AbstractMultiThreadedConvolution: fix issue 77 (part 1/2)

See #77

Maximum pool size for Executors.newCachedThreadPool() is Integer.MAX_VALUE.
This is not useful, when you want to decide, on how many threads a task
should be distributed.
Use the number of available processor as an upper limit in that case.

Author: maarzt

src/main/java/net/imglib2/algorithm/convolution/AbstractMultiThreadedConvolution.java

@@ -39,7 +39,7 @@ final public void process( final RandomAccessible< ? extends T > source, final R
 	{
 		if ( executor == null )
 		{
-			final int numThreads = suggestNumThreads();
+			final int numThreads = Runtime.getRuntime().availableProcessors();
 			final ExecutorService executor = Executors.newFixedThreadPool( numThreads );
 			try
 			{
@@ -56,15 +56,13 @@ final public void process( final RandomAccessible< ? extends T > source, final R
 		}
 	}
 
-	private int getNumThreads( final ExecutorService executor )
+	static int getNumThreads( final ExecutorService executor )
 	{
-		if ( executor instanceof ThreadPoolExecutor )
-			return ( ( ThreadPoolExecutor ) executor ).getMaximumPoolSize();
-		return suggestNumThreads();
+		int maxPoolSize = ( executor instanceof ThreadPoolExecutor ) ?
+				( ( ThreadPoolExecutor ) executor ).getMaximumPoolSize() :
+				Integer.MAX_VALUE;
+		int availableProcessors = Runtime.getRuntime().availableProcessors();
+		return Math.max(1, Math.min(availableProcessors, maxPoolSize));
 	}
 
-	private int suggestNumThreads()
-	{
-		return Runtime.getRuntime().availableProcessors();
-	}
 }

src/test/java/net/imglib2/algorithm/convolution/AbstractMultiThreadedConvolutionTest.java

@@ -0,0 +1,35 @@
+package net.imglib2.algorithm.convolution;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import org.junit.Test;
+
+import static junit.framework.TestCase.assertTrue;
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
+
+/**
+ * Tests {@link AbstractMultiThreadedConvolution}
+ *
+ * @author Matthias Arzt
+ */
+public class AbstractMultiThreadedConvolutionTest
+{
+	@Test
+	public void testSuggestNumTasksFixedThreadPool() {
+		if ( Runtime.getRuntime().availableProcessors() < 3 )
+			return;
+		final ExecutorService executor = Executors.newFixedThreadPool( 3 );
+		int result = AbstractMultiThreadedConvolution.getNumThreads( executor );
+		assertEquals( 3, result );
+	}
+
+	@Test
+	public void testSuggestNumTasksCachedThreadPool() {
+		final ExecutorService executor = Executors.newCachedThreadPool();
+		int result = AbstractMultiThreadedConvolution.getNumThreads( executor );
+		assertTrue( Runtime.getRuntime().availableProcessors() >= result );
+	}
+}