Limit max pool size for dynamic parallel execution

documentation/src/docs/asciidoc/release-notes/release-notes-5.10.0-M1.adoc

@@ -36,6 +36,7 @@ repository on GitHub.
   of the former. Please refer to the
   <<../user-guide/index.adoc#launcher-api-launcher-interceptors-custom, User Guide>> for
   details.
+* Support for limiting the `max-pool-size-factor` for parallel execution via a configuration parameter.
 
 [[release-notes-5.10.0-M1-junit-jupiter]]
 === JUnit Jupiter
@@ -54,7 +55,8 @@ repository on GitHub.
 
 ==== Deprecations and Breaking Changes
 
-* ❓
+* The `dynamic` parallel execution strategy now allows the thread pool to be saturated by
+default.
 
 ==== New Features and Improvements
 
@@ -68,7 +70,10 @@ repository on GitHub.
   `AnnotationConsumer` interfaces.
 * New `AnnotationBasedArgumentConverter` that implements both `ArgumentConverter` and
   `AnnotationConsumer` interfaces.
-
+* New `junit.jupiter.execution.parallel.config.dynamic.max-pool-size-factor` configuration
+parameter to set the maximum pool size factor.
+* New `junit.jupiter.execution.parallel.config.dynamic.saturate` configuration
+parameter to disable pool saturation.
 
 [[release-notes-5.10.0-M1-junit-vintage]]
 === JUnit Vintage

documentation/src/docs/asciidoc/user-guide/writing-tests.adoc

@@ -2354,6 +2354,8 @@ configuration parameter to one of the following options.
   Computes the desired parallelism based on the number of available processors/cores
   multiplied by the `junit.jupiter.execution.parallel.config.dynamic.factor`
   configuration parameter (defaults to `1`).
+  The optional `junit.jupiter.execution.parallel.config.dynamic.max-pool-size-factor`
+  configuration parameter can be used to limit the maximum number of threads.
 
 `fixed`::
   Uses the mandatory `junit.jupiter.execution.parallel.config.fixed.parallelism`
@@ -2377,8 +2379,8 @@ of the synchronization mechanisms described in the next section, the `ForkJoinPo
 is used behind the scenes may spawn additional threads to ensure execution continues with
 sufficient parallelism.
 If you require such guarantees, with Java 9+, it is possible to limit the maximum number
-of concurrent threads by controlling the maximum pool size of the `fixed` and `custom`
-strategies.
+of concurrent threads by controlling the maximum pool size of the `dynamic`, `fixed` and
+`custom` strategies.
 
 [[writing-tests-parallel-execution-config-properties]]
 ===== Relevant properties
@@ -2425,6 +2427,14 @@ The following table lists relevant properties for configuring parallel execution
 | a positive decimal number
 | ```1.0```
 
+| ```junit.jupiter.execution.parallel.config.dynamic.max-pool-size.factor```
+| Factor to be multiplied by the number of available processors/cores and the value of
+  `junit.jupiter.execution.parallel.config.dynamic.factor` to determine the desired
+  parallelism for the ```dynamic``` configuration strategy
+| a positive decimal number, must be greater than or equal to `1.0`
+| 256 + the value of `junit.jupiter.execution.parallel.config.dynamic.factor` multiplied
+  by the number of available processors/cores
+
 | ```junit.jupiter.execution.parallel.config.fixed.parallelism```
 | Desired parallelism for the ```fixed``` configuration strategy
 | a positive integer
@@ -2433,7 +2443,7 @@ The following table lists relevant properties for configuring parallel execution
 | ```junit.jupiter.execution.parallel.config.fixed.max-pool-size```
 | Desired maximum pool size of the underlying fork-join pool for the ```fixed```
   configuration strategy
-| a positive integer, must greater than or equal to `junit.jupiter.execution.parallel.config.fixed.parallelism`
+| a positive integer, must be greater than or equal to `junit.jupiter.execution.parallel.config.fixed.parallelism`
 | 256 + the value of `junit.jupiter.execution.parallel.config.fixed.parallelism`
 
 | ```junit.jupiter.execution.parallel.config.fixed.saturate```

junit-platform-engine/src/main/java/org/junit/platform/engine/support/hierarchical/DefaultParallelExecutionConfigurationStrategy.java

@@ -71,8 +71,20 @@ public ParallelExecutionConfiguration createConfiguration(ConfigurationParameter
 			int parallelism = Math.max(1,
 				factor.multiply(BigDecimal.valueOf(Runtime.getRuntime().availableProcessors())).intValue());
 
-			return new DefaultParallelExecutionConfiguration(parallelism, parallelism, 256 + parallelism, parallelism,
-				KEEP_ALIVE_SECONDS, null);
+			int maxPoolSize = configurationParameters.get(CONFIG_DYNAMIC_MAX_POOL_SIZE_FACTOR_PROPERTY_NAME,
+				BigDecimal::new).map(maxPoolSizeFactor -> {
+					Preconditions.condition(maxPoolSizeFactor.compareTo(BigDecimal.ONE) >= 0,
+						() -> String.format(
+							"Factor '%s' specified via configuration parameter '%s' must be greater than or equal to 1",
+							factor, CONFIG_DYNAMIC_FACTOR_PROPERTY_NAME));
+					return maxPoolSizeFactor.multiply(BigDecimal.valueOf(parallelism)).intValue();
+				}).orElseGet(() -> 256 + parallelism);
+
+			boolean saturate = configurationParameters.get(CONFIG_DYNAMIC_SATURATE_PROPERTY_NAME,
+				Boolean::valueOf).orElse(true);
+
+			return new DefaultParallelExecutionConfiguration(parallelism, parallelism, maxPoolSize, parallelism,
+				KEEP_ALIVE_SECONDS, __ -> saturate);
 		}
 	},
 
@@ -154,12 +166,46 @@ public ParallelExecutionConfiguration createConfiguration(ConfigurationParameter
 	 * Property name of the factor used to determine the desired parallelism for the
 	 * {@link #DYNAMIC} configuration strategy.
 	 *
-	 * <p>Value must be a decimal number; defaults to {@code 1}.
+	 * <p>Value must be a non-negative decimal number; defaults to {@code 1}.
 	 *
 	 * @see #DYNAMIC
 	 */
 	public static final String CONFIG_DYNAMIC_FACTOR_PROPERTY_NAME = "dynamic.factor";
 
+	/**
+	 * Property name of the factor used to determine the maximum pool size of
+	 * the underlying fork-join pool for the {@link #DYNAMIC} configuration
+	 * strategy.
+	 *
+	 * <p>Value must be a decimal number equal and greater than or equal to
+	 * {@code 1}. When set the maximum pool size is calculated as
+	 * {@code dynamic.max-pool-size-factor * dynamic.factor * Runtime.getRuntime().availableProcessors()}
+	 * When not set the maximum pool size is calculated as
+	 * {@code 256 + dynamic.factor * Runtime.getRuntime().availableProcessors()}
+	 * instead.
+	 *
+	 * @since 1.10
+	 * @see #DYNAMIC
+	 */
+	@API(status = EXPERIMENTAL, since = "1.10")
+	public static final String CONFIG_DYNAMIC_MAX_POOL_SIZE_FACTOR_PROPERTY_NAME = "dynamic.max-pool-size-factor";
+
+	/**
+	 * Property name used to disable saturation of the underlying fork-join pool
+	 * for the {@link #DYNAMIC} configuration strategy.
+	 *
+	 * <p>When set to {@code false} the underlying fork-join pool will reject
+	 * additional tasks if all available workers are busy and the maximum
+	 * pool-size would be exceeded.
+	 * <p>Value must either {@code true} or {@code false}; defaults to {@code true}.
+	 *
+	 * @since 1.10
+	 * @see #DYNAMIC
+	 * @see #CONFIG_DYNAMIC_FACTOR_PROPERTY_NAME
+	 */
+	@API(status = EXPERIMENTAL, since = "1.10")
+	public static final String CONFIG_DYNAMIC_SATURATE_PROPERTY_NAME = "dynamic.saturate";
+
 	/**
 	 * Property name used to specify the fully qualified class name of the
 	 * {@link ParallelExecutionConfigurationStrategy} to be used by the

platform-tests/src/test/java/org/junit/platform/engine/support/hierarchical/DefaultParallelExecutionConfigurationStrategyTests.java

@@ -30,7 +30,7 @@
  */
 class DefaultParallelExecutionConfigurationStrategyTests {
 
-	private ConfigurationParameters configParams = mock();
+	final ConfigurationParameters configParams = mock();
 
 	@BeforeEach
 	void setUp() {
@@ -78,7 +78,25 @@ void dynamicStrategyCreatesValidConfiguration() {
 		assertThat(configuration.getMinimumRunnable()).isEqualTo(availableProcessors * 2);
 		assertThat(configuration.getMaxPoolSize()).isEqualTo(256 + (availableProcessors * 2));
 		assertThat(configuration.getKeepAliveSeconds()).isEqualTo(30);
-		assertThat(configuration.getSaturatePredicate()).isNull();
+		assertThat(configuration.getSaturatePredicate().test(null)).isTrue();
+	}
+
+	@Test
+	void dynamicSaturateStrategyCreatesValidConfiguration() {
+		when(configParams.get("dynamic.factor")).thenReturn(Optional.of("2.0"));
+		when(configParams.get("dynamic.max-pool-size-factor")).thenReturn(Optional.of("3.0"));
+		when(configParams.get("dynamic.saturate")).thenReturn(Optional.of("false"));
+
+		ParallelExecutionConfigurationStrategy strategy = DefaultParallelExecutionConfigurationStrategy.DYNAMIC;
+		var configuration = strategy.createConfiguration(configParams);
+
+		var availableProcessors = Runtime.getRuntime().availableProcessors();
+		assertThat(configuration.getParallelism()).isEqualTo(availableProcessors * 2);
+		assertThat(configuration.getCorePoolSize()).isEqualTo(availableProcessors * 2);
+		assertThat(configuration.getMinimumRunnable()).isEqualTo(availableProcessors * 2);
+		assertThat(configuration.getMaxPoolSize()).isEqualTo(availableProcessors * 6);
+		assertThat(configuration.getKeepAliveSeconds()).isEqualTo(30);
+		assertThat(configuration.getSaturatePredicate().test(null)).isFalse();
 	}
 
 	@Test