Add search optimization document

docs/performance/tuning-search-performance.md

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+# Tuning Search Performance
+
+ANN is fast, but sometimes it can be improved more by tuning parameters.
+This page shows the essence for improve ANN search by the Vald cluster.
+
+## Tuning Guideline
+
+When the search results do NOT satisfy the expected result, it can be improved by tuning parameters.
+
+First of all, we recommend tuning by following the steps below without doing it blindly.
+
+```mermaid
+flowchart TD
+    A[Perform Linear Search API]
+    B{Is satisfies?}
+    C[Tuning Parameters to improve precision]
+    D[Tuning Embedding Models]
+    E[Perform Search API]
+    F[Tuning Parameters to improve latency]
+    A-->B
+    B-- Yes -->C
+    B-- No -->D
+    C--> E
+    E--> C
+    E--> F
+    F--> E
+```
+
+The best practice is:
+
+1. Measure [Linear Search](../../docs/api/search.md#linearsearch-rpc) performance and use it as a baseline for Search API
+1. Repeat tuning to improve precision and measure Search API until the conditions are met
+1. Repeat tuning to improve latency and measure Search API until the conditions are met
+
+<div class="notice">
+When the results are not good by Linear Search API, it may need to rethink the embedding model for vectorization.
+</div>
+
+## Tuning parameters
+
+There are two viewpoints, client-side and cluster-side, for improving search performance.
+
+<div class="notice">
+There is a trade-off between search speed and accuracy, so tuning accuracy at first is recommended.
+</div>
+
+### Client side
+
+On the client side, parameters of [`Search.Config`](../../docs/api/search.md#input) will affect the search result.
+
+|             | description                                                                                                                   | how does it affect?                                                                                                                                                                                                    | memo                                                          |
+| :---------- | :---------------------------------------------------------------------------------------------------------------------------- | :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :------------------------------------------------------------ |
+| radius      | the search radius for NGT<BR>[ref: search_radius](https://github.com/yahoojapan/NGT/tree/main/bin/ngt#search)                 | Define the search range when NGT searches the nearest neighbors                                                                                                                                                        | recommended value: `-1`                                       |
+| epsilon     | the search coefficient for NGT<BR>[ref: search_range_coefficient](https://github.com/yahoojapan/NGT/tree/main/bin/ngt#search) | Expansion factor of the NGT search range.<BR>Search operation time increases when the epsilon is big.                                                                                                                  | recommended value range: `0.01 ~ 0.3`<BR>default value: `0.1` |
+| timeout(ns) | max time duration until receiving search results.                                                                             | An error will be returned if the set `num` search results cannot be obtained within the set time.<BR>By setting `min_num`, the search results will be returned if more than `min_num` can be searched within the time. | default value: `3,000,000,000ns`                              |
+
+### Cluster-side
+
+On the cluster side, these parameters can be set by `values.yaml`, affect the search result.
+
+|                              | description                                                                                                                                                  | how does it affect?                                                                                                                                                                        | Memo                |
+| :--------------------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :------------------ |
+| agent.ngt.creation_edge_size | Number of nodes connected to one node<BR>[ref: no_of_edges](https://github.com/yahoojapan/NGT/tree/main/bin/ngt#create)                                      | It helps reduce unreachable edges.<BR>The larger it is, the denser the graph structure will be, but the memory usage, search speed, and index construction time will increase accordingly. | default value: `20` |
+| agent.ngt.search_edge_size   | Number of nodes to search from the origin node when searching<BR>[ref: no_of_edges_at_search_at](https://github.com/yahoojapan/NGT/tree/main/bin/ngt#create) | The number of nodes to search will increase.<BR>Accuracy will be higher, but speed will be lower.<BR>Adjust if adjusting the radius and epsilon does not improve the situation.            | default value: `10` |