[jira] [Created] (LUCENE-10404) Use hash set for visited nodes in HNSW search?

["Julie Tibshirani (Jira)" <ji...@apache.org>]

Julie Tibshirani created LUCENE-10404:
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             Summary: Use hash set for visited nodes in HNSW search?
                 Key: LUCENE-10404
                 URL: https://issues.apache.org/jira/browse/LUCENE-10404
             Project: Lucene - Core
          Issue Type: Improvement
            Reporter: Julie Tibshirani


While searching each layer, HNSW tracks the nodes it has already visited using a BitSet. We could look into using something like IntHashSet instead. I tried out the idea quickly by switching to IntIntHashMap (which has already been copied from hppc) and saw an improvement in index performance. 

**Baseline:** 760896 msec to write vectors
**Using IntIntHashMap:** 733017 msec to write vectors

I noticed search performance actually got a little bit worse with the change -- that is something to look into.

For background, it's good to be aware that HNSW can visit a lot of nodes. For example, on the glove-100-angular dataset with ~1.2 million docs, HNSW search visits ~1000 - 15,000 docs depending on the recall. This number can increase when searching with deleted docs, especially if you hit a "pathological" case where the deleted docs happen to be closest to the query vector.



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