title: what's the difference between airflow's 'parallelism' and 'dag_concurrency'
body: <p>I can't understand the difference between <code>dag_concurrency</code> and <code>parallelism</code>. documentation and some of the related posts here somehow contradicts my findings.</p>
  
  <p>The understanding I had before was that the <code>parallelism</code> parameter allows you to set the MAX number of global(across all DAGs) TaskRuns possible in airflow and <code>dag_concurrency</code> to mean the MAX number of TaskRuns possible for a single Dag.</p>
  
  <p>So I set the <code>parallelism</code> to 8 and <code>dag_concurrency</code> to 4 and ran a single Dag. And I found out that it was running 8 TIs at a time but I was expecting it to run 4 at a time. </p>
  
  <ol>
  <li><p>How is that possible? </p></li>
  <li><p>Also, if it helps, I have set the pool size to 10 or so for these tasks. But that shouldn't have mattered as "config" parameters are given higher priorities than the pool's, Right?</p></li>
  </ol>