Reuse SpanBuilder within a Thread

dd-trace-api/src/main/java/datadog/trace/api/config/GeneralConfig.java

@@ -107,6 +107,7 @@ public final class GeneralConfig {
   public static final String OPTIMIZED_MAP_ENABLED = "optimized.map.enabled";
   public static final String TAG_NAME_UTF8_CACHE_SIZE = "tag.name.utf8.cache.size";
   public static final String TAG_VALUE_UTF8_CACHE_SIZE = "tag.value.utf8.cache.size";
+  public static final String SPAN_BUILDER_REUSE_ENABLED = "span.builder.reuse.enabled";
   public static final String STACK_TRACE_LENGTH_LIMIT = "stack.trace.length.limit";
 
   public static final String SSI_INJECTION_ENABLED = "injection.enabled";

dd-trace-core/src/jmh/java/datadog/trace/core/CoreTracerBenchmark.java

@@ -0,0 +1,49 @@
+package datadog.trace.core;
+
+import static java.util.concurrent.TimeUnit.MICROSECONDS;
+
+import datadog.trace.bootstrap.instrumentation.api.AgentSpan;
+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.Scope;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.Threads;
+import org.openjdk.jmh.annotations.Warmup;
+
+/**
+ * Benchmark of key operations of the CoreTracer
+ *
+ * <p>NOTE: This is a multi-threaded benchmark; single threaded benchmarks don't accurately reflect
+ * some of the optimizations.
+ *
+ * <p>Use -t 1, if you'd like to do a single threaded run
+ */
+@State(Scope.Benchmark)
+@Warmup(iterations = 3)
+@Measurement(iterations = 5)
+@BenchmarkMode(Mode.Throughput)
+@Threads(8)
+@OutputTimeUnit(MICROSECONDS)
+@Fork(value = 1)
+public class CoreTracerBenchmark {
+  static final CoreTracer TRACER = CoreTracer.builder().build();
+
+  @Benchmark
+  public AgentSpan startSpan() {
+    return TRACER.startSpan("foo", "bar");
+  }
+
+  @Benchmark
+  public AgentSpan buildSpan() {
+    return TRACER.buildSpan("foo", "bar").start();
+  }
+
+  @Benchmark
+  public AgentSpan singleSpanBuilder() {
+    return TRACER.singleSpanBuilder("foo", "bar").start();
+  }
+}