Gang Scheduling

Gang scheduling ensures that a group of Pods are scheduled on an "all-or-nothing" basis. If the cluster cannot accommodate the entire group (or a defined minimum number of Pods), none of the Pods are bound to a node.

This feature depends on the PodGroup API. Ensure the GenericWorkload feature gate and the scheduling.k8s.io/v1alpha2 API group are enabled in the cluster.

How it works

When the GangScheduling plugin is enabled, the scheduler alters the lifecycle for Pods belonging to a PodGroup that has a gang scheduling policy. The process follows these steps for each PodGroup:

1. The scheduler holds Pods in the PreEnqueue phase until:

   • The referenced PodGroup object exists.
   • The number of Pods created for the PodGroup is at least equal to minCount.

   Pods do not enter the active scheduling queue until both conditions are met.

2. Once the quorum is met, the scheduler attempts to find placements for all Pods in the group. It utilizes the PodGroup scheduling cycle to make a single, atomic scheduling decision. GangScheduling plugin implements a Permit extension point that is evaluated for each schedulable Pod during the cycle. This is used to determine whether the minCount constraint is satisfied, by comparing the number of successfully placed pods against the minCount value.

3. If the scheduler finds valid placements for at least the minCount number of Pods, it allows those successfully placed Pods to be bound to their assigned nodes. If it cannot find enough placements to satisfy the minCount requirement, none of the Pods are scheduled. Instead, they are moved to the unschedulable queue to wait for cluster resources to free up, allowing other workloads to be scheduled in the meantime.

What's next

• Learn about the PodGroup API and its lifecycle.
• Read about PodGroup scheduling policies.
• Read about PodGroup scheduling.