Limit max pool size for dynamic parallel execution

[mpkorstanje]

Overview

This is a followup of #3044 for the dynamic strategy.

With Java 9+ the ForkJoinPool used by JUnit can be configured with a maximum pool size via the junit.jupiter.execution.parallel.config.dynamic.max-pool-size-factor property.

While the number of concurrently executing tests may exceed the configured parallelism factor when tests become blocked, it will not exceed the maximum pool size factor in these circumstances.

With the following configuration:

junit.jupiter.execution.parallel.enabled=true
junit.jupiter.execution.parallel.config.strategy=dynamic
junit.jupiter.execution.parallel.mode.default=concurrent
junit.jupiter.execution.parallel.config.dynamic.factor=0.1667

This example will report between 2 and 4 tests running concurrently on a system with 12 available cores while only (12 * 0.1667 =) 2 would be expected.

class ExampleTest {

    private static final AtomicInteger running = new AtomicInteger();

    @BeforeEach
    void increment() {
        System.out.println("Running " + running.incrementAndGet());
    }

    @AfterEach
    void decrement() {
        running.decrementAndGet();
    }

    static IntStream numbers() {
        return IntStream.range(0, 1000);
    }

    @ParameterizedTest
    @MethodSource("numbers")
    void test(int i) throws ExecutionException, InterruptedException {
        Runnable sleep = () -> {
            try {
                Thread.sleep(600);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        };
        ForkJoinPool.commonPool().submit(sleep).get();
    }
}

By also configuring the max-pool-size-factor we can ensure the concurrently executing test does not exceed the expected 2 .

junit.jupiter.execution.parallel.config.dynamic.max-pool-size-factor=1

Additionally, because the ForkJoinPool will by default reject tasks that would exceed the maximum pool size the
junit.jupiter.execution.parallel.config.dynamic.saturate property has been added and will default to true. There appears to be no reason to ever set this false but it is there should someone depend on the old behavior.

Fixes: #3205

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
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[mpkorstanje]

There are two ways to calculate the max pool size:

maxPoolSize = dynamic.max-pool-size-factor * dynamic.factor * Runtime.getRuntime().availableProcessors() 
maxPoolSize = dynamic.max-pool-size-factor * Runtime.getRuntime().availableProcessors() 

And the choice is a bit arbitrary. But I think reasoning in terms of "threads per degree of parallelism" is a bit easier so I prefer the former.