public synchronized void stop() {
    tasks.forEach(task -> task.cancel(true));
    tasks.clear();
    stopWatcher.getAndSet(null).cancel(true);
  }
Cancels all current running threads.
The thread pool remains open, and this contention service can be reused.
geode-junit/src/main/java/org/apache/geode/test/concurrent/CPUContentionService.java