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A set of benchmarks designed to show the cost of Kotlin language features.

Principle Of Operation

Kostings uses the JMH microbenchmark framework to measure the throughput of benchmark methods. It then uses JUnit to test assertions about the results.

To Run Benchmarks

JMH requires a sophisticated Maven build, with code-generation-n-shit. After every change to actual benchmark code or its dependencies, rebuild the benchmarks with

mvn clean package

To run the default set of benchmarks with the default number of runs, read the section on Running Reliable Benchmarks and then run


This will tie your computer up for hours to populate the results directory and plot some nice graphs in results/images.

The results directory is then used as a cache - results for a given benchmark configuration will be read from the directory or rebuilt if they are not there.

To Run Performance Tests

Kostings allow you to write performance tests in the form of JUnit tests with assertions about the relative performance of benchmark methods.

class BaselineTests {

    fun `java is quicker but not by much`() {
        assertThat(JavaBaseline::baseline, probablyFasterThan(KotlinBaseline::baseline, byAFactorOf = 0.005))
        assertThat(JavaBaseline::baseline, ! probablyFasterThan(KotlinBaseline::baseline, byAFactorOf = 0.01))


The performance tests assume that the benchmarks have already been run - this allows fast iteration on the tests once data has been gathered.

Because benchmarks may be run with different parameters and in different batches, the tests don't load results from the results directory. Instead they load from canonical-results. For now, just copy the files from results to canonical-results. (If more than one file has results for a benchmark, the results from all files will be merged).

To run the performance tests (after you have collected some data in canonical-results)

mvn integration-test

or point your IDE at performance-tests/java and run the tests there.

Note that the matchers are becoming statistically sophisticated. probablyFasterThan/probablySlowerThan/probablyDifferentTo all use T-Tests to statistically compare the means between benchmarks. Optional byAFactorOf parameters allow testing of the scale of the difference.

Aspects of Performance to Investigate

These are the sort of things I'd like to write tests for

  • Nullable primitives (boxing)
  • Non-nullable references (assertions)
  • String interpolation
  • Collections of primitives
  • Use of TreeMap TreeSet etc
  • Val rather than fields
  • Iterables vs Sequences
  • Inlining
  • Calling Lambda
  • Null-coalesing and elvis
  • Method references
  • Java 6 vs 8
  • Top-level functions
  • Companion objects
  • Synchronisation (delegates)

Running Reliable Benchmarks

To get reliably results, you want your computer to be doing as little else as possible. Close your IDE and all other extraneous apps, and run the benchmark from a vanilla Bash prompt. More tips