Add benchmark to measure performance of CustomBinaryDocValuesField

Benchmarks show that ArrayList performs better than TreeSet. Added a comment
on where to find the results and benchmark.

Signed-off-by: Kiran Reddy <kkreddy@amazon.com>

benchmarks/src/main/java/org/opensearch/benchmark/index/mapper/CustomBinaryDocValuesFieldBenchmark.java

@@ -0,0 +1,81 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.benchmark.index.mapper;
+
+import org.apache.lucene.util.BytesRef;
+import org.opensearch.index.mapper.BinaryFieldMapper;
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Fork;
+import org.openjdk.jmh.annotations.Measurement;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+import org.openjdk.jmh.annotations.Param;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.Warmup;
+import org.openjdk.jmh.infra.Blackhole;
+
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.concurrent.TimeUnit;
+
+@Warmup(iterations = 1)
+@Measurement(iterations = 1)
+@Fork(1)
+@BenchmarkMode(Mode.Throughput)
+@OutputTimeUnit(TimeUnit.MILLISECONDS)
+@State(Scope.Thread)
+@SuppressWarnings("unused") // invoked by benchmarking framework
+public class CustomBinaryDocValuesFieldBenchmark {
+
+    static final String FIELD_NAME = "dummy";
+    static final String SEED_VALUE = "seed";
+
+    @Benchmark
+    public void add(CustomBinaryDocValuesFieldBenchmark.BenchmarkParameters parameters, Blackhole blackhole) {
+        // Don't use the parameter binary doc values object.
+        // Start with a fresh object every call and add maximum number of entries
+        BinaryFieldMapper.CustomBinaryDocValuesField customBinaryDocValuesField = new BinaryFieldMapper.CustomBinaryDocValuesField(
+            FIELD_NAME,
+            new BytesRef(SEED_VALUE).bytes
+        );
+        for (int i = 0; i < parameters.maximumNumberOfEntries; ++i) {
+            ThreadLocalRandom.current().nextBytes(parameters.bytes);
+            customBinaryDocValuesField.add(parameters.bytes);
+        }
+    }
+
+    @Benchmark
+    public void binaryValue(CustomBinaryDocValuesFieldBenchmark.BenchmarkParameters parameters, Blackhole blackhole) {
+        blackhole.consume(parameters.customBinaryDocValuesField.binaryValue());
+    }
+
+    @State(Scope.Benchmark)
+    public static class BenchmarkParameters {
+        @Param({ "8", "32", "128", "512" })
+        int maximumNumberOfEntries;
+
+        @Param({ "8", "32", "128", "512" })
+        int entrySize;
+
+        BinaryFieldMapper.CustomBinaryDocValuesField customBinaryDocValuesField;
+        byte[] bytes;
+
+        @Setup
+        public void setup() {
+            customBinaryDocValuesField = new BinaryFieldMapper.CustomBinaryDocValuesField(FIELD_NAME, new BytesRef(SEED_VALUE).bytes);
+            bytes = new byte[entrySize];
+            for (int i = 0; i < maximumNumberOfEntries; ++i) {
+                ThreadLocalRandom.current().nextBytes(bytes);
+                customBinaryDocValuesField.add(bytes);
+            }
+        }
+    }
+}

server/src/main/java/org/opensearch/index/mapper/BinaryFieldMapper.java

@@ -242,6 +242,10 @@ protected String contentType() {
      */
     public static class CustomBinaryDocValuesField extends CustomDocValuesField {
 
+        // We considered using a TreeSet instead of an ArrayList here.
+        // Benchmarks show that ArrayList performs much better
+        // For details, see: https://github.com/opensearch-project/OpenSearch/pull/9426
+        // Benchmarks are in CustomBinaryDocValuesFiledBenchmark
         private final ArrayList<byte[]> bytesList;
 
         public CustomBinaryDocValuesField(String name, byte[] bytes) {