This section describes how to write performance regression tests in ScalaMeter. In particular, it focuses on how to do performance test tuning. This encompasses two things. First, the test should have an appropriate running time. This means that it should be big enough to be observable, and not too long, so that it completes relatively quickly. Second, the test results should be reproducible. This means that you might have to tweak the configuration so that the running time can be reproduced across different test invocations.

Test tuning is something we do with unit tests as well – we have to at least run them in order to ensure they pass the first time. With performance tests the time you invest to tune the test becomes more noticeable.

Note

This is not a holy grail of performance testing.
Just like normal unit testing does not test your code against your specification, and inevidably leaves some of the cases left out, the performance regression testing is likely to overlook certain kinds of performance regressions, depending on how you write your tests. The goal of this section is to show how you can achieve reasonable test coverage, while having repeatable and reliable tests, with as few false positives as possible.

Lets write a performance regression test the foreach method on the Array class.

import org.scalameter.api._

class RegressionTest extends Bench.Regression {
  def persistor = new SerializationPersistor
  val sizes = Gen.range("size")(1000000, 5000000, 2000000)
  val arrays = for (sz <- sizes) yield (0 until sz).toArray

  performance of "Array" in {
    measure method "foreach" in {
      using(arrays) config (
        exec.independentSamples -> 1
      ) in { xs =>
        var sum = 0
        xs.foreach(x => sum += x)
      }
    }
  }    
}

We extend the Bench.Regression class this time, which is a preconfigured template for doing regression testing. This kind of a test has several smart facilities already in place to ensure more stable and reproducible testing. One of them is garbage collection cycle detection – measurements during which a GC occurs are discarded. For those benchmarks where a GC is inherent, once there are more discarded measurements than the desired number of measurements, all of them become viable measurements (this is to ensure termination). We discuss other facilities in more depth below.

Since a regression test needs a basis for comparison, it will continually persist running time results each time it is run. This persistence is done by the persistor part of the testing pipeline we’ve mentioned earlier. We instantiate a simple SerializationPersistor which will serialize the results and store them into files in the reports.resultDir directory.

Next, we define our input data, specifically, the arrays generator. We then define a test group Array and a nested test group foreach, within which we put our test.

using(arrays) config (
  exec.independentSamples -> 1
) in { xs =>
  var sum = 0
  xs.foreach(x => sum += x)
}

The test itself uses the arrays generator in a snippet in a way we’ve seen this before. A novelty here is a new parameter in the config block. The Regression class uses the SeparateJvmsExecutor – this executor invokes multiple instances of the JVM, warms up each one and runs the tests. The tests are by default divided between 9 JVM instances. You can set the number of JVM instances with the exec.independentSamples parameter.

:::Summary of regression test results - OverlapIntervals():::
Test group: Array.foreach
- Array.foreach.Test-0 measurements:
  - at size -> 1000000, 1 alternatives: passed
    (ci = <3.00, 3.00>, significance = 1.0E-10)
  - at size -> 3000000, 1 alternatives: passed
    (ci = <8.95, 10.05>, significance = 1.0E-10)
  - at size -> 5000000, 1 alternatives: passed
    (ci = <14.90, 16.21>, significance = 1.0E-10)

The first time we run the test, there is not history for it yet. This means that the test will be successful irrespective of the running time. The next time we run the test, we might get entirely different running times:

:::Summary of regression test results - OverlapIntervals():::
Test group: Array.foreach
- Array.foreach.Test-0 measurements:
  - at size -> 1000000, 1 alternatives: passed
    (ci = <3.00, 3.00>, significance = 1.0E-10)
  - at size -> 3000000, 1 alternatives: passed
    (ci = <8.84, 9.89>, significance = 1.0E-10)
  - at size -> 5000000, 1 alternatives: passed
    (ci = <14.98, 16.07>, significance = 1.0E-10)

However, after a few runs of the test on our machine, the test fails!

:::Summary of regression test results - OverlapIntervals():::
Test group: Array.foreach
- Array.foreach.Test-0 measurements:
  - at size -> 1000000, 5 alternatives: passed
    (ci = <2.88, 4.28>, significance = 1.0E-10)
  - at size -> 3000000, 5 alternatives: failed
    (ci = <10.00, 11.77>, significance = 1.0E-10)
      Failed confidence interval test: <-2.43, -0.35>
      Previous (mean = 9.50, stdev = 0.51, ci = <8.95, 10.05>): 9, 9, 9,
         9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 10, 10, 10, ...
      Latest   (mean = 10.89, stdev = 0.82, ci = <10.00, 11.77>): 10, 10, 10,
         10, 10, 10, 10, 10, 11, 11, 11, 11, 11, 11, 11, 11, 11, ...
...

It appears that our latest run of the benchmark is consistently slightly slower in that JVM instance. Effects like this are hard to predict – they may be due to a memory allocation pattern, decisions taken by the JIT compiler when optimizing the code or something entirely different.

The result times of failed tests are not saved into history and running the test again ends successfully on our machine.

Note

Reproducing this effect really depends on your environment. It may happen every single time you run the test, or it may not happen at all. Make sure that you have nothing in your environment that affects CPU performances -- check for background processes or that your laptop is not running on battery power.
If it does not happen at all for this example, you might have a different JVM or OS version.

So, how do we fight this? There are several things we can do.

  1. We can run several JVM instances per each test, instead of running just a single one. The probability that each of them will perform badly greatly decreases.

  2. We can reinstantiate the data for the test (in this case the array), multiple times. Different allocation patterns can yield very different results.

  3. We can add artificial Gaussian noise to each measurement in the series (this is controlled by the exec.noise.magnitude parameter). As a result, the confidence interval of each test will increase, making the tests less sensitive to random variations.

  4. We can artifically increase confidence intervals while doing a regression check (this is controlled by the reports.regression.noiseMagnitude parameter). This again results in the tests being less sensitive to random variations.

We can experiment easily with the first option. After setting the number of independent samples to 6 we are no longer able to reproduce the effect by running the benchmark many times. We also note that the confidence intervals now became larger, since there is a higher degree of variance in the tests.

Before we move on to the next example, try to experiment with different foreach bodies to see how the code in the body affects the observed performance, and which point you can trigger a performance regression failure.

Next, lets try to benchmark the map method on Lists. The nature of this benchmark is very different than the previous one, because it will allocate many small objects during the course of its run.

val sizes = Gen.range("size")(1000000, 2000000, 500000)
  
val lists = for (sz <- sizes) yield (0 until sz).toList
  
performance of "List" in {
  measure method "map" in {
    using(lists) config (
      exec.benchRuns -> 20,
      exec.independentSamples -> 1
    ) in { xs =>
      xs.map(_ + 1)
    }
  }
}

Note that we now do 20 repetitions within 1 JVM instance.

Running this test yields:

:::Summary of regression test results - OverlapIntervals():::
Test group: List.map
- List.map.Test-0 measurements:
  - at size -> 1000000, 1 alternatives: passed
    (ci = <1.60, 27.60>, significance = 1.0E-10)
  - at size -> 1500000, 1 alternatives: passed
    (ci = <12.59, 29.51>, significance = 1.0E-10)
  - at size -> 2000000, 1 alternatives: passed
    (ci = <17.85, 26.95>, significance = 1.0E-10)

The running times obviously have a high degree of variance, since the confidence intervals range up to 30 ms!

We use the -verbose option to see a bit more details about what happened. In particular, the running times from the verbose output look like this:

size -> 1000000: 9, 9, 9, 9, 9, 9, 10, 10, 17, 17, 17,
  17, 18, 18, 18, 18, 19, 19, 19, 21
size -> 1500000: 14, 17, 18, 19, 19, 19, 19, 20, 21, 21,
  21, 21, 23, 23, 23, 24, 24, 25, 25, 25
size -> 2000000: 20, 20, 20, 20, 21, 21, 22, 22, 22, 23,
  23, 23, 23, 23, 24, 24, 24, 24, 24, 25

There’s our problem. The running times differ by a great deal. This may be due to a number of factors, but typically it’s due to the fact that garbage collection cycles have been occurring during repetitions and/or the list on which a map is called has been allocated in a particularly bad memory pattern, making its traversal slow.

Each Executor has a Measurer component which is responsible for instantiating the test input and running the tests. This component decides how often it should call the generator to reinstantiate the value for the test, in this case the list. Doing this too often slows down your test, and never doing it may result in always measuring the badly allocated list. In the Regression benchmarks it does this every 12 repetitions by default, but you can customize this:

exec.reinstantiation.frequency -> 2

By instantiating the list more often running times stabilize, as we can see from the confidence intervals:

:::Summary of regression test results - OverlapIntervals():::
Test group: List.map
- List.map.Test-0 measurements:
  - at size -> 1000000, 1 alternatives: passed
    (ci = <7.74, 12.26>, significance = 1.0E-10)
  - at size -> 1500000, 1 alternatives: passed
    (ci = <13.35, 18.95>, significance = 1.0E-10)
  - at size -> 2000000, 1 alternatives: passed
    (ci = <16.93, 23.17>, significance = 1.0E-10)

We can also see this from the verbose output:

size -> 1000000: 9, 9, 9, 9, 9, 9, 10, 10, 10, 10, 10,
  10, 10, 10, 11, 11, 11, 11, 11, 11
size -> 1500000: 15, 15, 15, 15, 15, 16, 16, 16, 16,
  16, 16, 16, 16, 16, 17, 17, 17, 17, 17, 19
size -> 2000000: 18, 19, 19, 19, 19, 19, 20, 20, 20,
  20, 20, 20, 20, 20, 20, 21, 21, 22, 22, 22

Note

You may conclude from the verbose output that the running times were increasing for some strange reason. This is not the case at all -- the reason they are sorted is that it was done by the previously mentioned `Measurer` component to eliminate outliers.

Lets push the envelope a little bit this time – lets test a groupBy method which should be very allocation intensive, and lets increase the input test size:

val sizes = Gen.single("size")(5000000)
  
val lists = for (sz <- sizes) yield (0 until sz).toList
  
performance of "List" in {
  measure method "groupBy" in {
    using(lists) config (
      exec.benchRuns -> 20,
      exec.independentSamples -> 1
    ) in { xs =>
      xs.groupBy(_ % 10)
    }
  }
}

The output is:

:::Summary of regression test results - OverlapIntervals():::
Test group: List.groupBy
- List.groupBy.Test-0 measurements:
  - at size -> 5000000, 1 alternatives: passed
    (ci = <29.48, 352.02>, significance = 1.0E-10)

A confidence interval of this size is almost useless – we should try to stabilize the running times which differ by nearly 200%:

size -> 5000000: 125, 142, 144, 145, 146, 148, 149, 155,
  162, 164, 164, 164, 205, 210, 218, 224, 277, 281, 282, 310

If we examine the verbose output a bit more, we might notice something like this:

measurements: 218, 164, 281, 224, 164, 310, 818, 142, 210, 146, 277,
  205, 149, 282, 155, 148, 144, 162, 145, 164
Detected 1 outlier(s): 142, 144, 145, 146, 148, 149, 155, 162, 164, 164,
  164, 205, 210, 218, 224, 277, 281, 282, 310, 818
measurements: 125
After outlier elimination: 125, 142, 144, 145, 146, 148, 149, 155, 162, 164,
  164, 164, 205, 210, 218, 224, 277, 281, 282, 310

The Measurer component of the executor has done some measurements above, then sorted them and noticed that the last measurement is extremely different compared to the others. It then eliminated that measurement and redid it. This process is called outlier elimination.

We can take advantage of the outlier elimination to stabilize our tests. What the outlier elimination does is it takes a look at the suffixes of the sorted set of measurements. If it observes that some suffix of measurements changes the variance of the entire set of measurements by more than some factor (called coefficient of variance multiplier) then it discards that suffix and redoes those measurements. The process is repeated up to a certain number of times.

We can notice three parameters in this description:

  • exec.outliers.suspectPercent – the percentage size of the largest suffix to examine
  • exec.outliers.covMultiplier – the minimum factor by which removing the suffix changes the variance
  • exec.outliers.retries – the number of times to repeat the process

Lets tweak these a bit.

exec.outliers.covMultiplier -> 1.5,
exec.outliers.suspectPercent -> 40

We show the result below:

Detected 1 outlier(s): 140, 141, 143, ..., 206, 211, 268, 270, 275, 288, 876
All GC time ignored, accepted: 1, ignored: 0
measurements: 119
Detected 4 outlier(s): 119, 140, 141, ..., 203, 206, 211, 268, 270, 275, 288
All GC time ignored, accepted: 4, ignored: 7
measurements: 199, 144, 144, 140
Detected 5 outlier(s): 119, 140, 140, ..., 162, 166, 199, 203, 203, 206, 211
Some GC time recorded, accepted: 5, ignored: 7
measurements: 324, 1466, 140, 250, 153
Detected 1 outlier(s): 119, 140, 140, ..., 156, 162, 162, 166, 250, 324, 1466
Some GC time recorded, accepted: 1, ignored: 2
measurements: 157
Detected 1 outlier(s): 119, 140, 140, ..., 156, 157, 162, 162, 166, 250, 324
Some GC time recorded, accepted: 1, ignored: 2
measurements: 239
Detected 2 outlier(s): 119, 140, 140, ..., 156, 157, 162, 162, 166, 239, 250
Some GC time recorded, accepted: 2, ignored: 4
measurements: 260, 140
Detected 1 outlier(s): 119, 140, 140, ..., 153, 156, 157, 162, 162, 166, 260
Some GC time recorded, accepted: 1, ignored: 2
measurements: 290
Detected 1 outlier(s): 119, 140, 140, ..., 153, 156, 157, 162, 162, 166, 290
All GC time ignored, accepted: 1, ignored: 1
measurements: 147
After outlier elimination: 119, 140, ..., 147, 153, 156, 157, 162, 162, 166

At the expense of an increased running time, we’ve obtained more stable results:

:::Summary of regression test results - OverlapIntervals():::
Test group: List.groupBy
- List.groupBy.Test-0 measurements:
  - at size -> 5000000, 1 alternatives: passed
    (ci = <116.84, 176.46>, significance = 1.0E-10)

This should demonstrate that there are tests which are inherently unstable and that you can sometimes work around those instabilities by configuring how you do the measurement. Generally, understanding why the test is unstable and what is the cause of noise is very helpful in eliminating it. When trying to figure out the cause of an instability, always try to ask questions like:

  • is my test allocation intensive?
  • does my test allocate a huge chunk of memory?
  • what is the memory access pattern in my test?
  • does my test inherently trigger garbage collection cycles?
  • does my test allocate too much memory?

In the next section we examine different executors in more detail.

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