title: Concurrency optimization job in V8
body: <p>When concurrency optimization is triggered, a new optimization job will be dispatched to a child-thread to complete.</p>
  <p>How does Runtime know that the optimization job has been completed?</p>
  <p>I've found that in runtime function <code>StackGuard</code>, the optimized code will be set in JSFunction, but I don't know how this function is triggered.</p>