Fix bug of HNSW

[hhy3]

No description provided.

[mdouze]

Could you explain what this is?

[hhy3]

Could you explain what this is?

When popping the first element of the heap, it could be invalid. So in this case nvalid should not decrease by 1. So it should be checked first. Otherwise, the performance would be impaired.

[mdouze]

I believe this can happen only if a -1 id gets pushed on the heap, which does never happen, correct?

[hhy3]

I believe this can happen only if a -1 id gets pushed on the heap, which does never happen, correct?

It'll be set to -1 when popping the min element. https://github.com/facebookresearch/faiss/blob/main/faiss/impl/HNSW.cpp#L877
And after I made this modification the recall improved a lot.

[facebook-github-bot]

@alexanderguzhva has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[alexanderguzhva]

Confirmed for both the bug and the recall rate improvement.

[facebook-github-bot]

@alexanderguzhva merged this pull request in 159641a.

[mdouze]

Thanks a lot for this fix! it increases the accuracy of HNSW by a lot!