There are several predefined Reporters available in ScalaMeter. An abstract Reporter trait defines two overloaded methods called report:

trait Reporter[T] {
  def report(result: CurveData[T], persistor: Persistor): Unit
  def report(results: Tree[CurveData[T]], persistor: Persistor): Unit

The first report method is invoked by ScalaMeter whenever an Executor completes any single test. The second report method is invoked at the end when all of the tests are finished executing. If you plan to implement your own Reporter from scratch, then you should take a look at the ScalaMeter API to better understand what the parameters are, as well as how other important ScalaMeter datatypes work.
Otherwise, there are plenty of predefined reporters to choose from.

Null reporter

This reporter does not report anything at all. Any test result it receives will be ignored. If for whatever reason you don’t want any reports, use this reporter.

Composite reporter

This reporter is simply a composition of several reporters – when you need to report results with more than a single reporter, e.g. if you’d like command-line output as well as chart images for the tests, use this reporter.

Constructor arguments:

  • rs – a sequence of other reporters this reporter will

Configuration depends on the test parameters of the reporters in rs.

Logging reporter

Outputs the results of the tests into the terminal. This reporter is a fine choice when you need to output the results of your benchmark somewhere quickly.

Constructor arguments: (none)

Configuration: (none)

Chart reporter

Creates a chart for each test group. The chart can be a simple XY line chart, a histogram, a comparison of confidence intervals, a 3D chart or something else. A PNG file is created for each chart.

Constructor arguments:

  • drawer – a ChartFactory that determines the type of a chart to render


  • reports.resultDir – the directory in which the PNG files are generated

Chart factories

ChartReporter.ChartFactory is a common supertrait for objects that create charts. You can define your own chart factories by implementing this trait. There are also several predefined types of chart factories.

ChartFactory.XYLine renders a line chart for each curve in the test group. This factory only works correctly for data which depends on a single input axis which represents numeric value. For example, it works for the following generator:

for {
  size <- Gen.range("sizes")(200, 1000, 100)
} yield Array(0 until size: _*)

where there is a single input axis "size", but it does not work for:

for {
  size <- Gen.range("sizes")(200, 1000, 100)
  offset <- Gen.range("offset")(0, 1000, 100)
} yield Array((0 until size).map(_ + offset): _*)

where there are two input axes "size" and "offset".

ChartFactory.ConfidenceIntervals renders a confidence interval for each curve and the cummulative confidence interval of the previous runs of the same benchmarks. This is very useful when visually comparing the two alternatives since it gives insight not only in the mean value of each run, but also in the variance in the measurements.

This factory also only works correctly only for 2D data.

DSV reporter

Produces a DSV file for each curve with results that can be used for visualization. Every row represents an average obtained for one parameter combination for a specific test date. Confidence intervals are also included.

Constructor arguments:

  • delimiter – the character used to delimit columns


  • reports.resultDir – the directory in which the DSV files are stored
  • reports.regression.significance – the significance level for the statistical test (described below)

HTML reporter

Creates an HTML document with reports for all the tests. This reporter creates an interactive page which gives an overview of all test groups and curves. The page is capable of rendering charts in SVG format using the D3.js library. Performance data can be filtered by curve, date, and Generator dimensions. All filter parameters are set directly from within the HTML UI. Permalinks for specific filter configurations can be generated as a simple way of storing or sharing filter parameters.

HtmlReporter internally uses a DsvReporter to export performance data. It can either be exported to individual files for each curve, or embedded in the generated data.js file. The latter is particularly useful in cases where the JavaScript code has no access to the DSV files. This typically happens when opening the generated HTML document from the local file system. Most browsers enforce a same origin policy that prevents JavaScript code from accessing local files.

The HtmlReporter has to be used in combination with a RegressionReporter (see below) in order to have access to a history of running times. In the composition, RegressionReporter has to precede HtmlReporter in order for the history to include the most recent run, e.g. like this:

def reporter: Reporter = Reporter.Composite(
  new RegressionReporter(
    RegressionReporter.Historian.ExponentialBackoff() ),

Constructor arguments:

  • embedDsv – when set to true, data is embedded in data.js, otherwise a separate DSV file is created for each curve


  • reports.resultDir – the directory in which the report page and its resources are generated

Regression reporter

The RegressionReporter is a reporter that does performance regression testing. It compares the running time of each snippet against the previous running times and does some statistical analysis to decide whether the performance has changed.

If the reporter concludes there are no performance regressions, the running times of the test are persisted, to be taken into consideration during the subsequent runs of the test.

This reporter has to be used with a persistor different than Persistor.None. Otherwise, it will have nothing to compare the results against, and it will not be able to persist the results if the tests are successful.

We will explain this reporter in more detail in the section on performance regression testing, where we show how to do performance regression tests on a concrete example. In the meanwhile, we note that a regression reporter takes two parameters, namely, the Tester and the Historian. The former abstracts away the testing methodology, while the latter can prune the running time history so that it does not grow too large.

Constructor arguments:

  • test – the methodology used to detect performance regressions
  • historian – the policy used to prune old results


  • reports.resultDir – the directory in which the performance results are persisted
  • reports.regression.significance – the significance level for the statistical test, which is equal to 1 - confidenceLevel, where a confidence level is the probability that the real running time is in the computed confidence interval (the bottomline is – the smaller the significance level, the less likely the test reports false regressions, but may fail to report some real regressions)


The RegressionReporter.Tester trait represents the methodology used to test the results. There are several predefined implementations.

Tester.Accepter just accepts the test results each time. If you need to build a history of running times for the test, this is a tester of choice.

Tester.ANOVA applies the analysis of variance technique to detect whether there is at least one alternative in the history which is statistically different from all the other alternatives. This tester is very sensitive and is particularly applicable if your running time history has a fixed size (i.e. the oldest results are thrown away – see historians below). For unlimited history sizes, it might produce unreliable test results.

Tester.ConfidenceIntervals computes the confidence interval for each of the alternatives in the history and the most recent alternative, as well as the confidence interval of the difference between the most recent and any previous alternative. If the confidence interval of the difference includes zero, the performance test is successful. The strict parameter can be set to false, in which case this tester also passes the test if the confidence intervals of the most recent alternative and any previous alternative overlap. This makes the tests more solid and reliable, but the drawback is that certain types of regressions are harder or impossible to detect.
This is a recommended tester, and the one we will use in this overview – it works well for histories of any size.


The RegressionReporter.Historian trait represents the policy for removing the old test results from the history. Typically, the history grows every time a test is run, so we may want to limit it in practice.

Historian.Complete preserves the entire history. If you run the tests relatively rarely or you really want to preserve your entire history, this simple historian is ideal.

Historian.Window maintains a sliding window of the history. The size parameter determines the size of the sliding window – the number of previous results is always fixed. This historian is ideal if you run your tests very often. The downside is that it may not detect very slow performance regression trends – if each running time is only slightly slower than the previous one, the overall regression trend may be undetected.

Historian.ExponentialBackoff prunes the history exponentially. Assuming that the previous test runs are labeled 0 through 16, 0 being the oldest:

     1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16

Then the actual running times history that this historian actually preserves is:

     1                       9          13    15 16

If we add an additional measurement labeled 17, the history becomes:

   1                      9          13    15 16 17

Adding another measurement labeled 18 now eliminates the measurement labeled 16 because there are “two many” recent measurements:

 1                     9          13    15    17 18

This historian yields histories with unlimited size, but the size of the history is logarithmic in the number of the tests run. This detects slow regression trends accurately, while consuming very little space with each newly run test. We will mostly be using this historian.

Since performance regression testing may not be completely clear after this high level overview of the regression reporter, we show a coding example in the next section.