presto-main/src/main/java/io/prestosql/execution/executor/TaskExecutor.java/TaskExecutor/start
class TaskExecutor:    @PostConstruct
    public synchronized void start()
    {
        checkState(!closed, "TaskExecutor is closed");
        for (int i = 0; i < runnerThreads; i++) {
            addRunnerThread();
        }
    }