Having a large number of offline executors causes massive slowdown in  hudson.model.Queue. The maintain method is holding the queue lock over 80% of the time in some cases.

"AtmostOneTaskExecutor[Periodic Jenkins queue maintenance] 
   java.lang.Thread.State: RUNNABLE
        at org.jenkinsci.plugins.durabletask.executors.ContinuedTask$Scheduler.canTake(ContinuedTask.java:66)
        at hudson.model.Queue$JobOffer.getCauseOfBlockage(Queue.java:278)
        at hudson.model.Queue.maintain(Queue.java:1616)
        at hudson.model.Queue$1.call(Queue.java:325)
        at hudson.model.Queue$1.call(Queue.java:322)

Steps to reproduce:

1. install jenkins + job-dsl-plugin + matrix-project-plugin + ssh-slaves-plugin + workflow-durable-task-step (Pipeline: nodes and processes)
2. create a ssh node with 500 executors, add some random labels "a b c d e f g h i"
3. mark the node offline using configure->availability->"bring online according to schedule"
4. create the jobs using job dsl below
5. wait for the jobs to start, observe the sluggish queue, fire up jvisualvm to analyze

configs = []
for (int i = 0; i < 100; i++) {
  configs.add(String.valueOf(i))
}

for (int i = 0; i < 10; i++) {
  matrixJob("matrix-"+i) {
    axes {
      text('cfg', configs)
    }
    triggers {
      cron('* * * * *')
    }
    steps {
      shell('sleep 30')
    }
  }
}

It seems each "parked executor" causes a Queue$JobOffer to be created, which is turn triggers some getCauseOfBlockage analysis. This seems to do blockedItems * buildableItems operations which can get quite slow for a large job queue.

How it was found: We have ~80 nodes with 10 executors each. We took half of them offline during a hardware migration. Soon our jobs filled the queue with 2000 items. Jenkins started timing out due to queue lock contention - a single maintain() call took around 60sec.