src/Pure/Concurrent/isabelle_thread.scala

 
   def make_name(name: String = "", base: String = "thread"): String =
     "Isabelle." + proper_string(name).getOrElse(base + counter())
 
   def current_thread_group: ThreadGroup = Thread.currentThread.getThreadGroup
+
+  lazy val worker_thread_group: ThreadGroup =
+    new ThreadGroup(current_thread_group, "Isabelle worker")
 
   def create(
     main: Runnable,
     name: String = "",
     group: ThreadGroup = current_thread_group,

   {
     val m = Value.Int.unapply(System.getProperty("isabelle.threads", "0")) getOrElse 0
     val n = if (m > 0) m else (Runtime.getRuntime.availableProcessors max 1) min 8
     val executor =
       new ThreadPoolExecutor(n, n, 2500L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue[Runnable])
-    executor.setThreadFactory(create(_, name = make_name(base = "worker"), group = current_thread_group))
+    executor.setThreadFactory(
+      create(_, name = make_name(base = "worker"), group = worker_thread_group))
     executor
   }
 
 
   /* interrupt handlers */