Cannot create index with HNSW + PQ

Plugins k-NN
1

Versions (relevant - OpenSearch/Dashboard/Server OS/Browser):
2.9

Describe the issue:
I am trying to train model in order to create index using HNSQ and PQ:

{
    "training_index": "training-data-index",
    "training_field": "embedding_vector",
    "dimension": 512,
    "description": "HNSW with PQ",
    "method": {
        "name": "hnsw",
        "engine": "faiss",
        "space_type": "l2",
        "parameters": {
            "encoder": {
                "name": "pq",
                "parameters": {
                    "code_size": 8,
                     "m": 8
                }
            }
        }
    }
}

When trying to read status, I’m getting error about probably running out of memory or invalid parameters. According to logs, it may be the latter case:

[2023-08-23T21:17:09,996][ERROR][o.o.k.t.TrainingJob      ] [fa696978b96b] Failed to run training job for model "hnsw_pq_m8": Error in std::unique_ptr<faiss::Index> faiss::{anonymous}::index_factory_sub(int, std::string, faiss::MetricType) at /tmp/tmpzmr1s8yb/k-NN/jni/external/faiss/faiss/index_factory.cpp:781: Error: 'index' failed: could not parse HNSW code description PQ8x8 in HNSW16,PQ8x8

Any ideas what may be wrong with the parameters?

EDIT: I dug into faiss source code starting from the line number provided in log.
The code - in that case PQ8x8 is parsed and matched sequentially to couple of regular expressions. And I confirm it does not match any. PQ8x8 would be valid for IVF, but not HNSW.

Configuration:

Relevant Logs or Screenshots:

2

Thanks @Tomex. You are correct. This is a bug. I created an issue in faiss to see if it can be further parametrized: Can HNSWPQ nbits parameter be made configurable? · Issue #3027 · facebookresearch/faiss · GitHub. Ill create an issue in the k-NN repo to track as well.