Know About Kubernetes Advanced Scheduling Techniques

Orchestrators like Kubernetes have abstracted servers away, and now you can manage your whole infrastructure in a multi-tenant, heterogeneous Kubernetes cluster. You package your application in containers, and then Kubernetes takes care of availability, scaling, monitoring, and more across various nodes featuring specialized hardware or logical isolation.

Kubernetes has a lot of options and flexibility depending on what you need from it. While the basic scheduling capabilities of Kubernetes are robust, advanced scheduling techniques can further optimize resource utilization and improve application performance. In this post, will explore some advanced scheduling techniques that can unlock the full potential of Kubernetes.

Advanced Scheduling Techniques

Node Affinity and Anti-Affinity: Node affinity and anti-affinity rules allow you to specify constraints on which nodes your pods can be scheduled. With node affinity, you can schedule pods onto specific nodes based on node labels or other node attributes. This is useful for scenarios where you want to deploy pods on nodes with specific hardware capabilities or geographical locations. On the other hand, node anti-affinity ensures that pods are not scheduled on the same node, providing improved fault tolerance and high availability.

Specification: nodeAffinity field in Pod spec.

Properties:

• requiredDuringSchedulingIgnoredDuringExecution: Specifies the rules that must be satisfied for pod scheduling.
• preferredDuringSchedulingIgnoredDuringExecution: Specifies the preferences for pod scheduling.
• Rules can be based on node labels, node selectors, or node affinity expressions.

Pod Affinity and Anti-Affinity: Similar to node affinity, pod affinity and anti-affinity rules enable you to define constraints on how pods are scheduled relative to other pods. With pod affinity, you can specify rules to ensure that pods are scheduled onto the same node or spread across different nodes based on labels or other pod attributes. This is beneficial when you have pods that need to co-locate on the same node for better performance or when you want to distribute pods across nodes for load balancing purposes.

Pod Affinity:

Specification: affinity field in Pod spec.

Properties:

• podAffinity: Specifies the rules for pod scheduling based on the presence of other pods.
• preferredDuringSchedulingIgnoredDuringExecution: Specifies the preferences for pod scheduling.
• Rules can be based on pod labels, pod selectors, or pod affinity expressions.

Pod Anti-Affinity:

Specification: affinity field in Pod spec.

Properties:

• podAntiAffinity: Specifies the rules for pod scheduling to avoid proximity with other pods.
• preferredDuringSchedulingIgnoredDuringExecution: Specifies the preferences for pod scheduling.
• Rules can be based on pod labels, pod selectors, or pod affinity expressions.

Resource Limits and Requests: Kubernetes allows you to set resource limits and requests for individual containers within a pod. Resource limits define the maximum amount of CPU and memory a container can use, while resource requests specify the minimum required resources for a container. By properly setting these values, Kubernetes can make intelligent scheduling decisions based on resource availability, ensuring that pods are scheduled on nodes with sufficient resources to meet their requirements.

Specification: resources field in Container spec.

Properties:

• limits: Specifies the maximum amount of CPU and memory that a container can use.
• requests: Specifies the minimum required amount of CPU and memory for a container.
• Limits and requests can be set for CPU (cpu) and memory (memory) resources.

Quality of Service (QoS) Classes: Kubernetes introduces three QoS classes for pods: BestEffort, Burstable, and Guaranteed. These classes provide a way to prioritize pods during scheduling and eviction. Pods with Guaranteed QoS have resource requests and limits set, ensuring they receive the allocated resources. Burstable QoS pods have resource requests set but can also use additional resources if available. BestEffort QoS pods have no resource requests or limits and are scheduled only when resources are available after serving higher priority pods.

Specification: resources field in Container spec.

Properties:

• requests.cpu, requests.memory: Set resource requests to define the Guaranteed QoS class.
• limits.cpu, limits.memory: Set resource limits to define the Burstable QoS class.
• No requests or limits specified define the BestEffort QoS class.

Custom Schedulers: Kubernetes allows you to develop custom schedulers that can implement sophisticated scheduling algorithms tailored to your specific needs. Custom schedulers can leverage application-specific metrics, policies, and external data sources to make intelligent scheduling decisions. This opens up possibilities for advanced scheduling techniques such as bin packing, inter-pod communication awareness, or even machine learning-based schedulers.

Specification: Custom scheduler implementation using the Kubernetes Scheduler framework.

Properties:

• Implement custom scheduling logic using a combination of filters, priority functions, and scoring algorithms.
• Interact with the Kubernetes API server to receive scheduling requests and update the pod status.

Horizontal Pod Autoscaling (HPA): Horizontal Pod Autoscaling is a built-in Kubernetes feature that automatically scales the number of pods based on resource utilization metrics. By configuring HPA, Kubernetes can dynamically adjust the number of replicas for a deployment, ensuring that the application can handle varying workloads efficiently. This technique optimizes resource allocation and can automatically distribute pods across nodes to accommodate increased demand.

Specification: HorizontalPodAutoscaler resource in Kubernetes.

Properties:

• targetCPUUtilizationPercentage: Specifies the target CPU utilization for autoscaling.
• minReplicas, maxReplicas: Defines the minimum and maximum number of pod replicas.
• Autoscaling can be based on CPU or custom metrics obtained from external metrics providers.

Taints and Tolerations: Node affinity is a property of Pods that attracts them to a set of nodes (either as a preference or a hard requirement). Taints are the opposite — they allow a node to repel a set of pods. Tolerations are applied to pods. Tolerations allow the scheduler to schedule pods with matching taints. Tolerations allow scheduling but don’t guarantee scheduling: the scheduler also evaluates other parameters as part of its function.  Taints and tolerations work together to ensure that pods are not scheduled onto inappropriate nodes. One or more taints are applied to a node; this marks that the node should not accept any pods that do not tolerate the taints.

Specification: tolaration field in container spec.

Properties:

• key: Set key name for the toleration.
• operator: Set the condition to match, either exists or equals.
• effect: Set the action to take, like NoExecute

These specifications explains the various fields, properties, and resources that are used to implement advanced scheduling techniques in Kubernetes. By utilizing these specifications effectively, you can optimize pod placement, resource allocation, fault tolerance, and scalability in your Kubernetes clusters.