Why and Why Not of Enabling Swap in Kubernetes
Introduction
Kubernetes, by default, requires swap to be disabled on nodes. This design choice stems from the need for predictable performance and resource management. However, with advancements in system memory management and the introduction of cgroups v2, the Kubernetes community is re-evaluating this stance. This article explores the reasons behind the original decision, the potential benefits and drawbacks of enabling swap, and the evolving support within Kubernetes.
Understanding Swap
Swap is a memory management technique where inactive memory pages are moved from RAM to disk storage. This process frees up physical memory for active processes, allowing systems to handle more applications than the available RAM would typically permit
Why Swap Was Disabled in Kubernetes
When setting up a Kubernetes node, if swap is enabled, the kubelet will fail to start, displaying an error message:
This restriction was implemented due to several concerns:
- Quality of Service (QoS) Interference: Swap can disrupt Kubernetes’ QoS guarantees, especially for Guaranteed Pods that expect consistent performance.
- Performance Degradation: Accessing data from swap is significantly slower than RAM, leading to increased latency and potential application slowdowns.
- Resource Isolation Challenges: Swap usage can blur the lines of resource allocation, making it harder to enforce strict resource limits and isolation between Pods.
The swap support issue was raised since K8s 1.8 (2017),
Original issue:
Reconsidering Swap: The Case for Enabling
Despite the initial concerns, there are scenarios where enabling swap can be beneficial:
Enhanced Memory Utilization: Swap allows for better utilization of system memory by offloading inactive pages, making room for active processes.
Improved System Stability: In situations where memory is constrained, swap can prevent the system from terminating processes due to Out-Of-Memory (OOM) errors.
Support for Memory-Intensive Applications: Certain applications, like those running on Java or Node.js, can benefit from swap during their initialization phases, leading to more efficient memory usage over time.
Edge and Low-Memory Devices: Devices with limited RAM, such as edge computing nodes or IoT devices, can leverage swap to handle workloads more effectively.
Advancements Facilitating Swap in Kubernetes
The introduction of cgroups v2 has provided more granular control over memory and swap usage. Notably, tools like Facebook’s oomd (Out-Of-Memory Daemon) Recommend the use of swap to enhance system stability. These advancements have prompted the Kubernetes community to revisit the swap policy.
Key developments include:
- KEP-2400: Node System Swap Support: This Kubernetes Enhancement Proposal outlines a phased approach to reintroducing swap support:
- v1.22: Alpha release with basic swap support.
- v1.28: Beta1, introducing more refined controls.
- v1.30: Beta2, further stabilizing swap functionalities.
- Selective Swap Usage: The proposal suggests enabling swap primarily for Pods in the Burstable QoS class, while keeping it disabled for Guaranteed and BestEffort Pods to maintain performance and reliability.
According to the best practices from the KEP-2400, the following best practices are recommended.
- Limit Swap to Burstable Pods: This approach ensures that only Pods that can tolerate variability in performance utilize swap, preserving the integrity of critical applications.
- Monitor Swap Usage: Implement monitoring tools to track swap usage and identify potential performance bottlenecks.
- Avoid Swap for Critical Workloads: Keep swap disabled for Guaranteed Pods to ensure consistent performance and adherence to resource limits.
- Stay Updated with Kubernetes Releases: As swap support evolves, regularly update Kubernetes versions to benefit from the latest improvements and fixes.
Conclusion
While Kubernetes initially mandated the disabling of swap to maintain predictable performance and resource management, evolving system capabilities and use cases have highlighted scenarios where swap can be advantageous. By selectively enabling swap and adhering to best practices, administrators can enhance system stability and resource utilization without compromising the performance of critical workloads.
