Add bench result files.

sql/core/benchmarks/MetricsAggregationBenchmark-jdk11-results.txt

@@ -0,0 +1,12 @@
+OpenJDK 64-Bit Server VM 11.0.4+11 on Linux 4.15.0-66-generic
+Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz
+metrics aggregation (50 metrics, 100000 tasks per stage):  Best Time(ms)   Avg Time(ms)   Stdev(ms)    Rate(M/s)   Per Row(ns)   Relative
+------------------------------------------------------------------------------------------------------------------------
+1 stage(s)                                          672            841         179          0.0   671888474.0       1.0X
+2 stage(s)                                         1700           1842         201          0.0  1699591662.0       0.4X
+3 stage(s)                                         2601           2776         247          0.0  2601465786.0       0.3X
+
+Stage Count    Stage Proc. Time    Aggreg. Time
+     1              436                164
+     2              537                354
+     3              480                602

sql/core/benchmarks/MetricsAggregationBenchmark-results.txt

@@ -0,0 +1,12 @@
+Java HotSpot(TM) 64-Bit Server VM 1.8.0_181-b13 on Linux 4.15.0-66-generic
+Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz
+metrics aggregation (50 metrics, 100000 tasks per stage):  Best Time(ms)   Avg Time(ms)   Stdev(ms)    Rate(M/s)   Per Row(ns)   Relative
+------------------------------------------------------------------------------------------------------------------------
+1 stage(s)                                          740            883         147          0.0   740089816.0       1.0X
+2 stage(s)                                         1661           1943         399          0.0  1660649192.0       0.4X
+3 stage(s)                                         2711           2967         362          0.0  2711110178.0       0.3X
+
+Stage Count    Stage Proc. Time    Aggreg. Time
+     1              405                179
+     2              375                414
+     3              364                644

sql/core/src/test/scala/org/apache/spark/sql/execution/ui/MetricsAggregationBenchmark.scala

@@ -34,6 +34,16 @@ import org.apache.spark.status.ElementTrackingStore
 import org.apache.spark.util.{AccumulatorMetadata, LongAccumulator, Utils}
 import org.apache.spark.util.kvstore.InMemoryStore
 
+/**
+ * Benchmark for metrics aggregation in the SQL listener.
+ * {{{
+ *   To run this benchmark:
+ *   1. without sbt: bin/spark-submit --class <this class> --jars <core test jar>
+ *   2. build/sbt "core/test:runMain <this class>"
+ *   3. generate result: SPARK_GENERATE_BENCHMARK_FILES=1 build/sbt "core/test:runMain <this class>"
+ *      Results will be written to "benchmarks/MetricsAggregationBenchmark-results.txt".
+ * }}}
+ */
 object MetricsAggregationBenchmark extends BenchmarkBase {
 
   private def metricTrackingBenchmark(
@@ -161,13 +171,12 @@ object MetricsAggregationBenchmark extends BenchmarkBase {
 
     val benchmark = new Benchmark(
       s"metrics aggregation ($metricCount metrics, $taskCount tasks per stage)", 1,
-      warmupTime = 0.seconds)
+      warmupTime = 0.seconds, output = output)
 
     // Run this outside the measurement code so that classes are loaded and JIT is triggered,
     // otherwise the first run tends to be much slower than others. Also because this benchmark is a
     // bit weird and doesn't really map to what the Benchmark class expects, so it's a bit harder
     // to use warmupTime and friends effectively.
-    benchmark.out.printf("Warming up...\n")
     stageCounts.foreach { count =>
       metricTrackingBenchmark(new Benchmark.Timer(-1), metricCount, taskCount, count)
     }