/1295#issuecomment-592287771 > I believe in practice that results. max size is always set to ef, so there shouldn't be any real issue. I agree that the interface doesn't make that plain; we should enforce this invariant by API contract I agree that the max size is always set to _ef_, but _ef_ has different values in different layers. According to **Algorithm 5** of [papar](https://arxiv.org/pdf/1603.09320.pdf), HNSW searches the nearest one (namely, _ef_=1) neighbor from top layer to the 1st layer, and then finds the nearest _ef_ (_ef_=topK) neighbors from layer 0. In the implementation of Lucene HNSW, the actual size of result queue (Line 63, [HNSWGraphReader](https://github.com/apache/lucene-solr/blob/jira/lucene-9004-aknn-2/lucene/core/src/java/org/apache/lucene/util/hnsw/HNSWGraphReader.java)) is set to _ef_=topK when searching from top layer to the 1st layer, result in finding more neighbors than expected. Even if the parameter _ef_ is set to 1 in Line 66, [HNSWGraphReader](https://github.com/apache/lucene-solr/blob/jira/lucene-9004-aknn-2/lucene/core/src/java/org/apache/lucene/util/hnsw/HNSWGraphReader.java), the code `if (dist < f.distance() || results.size() < ef)` (Line 87, [HNSWGraph](https://github.com/apache/lucene-solr/blob/jira/lucene-9004-aknn-2/lucene/core/src/java/org/apache/lucene/util/hnsw/HNSWGraph.java)) allows insert more than 1 neighbor to the "results" when `dist < f.distance()` because the max size of "results" is _ef_=topK, which implies that actual size of "results" belongs to [1, topK]. The simplest way to check this problem is to print the actual size of neighbors. For example, add "System.out.println(neighbors.size());" after "visitedCount += hnsw.searchLayer(query, neighbors, 1, l, vectorValues);" (Line 66, [HNSWGraphReader](https://github.com/apache/lucene-solr/blob/jira/lucene-9004-aknn-2/lucene/core/src/java/org/apache/lucene/util/hnsw/HNSWGraphReader.java)), where the nearest one neighbor is expected, but the actual neighbor size would be range from 1~topK. Which also applies to [HNSWGraphWriter](https://github.com/apache/lucene-solr/blob/jira/lucene-9004-aknn-2/lucene/core/src/java/org/apache/lucene/util/hnsw/HNSWGraphWriter.java).
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