[Enhancement][Build-TimeReduction V1] Remove conversion of KNN Field to string if stored Fields not required

[navneet1v]

Description

While doing some benchmarks with Opensearch k-NN. I see that there is time going in to convert the kNN field to String even when storedFields is marked as false. On doing deep-dive I found out that, we convert the field to the string and then check if stored field is set as true or false.

Code ref:
https://github.com/opensearch-project/k-NN/blob/main/src/main/java/org/opensearch/knn/index/mapper/KNNVectorFieldMapper.java#L575

Here point.toString will convert the object to string which is expensive operation in high dimensionality use cases.

Refer the Flame graph for the same for Vector dimension: 1536 ingestion

[navneet1v]

The fix for the issue is pretty simple rather than doing toString we just need to pass the VectorField to the underline function.

[navneet1v]

Resolving the issue as the PR is merged.

[jmazanec15]

@navneet1v for the flame graph above, what was the setup? Trying to understand impact on ingestion

[navneet1v]

I created a 2.14 cluster and did the ingestion. I removed few more things from indexing path like adding recovery_source and parsing of vector field as an array. But you don't need all these changes. Just pick up a 1536D dataset and run it with and without this change

and I used a 1536D dataset with 5M records