Extend ParallelExecutionConfiguration to specify a predicate for determing if a pool is saturated

[klease]

I'm working on the Apache Camel project which uses parallel execution on a module with a large number of test cases (over 2400 classes and over 6000 tests.) After upgrading to 5.7.2 or later we were getting RejectedExecutionException which we solved by using a custom configuration with a large value for maxPoolSize.

The cause appears to be that the Tasks for the test classes all call ForkJoinPool.managedBlock and the tests themselves can take up to a few seconds to run. Fairly quickly all threads in the ForkJoinPool are busy and therefore each new call to managedBock causes a new thread to be allocated up to the maxPoolSize. Each of these new threads usually runs only 1 or 2 tests.

I found that by providing a predicate for saturated when creating the ForkJoinPool, and having it return true, it was possible to run all the tests with a much smaller maxPoolSize.

Currently it's not possible to control this parameter using a custom implementation of ParallelExecutionConfiguration.

So I propose to add a method to the interface such as the following:

/**
 * Get the predicate called when there the ForkJoinPool has reached MaxPoolSize
 * and there are no threads available, or null. The predicate should return true if the
 * blocking thread should try again. If the predicate is null or returns false a
 * RejectedExcecutionException is thrown.
 * @return a Predicate accepting a ForkJoinPool as the parameter or null.
 */
Predicate<? super ForkJoinPool> getSaturatePredicate();

The DefaultParallelExecutionConfiguration would return null.

The class ForkJoinPoolHierarchicalTestExecutorService would use configuration.getSaturatePredicate() when creating the ForkJoinPool.

[sbrannen]

How would one supply the Predicate?

[klease]

By implementing the new method in a custom implementation of ParallelExecutionConfiguration, for example:

@Override
public Predicate<? super ForkJoinPool> getSaturatePredicate() {
    return (ForkJoinPool p) -> true;
}

[sbrannen]

Thanks for the clarification.

Wouldn't getSaturatedPredicate() make more sense (saturated)?

[klease]

Yes, that does make more sense, even if the parameter name in the java.util.concurrent.ForkJoinPool constructor is named "saturate".

[marcphilipp]

Team decision: Add ParallelExecutionConfiguration.getSaturatePredicate()

@klease Would you be interested in submitting a PR for this?

[klease]

@klease Would you be interested in submitting a PR for this?

Yes, of course. It will be my first for junit5 so I'll try to follow the guidelines.