Kubernetes Resource Requests vs. Limits

In Kubernetes, requests and limits are configurations that help manage the amount of CPU and memory resources allocated to containers. These settings define how Kubernetes schedules and manages containers within a cluster.

Resource Requests

A resource request specifies the minimum amount of CPU or memory that a container needs in order to run. It defines the guaranteed resources for that container.

• CPU Request:

  • The amount of CPU that Kubernetes guarantees to a container. Kubernetes will ensure that this amount of CPU is allocated to the container when it starts.
  • It is measured in CPU cores. For instance, a request of 0.5 means the container is guaranteed half a CPU core, while a request of 1 means it is guaranteed a full CPU core.

• Memory Request:

  • The amount of memory Kubernetes guarantees to the container.
  • It is measured in units like MB or GB. For example, a request of 512Mi (512 MiB) guarantees that the container will receive at least that amount of memory.

Purpose of Requests:

• Scheduling: When Kubernetes schedules a pod, it uses the resource requests to determine which node should run the pod. The scheduler checks the available resources on each node and ensures that the node has enough resources to satisfy the request.

• Guaranteed Resources: The container will always have access to at least the amount of resources defined by the request, even under heavy load conditions.

Resource Limits

A resource limit specifies the maximum amount of CPU or memory that a container can use. If the container exceeds the defined limit, Kubernetes will take action based on the type of resource:

• CPU Limit:

  • The maximum amount of CPU a container can use. If the container tries to use more than the allocated CPU limit, it will be throttled (i.e., it won't be allowed to consume more CPU resources).
  • A container can never exceed its CPU limit, but it can use less CPU if not needed.

• Memory Limit:

  • The maximum amount of memory the container can use. If the container exceeds the memory limit, Kubernetes will terminate the container and potentially restart it, depending on the pod's restart policy.
  • Containers that exceed the memory limit are considered "out of memory" (OOM) and are killed by the system. This is an important behavior because it prevents any single container from consuming excessive memory, which could impact other containers on the same node.

Purpose of Limits

• Prevent Resource Hogging: By setting limits, you can prevent a container from using more resources than it should, which could potentially starve other containers on the same node.

• Pod Stability: Limits on memory prevent runaway processes from consuming all available memory, which would cause the node to run out of memory and potentially kill other critical processes.

Key Differences Between Requests and Limits

Aspect	Request	Limit
Definition	Minimum resources guaranteed for a pod	Maximum resources a pod can consume
CPU Behavior	Guarantees CPU allocation for the container	Throttles CPU usage once the limit is reached
Memory Behavior	Guarantees memory allocation for the container	Kills the container if memory usage exceeds the limit
Scheduler Influence	Affects pod scheduling and placement on nodes	No direct impact on pod scheduling; only affects container behavior
Best Practice	Set requests based on the container's typical resource usage	Set limits to prevent containers from consuming too many resources

How Requests and Limits Interact

• Without Limits: If you specify only the requests, Kubernetes will ensure the container gets the requested amount of resources, but there's no upper bound on how much it can consume. This can lead to inefficient resource utilization if a container unexpectedly uses more resources than anticipated.

• Without Requests: If you only specify limits, the container may be scheduled on a node without guarantees, leading to potentially less stable performance since the scheduler cannot factor in the minimum resource needs.

• With Both Requests and Limits: The ideal configuration is to specify both requests and limits. Requests ensure that the container has enough resources to run efficiently, while limits ensure that it doesn't consume more resources than necessary, preventing it from impacting other containers or the node itself.

Example of Resource Requests and Limits

apiVersion: v1
kind: Pod
metadata:
  name: web-pod
spec:
  containers:
    - name: web
      image: nginx
      resources:
        requests:
          memory: "256Mi"
          cpu: "500m"
        limits:
          memory: "512Mi"
          cpu: "1"

In this example:

• The container is guaranteed 256 MiB of memory and 500m (0.5 CPU cores).
• The container is allowed to use up to 512 MiB of memory and 1 CPU core.

Best Practices for Setting Requests and Limits

• Monitor and Adjust: Always monitor the resource usage of your containers using tools like Prometheus, and adjust the requests and limits based on actual performance.

• Set reasonable limits: Setting a very high limit might be a sign of improper resource management, and setting a very low limit might cause containers to be killed frequently. Balance the resources based on real-world usage.

• Requests vs Limits for CPU: CPU is not as critical as memory in terms of container stability, since containers can be throttled if they exceed their CPU limit. However, requests still play a vital role in determining how the scheduler places the container.

• Requests vs Limits for Memory: Memory limits should be set carefully to avoid the possibility of OOM (Out of Memory) kills, which could lead to pod failures and application downtime.

Conclusion

Understanding resource requests and limits is fundamental to managing workloads in a Kubernetes cluster. Requests ensure that your applications receive the resources they need, while limits protect the cluster from resource contention issues. The key is finding the right balance for each application by monitoring usage and adjusting the settings over time.