PSI Metrics for Kubernetes Graduates to Beta

By Haowei Cai (Google) |

As Kubernetes clusters grow in size and complexity, understanding the health and performance of individual nodes becomes increasingly critical. We are excited to announce that as of Kubernetes v1.34, Pressure Stall Information (PSI) Metrics has graduated to Beta.

What is Pressure Stall Information (PSI)?

Pressure Stall Information (PSI) is a feature of the Linux kernel (version 4.20 and later) that provides a canonical way to quantify pressure on infrastructure resources, in terms of whether demand for a resource exceeds current supply. It moves beyond simple resource utilization metrics and instead measures the amount of time that tasks are stalled due to resource contention. This is a powerful way to identify and diagnose resource bottlenecks that can impact application performance.

PSI exposes metrics for CPU, memory, and I/O, categorized as either some or full pressure:

some
The percentage of time that at least one task is stalled on a resource. This indicates some level of resource contention.
full
The percentage of time that all non-idle tasks are stalled on a resource simultaneously. This indicates a more severe resource bottleneck.
Diagram illustrating the difference between 'some' and 'full' PSI pressure.

PSI: 'Some' vs. 'Full' Pressure

These metrics are aggregated over 10-second, 1-minute, and 5-minute rolling windows, providing a comprehensive view of resource pressure over time.

PSI metrics in Kubernetes

With the KubeletPSI feature gate enabled, the kubelet can now collect PSI metrics from the Linux kernel and expose them through two channels: the Summary API and the /metrics/cadvisor Prometheus endpoint. This allows you to monitor and alert on resource pressure at the node, pod, and container level.

The following new metrics are available in Prometheus exposition format via /metrics/cadvisor:

  • container_pressure_cpu_stalled_seconds_total
  • container_pressure_cpu_waiting_seconds_total
  • container_pressure_memory_stalled_seconds_total
  • container_pressure_memory_waiting_seconds_total
  • container_pressure_io_stalled_seconds_total
  • container_pressure_io_waiting_seconds_total

These metrics, along with the data from the Summary API, provide a granular view of resource pressure, enabling you to pinpoint the source of performance issues and take corrective action. For example, you can use these metrics to:

  • Identify memory leaks: A steadily increasing some pressure for memory can indicate a memory leak in an application.
  • Optimize resource requests and limits: By understanding the resource pressure of your workloads, you can more accurately tune their resource requests and limits.
  • Autoscale workloads: You can use PSI metrics to trigger autoscaling events, ensuring that your workloads have the resources they need to perform optimally.

How to enable PSI metrics

To enable PSI metrics in your Kubernetes cluster, you need to:

  1. Ensure your nodes are running a Linux kernel version 4.20 or later and are using cgroup v2.
  2. Enable the KubeletPSI feature gate on the kubelet.

Once enabled, you can start scraping the /metrics/cadvisor endpoint with your Prometheus-compatible monitoring solution or query the Summary API to collect and visualize the new PSI metrics. Note that PSI is a Linux-kernel feature, so these metrics are not available on Windows nodes. Your cluster can contain a mix of Linux and Windows nodes, and on the Windows nodes the kubelet does not expose PSI metrics.

What's next?

We are excited to bring PSI metrics to the Kubernetes community and look forward to your feedback. As a beta feature, we are actively working on improving and extending this functionality towards a stable GA release. We encourage you to try it out and share your experiences with us.

To learn more about PSI metrics, check out the official Kubernetes documentation. You can also get involved in the conversation on the #sig-node Slack channel.