Add documentation for k-NN Faiss SQfp16

.github/vale/styles/Vocab/OpenSearch/Words/accept.txt

@@ -80,6 +80,7 @@ Levenshtein
 [Oo]nboarding
 pebibyte
 [Pp]erformant
+[Pp]laintext
 [Pp]luggable
 [Pp]reconfigure
 [Pp]refetch
@@ -91,6 +92,7 @@ pebibyte
 [Pp]reprocess
 [Pp]retrain
 [Pp]seudocode
+[Quantiz](e|ation|ing|er)
 [Rr]ebalance
 [Rr]ebalancing
 [Rr]edownload

_search-plugins/knn/knn-index.md

@@ -11,10 +11,65 @@ has_children: false
 
 The k-NN plugin introduces a custom data type, the `knn_vector`, that allows users to ingest their k-NN vectors into an OpenSearch index and perform different kinds of k-NN search. The `knn_vector` field is highly configurable and can serve many different k-NN workloads. For more information, see [k-NN vector]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/knn-vector/).
 
+To create a k-NN index, set the `settings.index.knn` parameter to `true`:
+
+```json
+PUT /test-index
+{
+  "settings": {
+    "index": {
+      "knn": true
+    }
+  },
+  "mappings": {
+    "properties": {
+      "my_vector1": {
+        "type": "knn_vector",
+        "dimension": 3,
+        "method": {
+          "name": "hnsw",
+          "space_type": "l2",
+          "engine": "lucene",
+          "parameters": {
+            "ef_construction": 128,
+            "m": 24
+          }
+        }
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
 ## Lucene byte vector
 
 Starting with k-NN plugin version 2.9, you can use `byte` vectors with the `lucene` engine in order to reduce the amount of storage space needed. For more information, see [Lucene byte vector]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/knn-vector#lucene-byte-vector).
 
+## SIMD optimization for the Faiss engine
+
+Starting with version 2.13, the k-NN plugin supports [Single Instruction Multiple Data (SIMD)](https://en.wikipedia.org/wiki/Single_instruction,_multiple_data) processing if the underlying hardware supports SIMD instructions (AVX2 on x64 architecture and Neon on ARM64 architecture). SIMD is supported by default on Linux machines only for the Faiss engine. SIMD architecture helps boost overall performance by improving indexing throughput and reducing search latency.
+
+SIMD optimization is applicable only if the vector dimension is a multiple of 8.
+{: .note}
+
+<!-- vale off -->
+### x64 architecture
+<!-- vale on -->
+
+For the x64 architecture, two different versions of the Faiss library are built and shipped with the artifact:
+
+- `libopensearchknn_faiss.so`: The non-optimized Faiss library without SIMD instructions. 
+- `libopensearchknn_faiss_avx2.so`: The Faiss library that contains AVX2 SIMD instructions. 
+
+If your hardware supports AVX2, the k-NN plugin loads the `libopensearchknn_faiss_avx2.so` library at runtime.
+
+To disable AVX2 and load the non-optimized Faiss library (`libopensearchknn_faiss.so`), specify the `knn.faiss.avx2.disabled` static setting as `true` in `opensearch.yml` (default is `false`). Note that to update a static setting, you must stop the cluster, change the setting, and restart the cluster. For more information, see [Static settings]({{site.url}}{{site.baseurl}}/install-and-configure/configuring-opensearch/index/#static-settings).
+
+### ARM64 architecture
+
+For the ARM64 architecture, only one performance-boosting Faiss library (`libopensearchknn_faiss.so`) is built and shipped. The library contains Neon SIMD instructions and cannot be disabled. 
+
 ## Method definitions
 
 A method definition refers to the underlying configuration of the Approximate k-NN algorithm you want to use. Method definitions are used to either create a `knn_vector` field (when the method does not require training) or [create a model during training]({{site.url}}{{site.baseurl}}/search-plugins/knn/api#train-model) that can then be used to [create a `knn_vector` field]({{site.url}}{{site.baseurl}}/search-plugins/knn/approximate-knn/#building-a-k-nn-index-from-a-model).
@@ -48,12 +103,12 @@ For nmslib, *ef_search* is set in the [index settings](#index-settings).
 An index created in OpenSearch version 2.11 or earlier will still use the old `ef_construction` value (`512`).
 {: .note}
 
-### Supported faiss methods
+### Supported Faiss methods
 
 Method name | Requires training | Supported spaces | Description
 :--- | :--- | :--- | :---
 `hnsw` | false | l2, innerproduct | Hierarchical proximity graph approach to Approximate k-NN search.
-`ivf` | true | l2, innerproduct | Bucketing approach where vectors are assigned different buckets based on clustering and, during search, only a subset of the buckets is searched.
+`ivf` | true | l2, innerproduct | Stands for _inverted file index_. Bucketing approach where vectors are assigned different buckets based on clustering and, during search, only a subset of the buckets is searched.
 
 For hnsw, "innerproduct" is not available when PQ is used.
 {: .note}
@@ -107,25 +162,21 @@ An index created in OpenSearch version 2.11 or earlier will still use the old `e
 {: .note}
 
 ```json
-{
-    "type": "knn_vector",
-    "dimension": 100,
-    "method": {
-        "name":"hnsw",
-        "engine":"lucene",
-        "space_type": "l2",
-        "parameters":{
-            "m":2048,
-            "ef_construction": 245
-        }
+"method": {
+    "name":"hnsw",
+    "engine":"lucene",
+    "space_type": "l2",
+    "parameters":{
+        "m":2048,
+        "ef_construction": 245
     }
 }
 ```
 
-### Supported faiss encoders
+### Supported Faiss encoders
 
-You can use encoders to reduce the memory footprint of a k-NN index at the expense of search accuracy. faiss has
-several encoder types, but the plugin currently only supports *flat* and *pq* encoding.
+You can use encoders to reduce the memory footprint of a k-NN index at the expense of search accuracy. The k-NN plugin currently supports the
+`flat`, `pq`, and `sq` encoders in the Faiss library.
 
 The following example method definition specifies the `hnsw` method and a `pq` encoder:
 
@@ -151,11 +202,27 @@ The `hnsw` method supports the `pq` encoder for OpenSearch versions 2.10 and lat
 
 Encoder name | Requires training | Description
 :--- | :--- | :---
-`flat` | false | Encode vectors as floating point arrays. This encoding does not reduce memory footprint.
+`flat` (Default) | false | Encode vectors as floating-point arrays. This encoding does not reduce memory footprint.
 `pq` | true | An abbreviation for _product quantization_, it is a lossy compression technique that uses clustering to encode a vector into a fixed size of bytes, with the goal of minimizing the drop in k-NN search accuracy. At a high level, vectors are broken up into `m` subvectors, and then each subvector is represented by a `code_size` code obtained from a code book produced during training. For more information about product quantization, see [this blog post](https://medium.com/dotstar/understanding-faiss-part-2-79d90b1e5388).
+`sq` | false | An abbreviation for _scalar quantization_. Starting with k-NN plugin version 2.13, you can use the `sq` encoder to quantize 32-bit floating-point vectors into 16-bit floats. In version 2.13, the built-in `sq` encoder is the SQFP16 Faiss encoder. The encoder reduces memory footprint with a minimal loss of precision and improves performance by using SIMD optimization (using AVX2 on x86 architecture or Neon on ARM architecture). For more information, see [Faiss scalar quantization]({{site.url}}{{site.baseurl}}/search-plugins/knn/knn-vector-quantization#faiss-scalar-quantization).
 
-#### Examples
+#### PQ parameters
+
+Parameter name | Required | Default | Updatable | Description
+:--- | :--- | :--- | :--- | :---
+`m` | false | 1 | false |  Determines the number of subvectors into which to break the vector. Subvectors are encoded independently of each other. This vector dimension must be divisible by `m`. Maximum value is 1,024.
+`code_size` | false | 8 | false | Determines the number of bits into which to encode a subvector. Maximum value is 8. For IVF, this value must be less than or equal to 8. For HNSW, this value can only be 8.
+
+#### SQ parameters
+
+Parameter name | Required | Default | Updatable | Description
+:--- | :--- | :-- | :--- | :---
+`type` | false | `fp16` | false |  The type of scalar quantization to be used to encode 32-bit float vectors into the corresponding type. As of OpenSearch 2.13, only the `fp16` encoder type is supported. For the `fp16` encoder, vector values must be in the [-65504.0, 65504.0] range. 
+`clip` | false | `false` | false | If `true`, then any vector values outside of the supported range for the specified vector type are rounded so that they are in the range. If `false`, then the request is rejected if any vector values are outside of the supported range. Setting `clip` to `true` may decrease recall.
+
+For more information and examples, see [Using Faiss scalar quantization]({{site.url}}{{site.baseurl}}/search-plugins/knn/knn-vector-quantization/#using-faiss-scalar-quantization).
 
+#### Examples
 
 The following example uses the `ivf` method  without specifying an encoder (by default, OpenSearch uses the `flat` encoder):
 
@@ -204,12 +271,46 @@ The following example uses the `hnsw` method without specifying an encoder (by d
 }
 ```
 
-#### PQ parameters
+The following example uses the `hnsw` method with an `sq` encoder of type `fp16` with `clip` enabled:
 
-Paramater Name | Required | Default | Updatable | Description
-:--- | :--- | :--- | :--- | :---
-`m` | false | 1 | false |  Determines the number of subvectors into which to break the vector. Subvectors are encoded independently of each other. This dimension of the vector must be divisible by `m`. Maximum value is 1,024.
-`code_size` | false | 8 | false | Determines the number of bits into which to encode a subvector. Maximum value is 8. For IVF, this value must be less than or equal to 8. For HNSW, this value can only be 8.
+```json
+"method": {
+  "name":"hnsw",
+  "engine":"faiss",
+  "space_type": "l2",
+  "parameters":{
+    "encoder": {
+      "name": "sq",
+      "parameters": {
+        "type": "fp16",
+        "clip": true
+      }  
+    },    
+    "ef_construction": 256,
+    "m": 8
+  }
+}
+```
+
+The following example uses the `ivf` method with an `sq` encoder of type `fp16`:
+
+```json
+"method": {
+  "name":"ivf",
+  "engine":"faiss",
+  "space_type": "l2",
+  "parameters":{
+    "encoder": {
+      "name": "sq",
+      "parameters": {
+        "type": "fp16",
+        "clip": false
+      }
+    },
+    "nprobes": 2
+  }
+}
+```
 
 ### Choosing the right method
 
@@ -221,6 +322,8 @@ If you want to use less memory and index faster than HNSW, while maintaining sim
 
 If memory is a concern, consider adding a PQ encoder to your HNSW or IVF index. Because PQ is a lossy encoding, query quality will drop.
 
+You can reduce the memory footprint by a factor of 2, with a minimal loss in search quality, by using the [`fp_16` encoder]({{site.url}}{{site.baseurl}}/search-plugins/knn/knn-vector-quantization/#faiss-scalar-quantization). If your vector dimensions are within the [-128, 127] byte range, we recommend using the [byte quantizer]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/knn-vector/#lucene-byte-vector) in order to reduce the memory footprint by a factor of 4. To learn more about vector quantization options, see [k-NN vector quantization]({{site.url}}{{site.baseurl}}/search-plugins/knn/knn-vector-quantization/). 
+
 ### Memory estimation
 
 In a typical OpenSearch cluster, a certain portion of RAM is set aside for the JVM heap. The k-NN plugin allocates
@@ -230,6 +333,9 @@ the `circuit_breaker_limit` cluster setting. By default, the limit is set at 50%
 Having a replica doubles the total number of vectors.
 {: .note }
 
+For information about using memory estimation with vector quantization, see the [vector quantization documentation]({{site.url}}{{site.baseurl}}/search-plugins/knn/knn-vector-quantization/#memory-estimation).
+{: .note }
+
 #### HNSW memory estimation
 
 The memory required for HNSW is estimated to be `1.1 * (4 * dimension + 8 * M)` bytes/vector.

_search-plugins/knn/knn-vector-quantization.md

@@ -0,0 +1,159 @@
+---
+layout: default
+title: k-NN vector quantization
+nav_order: 27
+parent: k-NN search
+grand_parent: Search methods
+has_children: false
+has_math: true
+---
+
+# k-NN vector quantization
+
+By default, the k-NN plugin supports the indexing and querying of vectors of type `float`, where each dimension of the vector occupies 4 bytes of memory. For use cases that require ingestion on a large scale, keeping `float` vectors can be expensive because OpenSearch needs to construct, load, save, and search graphs (for native `nmslib` and `faiss` engines). To reduce the memory footprint, you can use vector quantization.
+
+## Lucene byte vector
+
+Starting with k-NN plugin version 2.9, you can use `byte` vectors with the `lucene` engine in order to reduce the amount of required memory. This requires quantizing the vectors outside of OpenSearch before ingesting them into an OpenSearch index. For more information, see [Lucene byte vector]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/knn-vector#lucene-byte-vector).
+
+## Faiss scalar quantization 
+
+Starting with version 2.13, the k-NN plugin supports performing scalar quantization for the Faiss engine within OpenSearch. Within the Faiss engine, a scalar quantizer (SQfp16) performs the conversion between 32-bit and 16-bit vectors. At ingestion time, when you upload 32-bit floating-point vectors to OpenSearch, SQfp16 quantizes them into 16-bit floating-point vectors and stores the quantized vectors in a k-NN index. At search time, SQfp16 decodes the vector values back into 32-bit floating-point values for distance computation. The SQfp16 quantization can decrease the memory footprint by a factor of 2. Additionally, it leads to a minimal loss in recall when differences between vector values are large compared to the error introduced by eliminating their two least significant bits. When used with [SIMD optimization]({{site.url}}{{site.baseurl}}/search-plugins/knn/knn-index#simd-optimization-for-the-faiss-engine), SQfp16 quantization can also significantly reduce search latencies and improve indexing throughput. 
+
+SIMD optimization is not supported on Windows. Using Faiss scalar quantization on Windows can lead to a significant drop in performance, including decreased indexing throughput and increased search latencies.
+{: .warning} 
+
+### Using Faiss scalar quantization
+
+To use Faiss scalar quantization, set the k-NN vector field's `method.parameters.encoder.name` to `sq` when creating a k-NN index:
+
+```json
+PUT /test-index
+{
+  "settings": {
+    "index": {
+      "knn": true,
+      "knn.algo_param.ef_search": 100
+    }
+  },
+  "mappings": {
+    "properties": {
+      "my_vector1": {
+        "type": "knn_vector",
+        "dimension": 3,
+        "method": {
+          "name": "hnsw",
+          "engine": "faiss",
+          "space_type": "l2",
+          "parameters": {
+            "encoder": {
+              "name": "sq",
+            },
+            "ef_construction": 256,
+            "m": 8
+          }
+        }
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+Optionally, you can specify the parameters in `method.parameters.encoder`. For more information about `encoder` object parameters, see [SQ parameters]({{site.url}}{{site.baseurl}}/search-plugins/knn/knn-index/#sq-parameters).
+
+The `fp16` encoder converts 32-bit vectors into their 16-bit counterparts. For this encoder type, the vector values must be in the [-65504.0, 65504.0] range. To define how to handle out-of-range values, the preceding request specifies the `clip` parameter. By default, this parameter is `false`, and any vectors containing out-of-range values are rejected. When `clip` is set to `true` (as in the preceding request), out-of-range vector values are rounded up or down so that they are in the supported range. For example, if the original 32-bit vector is `[65510.82, -65504.1]`, the vector will be indexed as a 16-bit vector `[65504.0, -65504.0]`.
+
+We recommend setting `clip` to `true` only if very few elements lie outside of the supported range. Rounding the values may cause a drop in recall.
+{: .note}
+
+The following example method definition specifies the Faiss SQfp16 encoder, which rejects any indexing request that contains out-of-range vector values (because the `clip` parameter is `false` by default):
+
+```json
+PUT /test-index
+{
+  "settings": {
+    "index": {
+      "knn": true,
+      "knn.algo_param.ef_search": 100
+    }
+  },
+  "mappings": {
+    "properties": {
+      "my_vector1": {
+        "type": "knn_vector",
+        "dimension": 3,
+        "method": {
+          "name": "hnsw",
+          "engine": "faiss",
+          "space_type": "l2",
+          "parameters": {
+            "encoder": {
+              "name": "sq",
+              "parameters": {
+                "type": "fp16"
+              }
+            },
+            "ef_construction": 256,
+            "m": 8
+          }
+        }
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+During ingestion, make sure each dimension of the vector is in the supported range ([-65504.0, 65504.0]):
+
+```json
+PUT test-index/_doc/1
+{
+  "my_vector1": [-65504.0, 65503.845, 55.82]
+}
+```
+{% include copy-curl.html %}
+
+During querying, there is no range limitation for the query vector:
+
+```json
+GET test-index/_search
+{
+  "size": 2,
+  "query": {
+    "knn": {
+      "my_vector1": {
+        "vector": [265436.876, -120906.256, 99.84],
+        "k": 2
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+## Memory estimation
+
+In the best-case scenario, 16-bit vectors produced by the Faiss SQfp16 quantizer require 50% of the memory that 32-bit vectors require. 
+
+#### HNSW memory estimation
+
+The memory required for HNSW is estimated to be `1.1 * (2 * dimension + 8 * M)` bytes/vector.
+
+As an example, assume that you have 1 million vectors with a dimension of 256 and M of 16. The memory requirement can be estimated as follows:
+
+```bash
+1.1 * (2 * 256 + 8 * 16) * 1,000,000 ~= 0.656 GB
+```
+
+#### IVF memory estimation
+
+The memory required for IVF is estimated to be `1.1 * (((2 * dimension) * num_vectors) + (4 * nlist * d))` bytes/vector.
+
+As an example, assume that you have 1 million vectors with a dimension of 256 and `nlist` of 128. The memory requirement can be estimated as follows:
+
+```bash
+1.1 * (((2 * 256) * 1,000,000) + (4 * 128 * 256))  ~= 0.525 GB
+```
+

_search-plugins/knn/settings.md

@@ -25,3 +25,4 @@ Setting | Default | Description
 `knn.model.index.number_of_shards`| 1 | The number of shards to use for the model system index, the OpenSearch index that stores the models used for Approximate Nearest Neighbor (ANN) search.
 `knn.model.index.number_of_replicas`| 1 | The number of replica shards to use for the model system index. Generally, in a multi-node cluster, this should be at least 1 to increase stability.
 `knn.advanced.filtered_exact_search_threshold`| null | The threshold value for the filtered IDs that is used to switch to exact search during filtered ANN search. If the number of filtered IDs in a segment is less than this setting's value, exact search will be performed on the filtered IDs.  
+`knn.faiss.avx2.disabled` | False | A static setting that specifies whether to disable the SIMD-based `libopensearchknn_faiss_avx2.so` library and load the non-optimized `libopensearchknn_faiss.so` library for the Faiss engine on machines with x64 architecture. For more information, see [SIMD optimization for the Faiss engine]({{site.url}}{{site.baseurl}}/search-plugins/knn/knn-index/#simd-optimization-for-the-faiss-engine).