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## Chapter 13. Benchmarking and tweaking

## 13.1. Finding the best `Solver` configuration

OptaPlanner supports several optimization algorithms, but you're probably wondering which is the best one? Although some optimization algorithms generally perform better than others, it really depends on your problem domain. Most solver phases have parameters which can be tweaked. Those parameters can influence the results a lot, even though most solver phases work pretty well out-of-the-box.

Luckily, OptaPlanner includes a benchmarker, which allows you to play out different solver phases with different settings against each other, so you can pick the best configuration for your planning problem.

## 13.2. Doing a benchmark

### 13.2.1. Adding a dependency on optaplanner-benchmark

The benchmarker is in a separate artifact called `optaplanner-benchmark`.

If you use Maven, add a dependency in your `pom.xml` file:

        <dependency>

          <groupId>org.optaplanner</groupId>

          <artifactId>optaplanner-benchmark</artifactId>

        </dependency>

This is similar for Gradle, Ivy and Buildr. The version must be exactly the same as the `optaplanner-core` version used (which is automatically the case if you import `optaplanner-bom`).

If you use ANT, you've probably already copied the required jars from the download zip's `binaries` directory.

### 13.2.2. Building and running a `PlannerBenchmark`

You can build a `PlannerBenchmark` instance with the `XmlPlannerBenchmarkFactory`. Configure it with a benchmark configuration xml file:

            PlannerBenchmarkFactory plannerBenchmarkFactory = new XmlPlannerBenchmarkFactory(

                    "/org/optaplanner/examples/nqueens/benchmark/nqueensBenchmarkConfig.xml");

            PlannerBenchmark plannerBenchmark = benchmarkFactory.buildPlannerBenchmark();

            plannerBenchmark.benchmark();

A basic benchmark configuration file looks something like this:

    <?xml version="1.0" encoding="UTF-8"?>

    <plannerBenchmark>

      <benchmarkDirectory>local/data/nqueens</benchmarkDirectory>

      <!--<parallelBenchmarkCount>AUTO</parallelBenchmarkCount>-->

      <warmUpSecondsSpend>30</warmUpSecondsSpend>

      <inheritedSolverBenchmark>

        <problemBenchmarks>

          <xstreamAnnotatedClass>org.optaplanner.examples.nqueens.domain.NQueens</xstreamAnnotatedClass>

          <inputSolutionFile>data/nqueens/unsolved/32queens.xml</inputSolutionFile>

          <inputSolutionFile>data/nqueens/unsolved/64queens.xml</inputSolutionFile>

          <problemStatisticType>BEST_SCORE</problemStatisticType>

        </problemBenchmarks>

        <solver>

          <solutionClass>org.optaplanner.examples.nqueens.domain.NQueens</solutionClass>

          <planningEntityClass>org.optaplanner.examples.nqueens.domain.Queen</planningEntityClass>

          <scoreDirectorFactory>

            <scoreDefinitionType>SIMPLE</scoreDefinitionType>

            <scoreDrl>/org/optaplanner/examples/nqueens/solver/nQueensScoreRules.drl</scoreDrl>

          </scoreDirectorFactory>

          <termination>

            <maximumSecondsSpend>20</maximumSecondsSpend>

          </termination>

          <constructionHeuristic>

            <constructionHeuristicType>FIRST_FIT_DECREASING</constructionHeuristicType>

            <forager>

              <pickEarlyType>FIRST_NON_DETERIORATING_SCORE</pickEarlyType>

            </forager>

          </constructionHeuristic>

        </solver>

      </inheritedSolverBenchmark>

      <solverBenchmark>

        <name>Entity tabu</name>

        <solver>

          <localSearch>

            <changeMoveSelector>

              <selectionOrder>ORIGINAL</selectionOrder>

            </changeMoveSelector>

            <acceptor>

              <entityTabuSize>5</entityTabuSize>

            </acceptor>

            <forager>

              <pickEarlyType>NEVER</pickEarlyType>

            </forager>

          </localSearch>

        </solver>

      </solverBenchmark>

      <solverBenchmark>

        <name>Value tabu</name>

        <solver>

          <localSearch>

            <changeMoveSelector>

              <selectionOrder>ORIGINAL</selectionOrder>

            </changeMoveSelector>

            <acceptor>

              <valueTabuSize>5</valueTabuSize>

            </acceptor>

            <forager>

              <pickEarlyType>NEVER</pickEarlyType>

            </forager>

          </localSearch>

        </solver>

      </solverBenchmark>

      <solverBenchmark>

        <name>Move tabu</name>

        <solver>

          <localSearch>

            <changeMoveSelector>

              <selectionOrder>ORIGINAL</selectionOrder>

            </changeMoveSelector>

            <acceptor>

              <moveTabuSize>5</moveTabuSize>

            </acceptor>

            <forager>

              <pickEarlyType>NEVER</pickEarlyType>

            </forager>

          </localSearch>

        </solver>

      </solverBenchmark>

    </plannerBenchmark>

This `PlannerBenchmark` will try 3 configurations (1 move tabu, 1 entity tabu and 1 value tabu) on 2 data sets (32 and 64 queens), so it will run 6 solvers.

Every `solverBenchmark` element contains a solver configuration (for example with a local search solver phase) and one or more `inputSolutionFile` elements. It will run the solver configuration on each of those unsolved solution files. The element `name` is optional, because it is generated if absent. The inputSolutionFile is read by a ProblemIO.

To lower verbosity, the common part of multiple `solverBenchmark` entities can be extracted to the `inheritedSolverBenchmark` element. Yet, every element can still be overwritten per `solverBenchmark` element. Note that inherited solver phases such as `<constructionHeuristic>` or `<localSearch>` are not overwritten but instead are added to the tail of the solver phases list.

You need to specify a `benchmarkDirectory` (relative to the working directory). A benchmark report will be written in that directory.

## Note

It's recommended that the `benchmarkDirectory` is a directory ignored for source control and not cleaned by your build system. This way the generated files are not bloating your source control and they aren't lost when doing a build. Usually that directory is called `local`.

### 13.2.3. ProblemIO: input and output of Solution files

#### 13.2.3.1. `ProblemIO` interface

The benchmarker needs to be able to read the input files to contain a `Solution` write the best `Solution` of each benchmark to an output file. For that it uses a class that implements the `ProblemIO` interface:

    public interface ProblemIO {

        String getFileExtension();

        Solution read(File inputSolutionFile);

        void write(Solution solution, File outputSolutionFile);

    }

## Warning

Your input files need to have been written with the same `ProblemIO` class as they are being read by the benchmarker.

#### 13.2.3.2. `XStreamProblemIO`: the default `ProblemIO`

By default, a benchmarker uses a `XStreamProblemIO` instance to read and write solutions.

You need to tell the benchmarker about your `Solution` class which is annotated with XStream annotations:

        <problemBenchmarks>

          <xstreamAnnotatedClass>org.optaplanner.examples.nqueens.domain.NQueens</xstreamAnnotatedClass>

          <inputSolutionFile>data/nqueens/unsolved/32queens.xml</inputSolutionFile>

          ...

        </problemBenchmarks>

Your input files need to have been written with a `XStreamProblemIO` instance, not just any `XStream` instance, because the `XStreamProblemIO` uses a customized `XStream` instance.

## Warning

XStream (and XML in general) is a very verbose format. Reading or writing large datasets in this format can cause an `OutOfMemoryError` and performance degradation.

#### 13.2.3.3. Custom `ProblemIO`

Alternatively, you can implement your own `ProblemIO` implementation and configure it with the `problemIOClass` element:

        <problemBenchmarks>

          <problemIOClass>org.optaplanner.examples.machinereassignment.persistence.MachineReassignmentProblemIO</problemIOClass>

          <inputSolutionFile>data/machinereassignment/import/model_a1_1.txt</inputSolutionFile>

          ...

        </problemBenchmarks>

## Warning

A `ProblemIO` implementation must be thread-safe.

### 13.2.4. Writing the output solution of the benchmark runs

The best solution of each benchmark run can be written to the in the `benchmarkDirectory`. By default, this is disabled, because the files are rarely used and considered bloat. Also, on large datasets, writing the best solution of each single benchmark can take quite some time and memory (causing an `OutOfMemoryError`), especially in a verbose format like XStream.

You can enable to write the output solution in the `benchmarkDirectory` with `writeOutputSolutionEnabled`:

        <problemBenchmarks>

          ...

          <writeOutputSolutionEnabled>true</writeOutputSolutionEnabled>

          ...

        </problemBenchmarks>

### 13.2.5. Warming up the HotSpot compiler

Without a warm up, the results of the first (or first few) benchmarks are not reliable, because they will have lost CPU time on HotSpot JIT compilation (and possibly DRL compilation too).

The avoid that distortion, the benchmarker can run some of the benchmarks for a specified amount of time, before running the real benchmarks. Generally, a warm up of 30 seconds suffices:

    <plannerBenchmark>

      ...

      <warmUpSecondsSpend>30</warmUpSecondsSpend>

      ...

    </plannerBenchmark>

## 13.3. Benchmark report

### 13.3.1. HTML report

After the running a benchmark, a HTML report will be written in the `benchmarkDirectory` with the filename `index.html`. Open it in your browser. It has a nice overview of your benchmark including:

- Summary statistics: graphs and tables

- Problem statistics per `inputSolutionFile`

- Each solver configuration (ranked): easy to copy and paste.

- Benchmark information

The HTML report will use your default locale to format numbers. If you need to share the benchmark report with people from another country, you might want to overwrite the `benchmarkReportLocale`:

    <plannerBenchmark>

      ...

      <benchmarkReportLocale>en_US</benchmarkReportLocale>

      ...

    </plannerBenchmark>

### 13.3.2. Summary statistics

#### 13.3.2.1. Best score summary (graph and table)

Shows the best score per `inputSolutionFile` for each solver configuration.

Useful for visualizing the best solver configuration.

Figure 13.1. Best score summary statistic

#### 13.3.2.2. Best score scalability summary (graph)

Shows the best score per problem scale for each solver configuration.

Useful for visualizing the scalability of each solver configuration.

#### 13.3.2.3. Winning score difference summary (graph and table)

Shows the winning score difference score per `inputSolutionFile` for each solver configuration. The winning score difference is the score difference with the score of the winning solver configuration for that particular `inputSolutionFile`.

Useful for zooming in on the results of the best score summary.

#### 13.3.2.4. Worst score difference percentage (ROI) summary (graph and table)

Shows the return on investment (ROI) per `inputSolutionFile` for each solver configuration if you'd upgrade from the worst solver configuration for that particular `inputSolutionFile`.

Useful for visualizing the return on investment (ROI) to decision makers.

#### 13.3.2.5. Average calculation count summary (graph and table)

Shows the score calculation speed: the average calculation count per second per problem scale for each solver configuration.

Useful for comparing different score calculators and/or score rule implementations (presuming that the solver configurations do not differ otherwise). Also useful to measure the scalability cost of an extra constraint.

#### 13.3.2.6. Time spend summary (graph and table)

Shows the time spend per `inputSolutionFile` for each solver configuration. This is pointless if it's benchmarking against a fixed time limit.

Useful for visualizing the performance of construction heuristics (presuming that no other solver phases are configured).

#### 13.3.2.7. Time spend scalability summary (graph)

Shows the time spend per problem scale for each solver configuration. This is pointless if it's benchmarking against a fixed time limit.

Useful for extrapolating the scalability of construction heuristics (presuming that no other solver phases are configured).

#### 13.3.2.8. Best score per time spend summary (graph)

Shows the best score per time spend for each solver configuration. This is pointless if it's benchmarking against a fixed time limit.

Useful for visualizing trade-off between the best score versus the time spend for construction heuristics (presuming that no other solver phases are configured).

### 13.3.3. Statistic per data set (graph and CSV)

#### 13.3.3.1. Enabling a problem statistic

The benchmarker supports outputting problem statistics as graphs and CSV (comma separated values) files to the `benchmarkDirectory`.

To configure graph and CSV output of a statistic, just add a `problemStatisticType` line:

    <plannerBenchmark>

      <benchmarkDirectory>local/data/nqueens/solved</benchmarkDirectory>

      <inheritedSolverBenchmark>

        <problemBenchmarks>

          ...

          <problemStatisticType>BEST_SCORE</problemStatisticType>

          <problemStatisticType>CALCULATE_COUNT_PER_SECOND</problemStatisticType>

        </problemBenchmarks>

        ...

      </inheritedSolverBenchmark>

      ...

    </plannerBenchmark>

Multiple `problemStatisticType` elements are allowed. Some statistic types might influence performance and benchmark results noticeably.

## Note

These statistic per data set can slow down the solver noticeably, which can affect the benchmark results. That's why they are optional and not enabled by default.

The non-optional summary statistics cannot slow down the solver noticeably.

The following types are supported:

#### 13.3.3.2. Best score over time statistic (graph and CSV)

To see how the best score evolves over time, add:

        <problemBenchmarks>

          ...

          <problemStatisticType>BEST_SCORE</problemStatisticType>

        </problemBenchmarks>

Figure 13.2. Best score over time statistic

The best score over time statistic is very useful to detect abnormalities, such as a potential score trap.

## Note

A time gradient based algorithm (such as Simulated Annealing) will have a different statistic if it's run with a different time limit configuration. That's because this Simulated Annealing implementation automatically determines its velocity based on the amount of time that can be spend. On the other hand, for the Tabu Search and Late Annealing, what you see is what you'd get.

#### 13.3.3.3. Step score over time statistic (graph and CSV)

To see how the step score evolves over time, add:

        <problemBenchmarks>

          ...

          <problemStatisticType>STEP_SCORE</problemStatisticType>

        </problemBenchmarks>

Figure 13.3. Step score over time statistic

Compare the step score statistic with the best score statistic (especially on parts for which the best score flatlines). If it hits a local optima, the solver should take deteriorating steps to escape it. But it shouldn't deteriorate too much either.

## Warning

The step score statistic has been seen to slow down the solver noticeably due to GC stress, especially for fast stepping algorithms (such as Simulated Annealing and Late Acceptance).

#### 13.3.3.4. Calculate count per second statistic (graph and CSV)

To see how fast the scores are calculated, add:

        <problemBenchmarks>

          ...

          <problemStatisticType>CALCULATE_COUNT_PER_SECOND</problemStatisticType>

        </problemBenchmarks>

Figure 13.4. Calculate count per second statistic

## Note

The initial high calculate count is typical during solution initialization: it's far easier to calculate the score of a solution if only a handful planning entities have been initialized, than when all the planning entities are initialized.

After those few seconds of initialization, the calculate count is relatively stable, apart from an occasional stop-the-world garbage collector disruption.

#### 13.3.3.5. Best solution mutation over time statistic (graph and CSV)

To see how much each new best solution differs from the previous best solution, by counting the number of planning variables which have a different value (not including the variables that have changed multiple times but still end up with the same value), add:

        <problemBenchmarks>

          ...

          <problemStatisticType>BEST_SOLUTION_MUTATION</problemStatisticType>

        </problemBenchmarks>

Figure 13.5. Best solution mutation over time statistic

Use Tabu Search - an algorithm that behaves like a human - to get an estimation on how difficult it would be for a human to improve the previous best solution to that new best solution.

#### 13.3.3.6. Move count per step statistic (graph and CSV)

To see how the selected and accepted move count per step evolves over time, add:

        <problemBenchmarks>

          ...

          <problemStatisticType>MOVE_COUNT_PER_STEP</problemStatisticType>

        </problemBenchmarks>

Figure 13.6. Move count per step statistic

## Warning

This statistic has been seen to slow down the solver noticeably due to GC stress, especially for fast stepping algorithms (such as Simulated Annealing and Late Acceptance).

#### 13.3.3.7. Memory use statistic (graph and CSV)

To see how much memory is used, add:

        <problemBenchmarks>

          ...

          <problemStatisticType>MEMORY_USE</problemStatisticType>

        </problemBenchmarks>

Figure 13.7. Memory use statistic

## Warning

The memory use statistic has been seen to affect the solver noticeably.

### 13.3.4. Ranking the `Solver`s

The benchmark report automatically ranks the solvers. The `Solver` with rank `0` is called the favorite `Solver`: it performs best overall, but it might not be the best on every problem. It's recommended to use that favorite `Solver` in production.

However, there are different ways of ranking the solvers. Configure it like this:

    <plannerBenchmark>

      ...

      <solverBenchmarkRankingType>TOTAL_SCORE</solverBenchmarkRankingType>

      ...

    </plannerBenchmark>

The following `solverBenchmarkRankingType`s are supported:

- `TOTAL_SCORE` (default): Maximize the overall score, so minimize the overall cost if all solutions would be executed.

- `WORST_SCORE`: Minimize the worst case scenario.

- `TOTAL_RANKING`: Maximize the overall ranking. Use this if your datasets differ greatly in size or difficulty, producing a difference in `Score` magnitude.

You can also use a custom ranking, by implementing a `Comparator`:

      <solverBenchmarkRankingComparatorClass>...TotalScoreSolverBenchmarkRankingComparator</solverBenchmarkRankingComparatorClass>

Or a weight factory:

      <solverBenchmarkRankingWeightFactoryClass>...TotalRankSolverBenchmarkRankingWeightFactory</solverBenchmarkRankingWeightFactoryClass>

## 13.4. Advanced benchmarking

### 13.4.1. Benchmarking performance tricks

#### 13.4.1.1. Parallel benchmarking on multiple threads

If you have multiple processors available on your computer, you can run multiple benchmarks in parallel on multiple threads to get your benchmarks results faster:

    <plannerBenchmark>

      ...

      <parallelBenchmarkCount>AUTO</parallelBenchmarkCount>

      ...

    </plannerBenchmark>

## Warning

Running too many benchmarks in parallel will affect the results of benchmarks negatively. Leave some processors unused for garbage collection and other processes.

We tweak `parallelBenchmarkCount` `AUTO` to maximize the reliability and efficiency of the benchmark results.

The following `parallelBenchmarkCount`s are supported:

- `1` (default): Run all benchmarks sequentially.

- `AUTO`: Let Planner decide how many benchmarks to run in parallel. This formula is based on experience. It's recommended to prefer this over the other parallel enabling options.

- Static number: The number of benchmarks to run in parallel.

      <parallelBenchmarkCount>2</parallelBenchmarkCount>

- JavaScript formula: Formula for the number of benchmarks to run in parallel. It can use the variable `availableProcessorCount`. For example:

      <parallelBenchmarkCount>(availableProcessorCount / 2) + 1</parallelBenchmarkCount>

## Note

The `parallelBenchmarkCount` is always limited to the number of available processors. If it's higher, it will be automatically decreased.

## Note

In the future, we will also support multi-JVM benchmarking. This feature is independent of multi-threaded solving or multi-JVM solving.

### 13.4.2. Template based benchmarking and matrix benchmarking

Matrix benchmarking is benchmarking a combination of value sets. For example: benchmark 4 `entityTabuSize` values (`5`, `7`, `11` and `13`) combined with 3 `acceptedCountLimit` values (`500`, `1000` and `2000`), resulting in 12 solver configurations.

To reduce the verbosity of such a benchmark configuration, you can use a Freemarker template for the benchmark configuration instead:

    <plannerBenchmark>

      ...

      <inheritedSolverBenchmark>

        ...

      </inheritedSolverBenchmark>

    <#list [5, 7, 11, 13] as entityTabuSize>

    <#list [500, 1000, 2000] as acceptedCountLimit>

      <solverBenchmark>

        <name>entityTabuSize ${entityTabuSize} acceptedCountLimit ${acceptedCountLimit}</name>

        <solver>

          <localSearch>

            <unionMoveSelector>

              <changeMoveSelector/>

              <swapMoveSelector/>

            </unionMoveSelector>

            <acceptor>

              <entityTabuSize>${entityTabuSize}</entityTabuSize>

            </acceptor>

            <forager>

              <acceptedCountLimit>${acceptedCountLimit}</acceptedCountLimit>

            </forager>

          </localSearch>

        </solver>

      </solverBenchmark>

    </#list>

    </#list>

    </plannerBenchmark>

And build it with the class `FreemarkerXmlPlannerBenchmarkFactory`:

            PlannerBenchmarkFactory plannerBenchmarkFactory = new FreemarkerXmlPlannerBenchmarkFactory(

                    "/org/optaplanner/examples/cloudbalancing/benchmark/cloudBalancingBenchmarkConfigTemplate.xml.ftl");

            PlannerBenchmark plannerBenchmark = benchmarkFactory.buildPlannerBenchmark();

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