[GitHub] [couchdb] nickva commented on issue #4633: maintenance_mode on high CPU usage

["nickva (via GitHub)" <gi...@apache.org>]

nickva commented on issue #4633:
URL: https://github.com/apache/couchdb/issues/4633#issuecomment-1593209456

   @sergey-safarov that would work in some cases but CPU load might be tricky to figure out in various environments (windows, linux, k8s, vms). 
   
   Often a proxy for system is overload is when internal process message queues start filling up and stay relatively high. That can be detected by `http $DB/_node/$nodename/_system` endpoint. It shows some of the `message_queues` and their length.
   
   ```
   http $DB/_node/_local/_system | jq '.message_queues'
   {
     "couch_file": {},
     "couch_db_updater": {},
     "couch_server": 0,
     "index_server": 0,
      ...
   }
   ```
   
   Specifically `couch_db_updater` is one to look for during document writes as it could back up. But that could also indicate a slow disk IO issue not necessary a CPU overload issue.
   
   Erlang VM also does some busy waiting, that is keeps schedulers working for a bit longer than necessary to trade-off CPU usage for latency. That can make it seem like it's running out of CPUs capacity as the OS visible CPU usage would be higher, but it may still be doing fine in that state. You can disable busy waiting with: `+sbwt none`   `+sbwtdcpu none`  `+sbwtdio none` `vm.args` settings.
   
   However, in general, it could be dangerous to automatically put nodes in maintenance mode. There are good chances whatever is causing it on one node, will start happening on another node as well. Especially if it now has to also process API requests from the nodes which are already put in maintenance mode. So it all could lead to a cascading failure until none of the nodes can access traffic.
   


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