Trace threadlocal entry is never removed, only nulled

changelog/@unreleased/pr-849.v2.yml

@@ -0,0 +1,5 @@
+type: improvement
+improvement:
+  description: Trace threadlocal entry is never removed, only nulled
+  links:
+  - https://github.com/palantir/tracing-java/pull/849

tracing-benchmarks/build.gradle

@@ -16,7 +16,7 @@ tasks.jmhCompileGeneratedClasses {
 }
 
 dependencies {
-    annotationProcessor 'org.openjdk.jmh:jmh-generator-annprocess'
+    jmhAnnotationProcessor 'org.openjdk.jmh:jmh-generator-annprocess'
     compileOnly 'org.openjdk.jmh:jmh-generator-annprocess'
     api 'org.immutables:value::annotations'
     api 'org.openjdk.jmh:jmh-core'

tracing-benchmarks/src/jmh/java/com/palantir/tracing/TracingBenchmark.java

@@ -43,7 +43,7 @@
 @OutputTimeUnit(TimeUnit.NANOSECONDS)
 @Warmup(iterations = 3, time = 3, timeUnit = TimeUnit.SECONDS)
 @Measurement(iterations = 3, time = 3, timeUnit = TimeUnit.SECONDS)
-@Fork(1)
+@Fork(value = 1, jvmArgsAppend = "-Dlog4j2.garbagefreeThreadContextMap=true")
 @Threads(4)
 @SuppressWarnings({"checkstyle:hideutilityclassconstructor", "checkstyle:VisibilityModifier"})
 public class TracingBenchmark {
@@ -88,6 +88,12 @@ public static void nestedSpans() {
         nestedSpans.run();
     }
 
+    @Benchmark
+    public static void traceWithSingleSpan() {
+        Tracer.fastStartSpan("benchmark");
+        Tracer.fastCompleteSpan();
+    }
+
     private static Runnable createNestedSpan(int depth) {
         if (depth <= 0) {
             return Runnables.doNothing();

tracing/src/main/java/com/palantir/tracing/Tracer.java

@@ -919,7 +919,7 @@ private static Trace getOrCreateCurrentTrace() {
     @VisibleForTesting
     static void clearCurrentTrace() {
         logClearingTrace();
-        currentTrace.remove();
+        currentTrace.set(null);
         MDC.remove(Tracers.TRACE_ID_KEY);
         MDC.remove(Tracers.TRACE_SAMPLED_KEY);
         MDC.remove(Tracers.REQUEST_ID_KEY);