Add configuration for ExecutionContext

src/library/scala/concurrent/impl/ExecutionContextImpl.scala

@@ -43,17 +43,31 @@ private[scala] class ExecutionContextImpl private[impl] (es: Executor, reporter:
       }
     }
   }
-
-  def createExecutorService: ExecutorService = try { new ForkJoinPool(
-          Runtime.getRuntime.availableProcessors(), //FIXME from config
-          forkJoinPoolThreadFactory,
-          null, //FIXME we should have an UncaughtExceptionHandler, see what Akka does
-          true) //FIXME I really think this should be async...
+
+  def createExecutorService: ExecutorService = try {
+    def getInt(name: String, f: String => Int): Int =
+      try f(System.getProperty(name)) catch { case e: Exception => Runtime.getRuntime.availableProcessors }
+    def range(floor: Int, desired: Int, ceiling: Int): Int =
+      if (ceiling < floor) range(ceiling, desired, floor) else scala.math.min(scala.math.max(desired, floor), ceiling)
+
+    new ForkJoinPool(
+      range(
+        getInt("scala.concurrent.ec.minThreads", _.toInt),
+        getInt("scala.concurrent.ec.numThreads", {
+          case null | "" => Runtime.getRuntime.availableProcessors
+          case s if s.charAt(0) == 'x' => (Runtime.getRuntime.availableProcessors * s.substring(1).toDouble).ceil.toInt
+          case other => other.toInt
+        }),
+        getInt("scala.concurrent.ec.maxThreads", _.toInt)
+      ),
+      forkJoinPoolThreadFactory,
+      null, //FIXME we should have an UncaughtExceptionHandler, see what Akka does
+      true) //FIXME I really think this should be async...
   } catch {
     case NonFatal(t) =>
       System.err.println("Failed to create ForkJoinPool for the default ExecutionContext, falling back to Executors.newCachedThreadPool")
       t.printStackTrace(System.err)
-      Executors.newCachedThreadPool(executorsThreadFactory)
+      Executors.newCachedThreadPool(executorsThreadFactory) //FIXME use the same desired parallelism here too?
   }
 
   def execute(runnable: Runnable): Unit = executor match {