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, specified by minCount), 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/v1beta1 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 (both already scheduled and unscheduled) is at least equal to minCount.

   The PodGroup does 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 unscheduled Pods in the group. It utilizes the PodGroup scheduling cycle to make a single, atomic scheduling decision. The GangScheduling plugin implements a PlacementFeasible extension point that is invoked for each evaluated Pod during the cycle. This is used to determine whether the minCount constraint is satisfied by comparing the number of successfully placed Pods (including those already scheduled in previous cycles) 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.

Hierarchical gang scheduling with CompositePodGroups

When the CompositePodGroup feature gate and the scheduling.k8s.io/v1alpha3 API group are enabled, gang scheduling extends its support to CompositePodGroups.

Contrary to PodGroups that group Pods, CompositePodGroups group child groups together - either PodGroups or other CompositePodGroups. CompositePodGroup specifies a scheduling policy that applies to its child groups during scheduling:

• gang policy with a minGroupCount field, specifying the minimum number of child groups (either CompositePodGroup or PodGroup objects) that must be scheduled together as a single unit atomically.
• basic policy which indicates that child groups can be scheduled independently.

gang policy is useful for multi-component workloads that require all-or-nothing scheduling across multiple child groups, ensuring that a minimum number of child groups are scheduled together. An example workload with such needs is replicated AI training.

basic policy can be used for workloads that comprise multiple groups of Pods each of which can be scheduled as an independent gang, e.g. for AI inference workloads.

Hierarchical quorum

The GangScheduling plugin holds the root CompositePodGroup from entering the active scheduling queue in the PreEnqueue phase until it satisfies the hierarchical quorum. This quorum is evaluated bottom-up, from leaf PodGroup objects up to the root CompositePodGroup:

• A leaf PodGroup satisfies quorum if and only if the PodGroup object exists and can potentially meet its scheduling policy criteria:
  • For a gang policy: at least minCount of its constituent Pods have been created.
  • For a basic policy: at least one of its constituent Pods has been created.
• A CompositePodGroup satisfies quorum if and only if the CompositePodGroup object exists and can potentially meet its scheduling policy criteria:
  • For a gang policy: at least minGroupCount of its direct child groups satisfy quorum.
  • For a basic policy: at least one of its direct child groups satisfies quorum.
• The overall hierarchical quorum is satisfied if and only if the root CompositePodGroup satisfies the quorum.

Ultimately, a root CompositePodGroup is admitted into the active scheduling queue if and only if it satisfies the hierarchical quorum and there is at least one pending Pod that belongs to one of its descendant PodGroups.

Placement feasibility

The GangScheduling plugin's PlacementFeasible method supports evaluation for both PodGroups and CompositePodGroups. It is invoked by the scheduling cycle before starting child evaluation and after evaluating each child group of a CompositePodGroup.

By taking into account the number of child groups that were successfully scheduled and the child groups that were not evaluated in the scheduling cycle just yet, PlacementFeasible determines whether the group's policy constraint is still achievable, allowing the scheduling cycle to abort the evaluation of the CompositePodGroup early if its underlying scheduling policy cannot be satisfied anymore.

What's next

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