Optimize reduceToTopK in ResultUtil by removing pre-filling and reducing peek calls

[junqiu-lei]

Description

This PR optimizes the reduceToTopK method by eliminating unnecessary pre-filling of the priority queue, reducing redundant peek() calls, and adding null safety checks for better performance.

Related Issues

Closes https://github.com/opensearch-project/k-NN/issues/2145

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [x] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

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[junqiu-lei]

Offline synced with @navneet1v and @jmazanec15, we now focused on optimizing reduceToTopK function in ResultUtil, updated the PR.

[junqiu-lei]

@junqiu-lei Did we compare removing the map with this approach to see which is faster? or is it not possible? I feel like its a simpler code that way if its yielding better results

• removing the map approach: See if we can preserve the order of the results for each leaf instead of converting it to map.

We might have to first check if KNNResults array is in order. but that way we can do something similar to what lucene does and just pick the top elements and stop at k in reduceToTopK

I think this is another place we can optimize, no we don't have comparing the difference so far. For this PR, we might can use micro benchmark for testing the improvement if possible.

[junqiu-lei]

So, I think the most optimal way to do this would be to require that each perLeafResults comes in order from top scores to worst scores. Then, a simple algorithm that basically just scans the top of each set of results and then only takes the top k. That being, that would require how results are returned in the different methods. Do you think we should do this @navneet1v ?

I can raise other PR for this part optimization.

[navneet1v]

So, I think the most optimal way to do this would be to require that each perLeafResults comes in order from top scores to worst scores. Then, a simple algorithm that basically just scans the top of each set of results and then only takes the top k. That being, that would require how results are returned in the different methods. Do you think we should do this @navneet1v ?

Yes I think we should change the return types