Use of IVF with HNSW32 coarse quantizer

[abhinavdangeti]

Bringing back the coarse quantizer into use per observations Aditi recorded earlier. A slight adjustment now is to deploy it only over a vector count of 100000.

[abhinavdangeti]

@metonymic-smokey would you share the numbers you have comparing the considered index classes for this doc count at the unit level.

[metonymic-smokey]

Numbers from go-faiss unit level testing -

nq = 100, k = 100

256 dims

nvecs	QPS without HNSW	QPS with HNSW
200k	425	311
500k	257	244
1M	170	145

512 dims

nvecs	QPS without HNSW	QPS with HNSW
1M	75	62

1024 dims

nvecs	QPS without HNSW	QPS with HNSW
1M	20	15

2048 dims

nvecs	QPS without HNSW	QPS with HNSW
200k	14	5
500k	11	5
1M	9	4

These numbers were prior to the latest go-faiss PRs being merged.
I'm not sure that 100k is a high enough number to use the coarse quantiser and the number should depend on the dims too since it's most effective for high dims.

[abhinavdangeti]

Hmm, but per the numbers you've shared it seems even at higher dims the QPS seems to fall with the coarse quantizer?