control http2writeQueue

	// streams maps stream ID to a queue.
	streams map[uint32]*http2writeQueue

	// stream queues are stored in a circular linked list.
	// head is the next stream to write, or nil if there are no streams open.
	head *http2writeQueue

	// pool of empty queues for reuse.
	queuePool http2writeQueuePool
}

  When the scheduling queue receives a cluster event, before moving each Pod from unschedulable pod pool to activeQ/backoffQ, it will call QueueingHintFn of plugins that rejected each Pod in the previous scheduling cycle.
  Depending on the value returned from QueueingHintFn, the scheduling queue changes how it queues each Pod:
  - if more than one QueueingHintFn returns QueueImmediately, it queues Pod to activeQ.

- Previously, the scheduling queue didn't notice any extenders' failures, potentially resulting in missed cluster events and Pods rejected by Extenders being stuck in the unschedulable pod pool for up to 5 minutes in the worst-case scenario. Now, the scheduling queue notices extenders' failures and requeues Pods rejected by Extenders appropriately.

- Sometimes, the scheduler incorrectly placed a pod in the "unschedulable" queue instead of the "backoff" queue. This happened when some plugin previously declared the pod as "unschedulable" and then in a later attempt encounters some other error. Scheduling of that pod then got delayed by up to five minutes, after which periodic flushing moved the pod back into the "active" queue. ([#120334](https://github.com/kubernetes/kubernetes/pull/120334), [@pohly](https://github.com/pohly))...

<a href="#Length_and_capacity"><code>cap</code></a> and
<a href="#Length_and_capacity"><code>len</code></a>
by any number of goroutines without further synchronization.
Channels act as first-in-first-out queues.
For example, if one goroutine sends values on a channel
and a second goroutine receives them, the values are
received in the order sent.
</p>

	// delete svc4. This will fail because the partial resync won't rewrite svc3's
	// rules and so the partial restore would leave a dangling jump from there to
	// svc4's endpoint. The proxier will then queue a full resync in response to the
	// partial resync failure, and the full resync will succeed (since it will rewrite
	// svc3's rules as well).
	//