title: How to limit Cassandra NTR queue size
body: <p>After upgrading to Cassandra 3.11.8 we are facing intermittent runaway NTR queue growth.
  The NTR queue reaches a size of 5M at which point the node can't recover from the resulting GC overhead.
  Having read <a href="https://cassandra.apache.org/blog/Improving-Apache-Cassandras-Front-Door-and-Backpressure.html" rel="nofollow noreferrer">Improving Apache Cassandra’s Front Door and Backpressure</a>, we tried configuring <code>native_transport_max_concurrent_requests_in_bytes</code> to 1MB, assuming that would prevent the NTR queue from exploding. However, we still saw the NTR queue reach a size above 1M requests (so surely above a size of 1MB).</p>
  <p>We have <code>-Dcassandra.max_queued_native_transport_requests=3072</code> vm argument set, and obviously that doesn't limit the queue size to 3072 requests. I'm also not sure if this setting still applies following <a href="https://issues.apache.org/jira/browse/CASSANDRA-15013" rel="nofollow noreferrer">this change in 3.11.5</a>.
  My questions are:</p>
  <ol>
  <li>What is the right way to limit the NTR queue size in Cassandra 3.11.8?</li>
  <li>If there is such a way, does it apply both to the inbound and outbound NTR queues? Or is the outbound queue really unbounded as stated <a href="https://cassandra.apache.org/blog/Improving-Apache-Cassandras-Front-Door-and-Backpressure.html" rel="nofollow noreferrer">here</a>?</li>
  </ol>