Always Set Pod Memory/Cpu Limits and Requests
The kubernetes scheduler is responsible for scheduling pods across the cluster of worker machines. The scheduler relies on every pod having mem/cpu limits set, so that it can calculate if a node is too full to schedule more pods on, or yet has enough spare capacity to assign a particular pod. If a pod has no mem/cpu limits set, then the pod is infinitesimally small according to the scheduler and thus can fit on any worker node. In addition, that pod is allowed to consume infinite resources, allowing the risk of out of memory issues on the worker node causing instability and having the Linux OOM killer trigger and kill processes at random.
Guidlines for setting Pod Memory/Cpu Limits and Requests:
- Set memory and cpu limits and requests are set on every pod
- Set default memory and cpu limits on every namespace using
- This allows pods without limits set to inherit a default setting.
- Set it to something low such as 100m cpu and 256Mi memory
- Always set “memory request” == “memory limit”
- This prevents oversubscription of memory on a node
- Out of memory conditions on a node creates unpredictable behavior (i.e. Kernel OOM Killer)
- Setting “cpu request” != “cpu limit” is generally okay.
- This allows cpu bursting, which is the nature of CPUs.
- For Java only:
- Always set “-Xmx”
- Java defaults to 25% of the available memory on the kube machines or 4GB depending on the size of machine.
- Always set “-Xmx” and “-Xms”, ensure they are the same
- Ensure “pod memory limit” is greater than “-Xmx” + “some headroom for OS caches and JVM (384MB)”
- Always set “-Xmx”
Use Pod PriorityClasses
Priority Classes allow for more important pods to preempt (evict) lower priority pods. There are race conditions in the Kubernetes multi-replica scheduler which sometimes cause nodes to be overbooked when they are first added to the cluster. Using PriorityClasses ensures that critical pods are always scheduled.
Example PriorityClass levels in priority order:
- “kube-system” pods (e.g. Kubelet)
- Monitoring and dashboards (e.g. Prometheus)
- Data stores (e.g. postgres)