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+# Scheduling queue in kube-scheduler
+
+Queueing mechanism is an integral part of the scheduler. It allows the scheduler
+to pick the most suitable pod for the next scheduling cycle. Given a pod can
+specify various conditions that have to be met at the time of scheduling,
+such as existence of a persistent volume, compliance with pod anti-affinity rules
+or toleration of node taints, the mechanism needs to be able to postpone
+the scheduling action until the cluster may meet all the conditions for
+the successful scheduling. The mechanism relies on three queues:
+- active ([activeQ](https://github.com/kubernetes/kubernetes/blob/4cc1127e9251fff364d5c77e2a9a9c3ad42383ab/pkg/scheduler/internal/queue/scheduling_queue.go#L130)): providing pods for immediate scheduling
+- unschedulable ([unschedulableQ](https://github.com/kubernetes/kubernetes/blob/4cc1127e9251fff364d5c77e2a9a9c3ad42383ab/pkg/scheduler/internal/queue/scheduling_queue.go#L135)): for parking pods which are waiting for certain condition(s) to happen
+- backoff ([podBackoffQ](https://github.com/kubernetes/kubernetes/blob/4cc1127e9251fff364d5c77e2a9a9c3ad42383ab/pkg/scheduler/internal/queue/scheduling_queue.go#L133)): exponentially postponing pods which failed
+  to be scheduled (e.g. volume still getting created) but are expected to get scheduled eventually.
+
+In addition, the scheduling queue mechanism has two periodical flushing goroutines
+running in the background responsible for moving pods to the active queue:
+- [flushUnschedulableQLeftover](https://github.com/kubernetes/kubernetes/blob/4cc1127e9251fff364d5c77e2a9a9c3ad42383ab/pkg/scheduler/internal/queue/scheduling_queue.go#L350): running every 30 seconds moving pods from unschedulable
+  queue to allow unschedulable pods that were not moved by any event
+  to be retried again. Pod has to stay for at least 30 seconds in the queue to get moved.
+  In the worst case it can take up to 60 seconds to have a pod moved.
+- [flushBackoffQCompleted](https://github.com/kubernetes/kubernetes/blob/4cc1127e9251fff364d5c77e2a9a9c3ad42383ab/pkg/scheduler/internal/queue/scheduling_queue.go#L324): running every second moving pods that were backed off
+  long enough to the active queue.
+
+Both retry periods for the goroutines are fixed and non-configurable.
+Also, in response to certain events, the scheduler
+move pods from either queue to the active queue (by invoking [MoveAllToActiveOrBackoffQueue](https://github.com/kubernetes/kubernetes/blob/4cc1127e9251fff364d5c77e2a9a9c3ad42383ab/pkg/scheduler/internal/queue/scheduling_queue.go#L493)).
+Example events include a node addition or update, an existing pod being deleted etc.
+
+![Pods moving between queues](scheduling_queues.png "Pods moving between queues")
+
+## Active queue (heap)
+
+A queue with the highest priority pod at the top by default. The ordering
+can be customized via QueueSort extension point. Newly created pods, with empty `.spec.nodeName`,
+are added to the queue as they come. In each scheduling cycle the scheduler takes
+one pod from the queue and tries to schedule it. In case the scheduling algorithm
+fails (e.g. plugins error, binding error), the pod is moved to the unschedulable queue.
+Or, moved to the backoff queue if a move request was issued at the same or newer time.
+The move request signals a move of pods from unschedulable to active, respectively backoff queue.
+If a pod is scheduled without an error, it is removed from all queues.
+
+## Backoff queue (heap)
+Queue keeping pods in a waiting state to avoid continuous retries. Queue ordering
+keeps a pod with the shortest backoff timeout at the top. The more times a pod gets
+backed off, the longer it takes for the pod to re-enter the active queue. The backoff
+timeout grows exponentially with each failed scheduling attempt until it reaches its maximum.
+Scheduler allows to configure initial backoff (set to 1 second by default) and maximum
+backoff (set to 10 seconds by default). A pod can get to the backoff queue
+when a move request (see below) is issued.
+
+As an example a pod with 3 failed attempts gets the target backoff timeout
+set to curTime + 2s^3 (8s). With 5 failed attempts the timeout gets set to curTime +2s^5 (32s).
+In case the maximum backoff is too low (e.g. the default 10s), a pod can get to the active
+queue too often. So it’s recommended to configure the maximum backoff to fit the workloads
+so the pods stay in the backoff queue long enough to avoid flooding the active queue
+with pods failing too often to be scheduled.
+
+## Unschedulable queue (map)
+Queue keeping all pods that failed to be scheduled and were not subject to a move request.
+Pods are kept in the queue until a move request is issued.
+
+## Moving request
+
+Moving request triggers an event responsible for moving pods from
+unschedulable queue to either the active or the backoff queue. Different cluster
+events can asynchronously trigger a moving request and make unschedulable
+pods (that were tried before) schedulable again. The events currently include
+changes in pods, nodes, services, PVs, PVCs, storage classes and CSI nodes.
+
+It’s possible that a pod fails to be scheduled while a moving request gets issued.
+Due to this event, the pod might now be schedulable and the following mechanism
+allows such pod to be retried. Every moving request operation stores the current
+scheduling cycle under [moveRequestCycle](https://github.com/kubernetes/kubernetes/blob/4cc1127e9251fff364d5c77e2a9a9c3ad42383ab/pkg/scheduler/internal/queue/scheduling_queue.go#L523) variable. After a pod fails scheduling,
+it is regularly put in the unschedulable queue. Unless moveRequestCycle
+is the current scheduling cycle, in which case the pod takes a shortcut
+and gets moved right under the backoff queue.
+
+**Examples**:
+- When a pod is scheduled, some pods in the unschedulable queue with matching
+  affinity can be made schedulable. If matching affinity is the only required
+  condition for scheduling, issuing a moving request for those pods will allow
+  them to get finally scheduled.
+- A pod is getting processed by filter plugins which give no nodes left for scheduling.
+  Meantime an asynchronous moving request gets issued as a reaction on a new node event.
+  Moving the pod under the backoff queue will allow the pod to be moved sooner
+  into the active queue and check if the new node is eligible for scheduling.
+
+## Metrics
+
+The scheduling queue populates two metrics:
+[pending_pods](https://github.com/kubernetes/kubernetes/blob/4cc1127e9251fff364d5c77e2a9a9c3ad42383ab/pkg/scheduler/metrics/metrics.go#L83-L89) and
+[queue_incoming_pods_total](https://github.com/kubernetes/kubernetes/blob/4cc1127e9251fff364d5c77e2a9a9c3ad42383ab/pkg/scheduler/metrics/metrics.go#L141-L147).
+All three queues count how many pods are pending in each queue and how many
+times a pod was enqueued into each queue. Including which event was responsible
+for the enqueueing. The events can include failed scheduling attempts,
+pod finishing backoff, node added, service updated, etc. The metrics allow us
+to see how many pods are present in each queue. Allowing to see how often pods
+are unschedulable, what’s the scheduler throughput, or which events are moving
+the pods from one queue to another most often.
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