CPU requests and limits in Kubernetes - Step-by-Step Execution

Process Flow - CPU requests and limits

Pod Spec Created

↓

CPU Requests Set?

No→Default Scheduler Behavior

Yes↓

Scheduler Checks Node Capacity

↓

Pod Scheduled on Node

↓

Container Starts Running

↓

CPU Usage Monitored

↓

CPU < Limit

↓

Runs Normally

The pod spec sets CPU requests and limits. Scheduler uses requests to place pods. Runtime enforces limits by throttling CPU if usage exceeds limit.

Execution Sample

Kubernetes

apiVersion: v1
kind: Pod
metadata:
  name: cpu-demo
spec:
  containers:
  - name: app
    resources:
      requests:
        cpu: "500m"
      limits:
        cpu: "1"

Defines a pod with CPU request of 0.5 CPU and limit of 1 CPU.

Process Table

Step	Action	CPU Request	CPU Limit	Scheduler Decision	Runtime Behavior
1	Pod spec created with CPU request 500m and limit 1	500m	1	Pending scheduling	Not started
2	Scheduler checks nodes for available CPU >= 500m	500m	1	Pod scheduled on node with enough CPU	Not started
3	Container starts running	500m	1	Scheduled	Container can use up to 1 CPU
4	Container CPU usage at 300m	500m	1	Scheduled	Runs normally, no throttling
5	Container CPU usage at 800m	500m	1	Scheduled	Runs normally, no throttling
6	Container CPU usage at 1200m	500m	1	Scheduled	CPU throttled to 1 CPU limit
7	Container CPU usage drops to 400m	500m	1	Scheduled	Runs normally, no throttling
8	Pod deleted or stopped	0	0	No scheduling	Container stopped

💡 Pod stops running or is deleted, ending CPU usage monitoring.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 6	After Step 8
CPU Request	None	500m	500m	500m	0
CPU Limit	None	1	1	1	0
CPU Usage	0	0	0	1200m	0
Pod State	Pending	Pending	Running	Running	Stopped
CPU Throttled	No	No	No	Yes	No

Key Moments - 3 Insights

Why does the scheduler use CPU requests and not limits to place pods?

The scheduler uses CPU requests as guaranteed minimum resources to ensure the node can handle the pod. Limits are enforced at runtime and do not affect scheduling decisions. See execution_table step 2.

What happens if the container tries to use more CPU than its limit?

The container is throttled by the runtime to not exceed the CPU limit, which means it cannot use more CPU than allowed. See execution_table step 6 where usage exceeds limit and throttling occurs.

Can a container use less CPU than its request?

Yes, the request is a minimum reserved amount, but the container can use less CPU if workload is low. See execution_table step 4 and 7 where usage is below request and runs normally.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the container start getting CPU throttled?

A Step 6

B Step 4

C Step 2

D Step 8

💡 Hint

Check the 'Runtime Behavior' column for when throttling starts.

According to the variable tracker, what is the CPU request value after step 3?

A 0

B 500m

C None

D 1

💡 Hint

Look at the 'CPU Request' row under 'After Step 3' column.

If the CPU limit was removed from the pod spec, how would the runtime behavior change at high CPU usage?

A Container would be throttled at 500m CPU

B Scheduler would fail to schedule the pod

C Container would never be throttled

D Pod would be deleted automatically

💡 Hint

Consider what happens when no CPU limit is set in the execution_table runtime behavior.

Concept Snapshot

CPU requests reserve minimum CPU for scheduling.
CPU limits cap max CPU usage at runtime.
Scheduler uses requests to place pods on nodes.
Runtime throttles CPU if usage exceeds limits.
Requests ensure guaranteed CPU, limits control max usage.

Full Transcript

In Kubernetes, CPU requests and limits control how pods use CPU resources. The CPU request is the minimum CPU the pod needs and is used by the scheduler to decide which node can run the pod. The CPU limit is the maximum CPU the pod can use at runtime. If the pod tries to use more CPU than its limit, the system throttles it to prevent overuse. The execution table shows a pod with 500m CPU request and 1 CPU limit. The scheduler places the pod on a node with enough CPU. The container runs normally when CPU usage is below the limit. When usage exceeds 1 CPU, throttling happens. Variables like CPU request, limit, usage, pod state, and throttling status change step by step. Key moments clarify why requests affect scheduling and limits affect runtime behavior. The quiz tests understanding of when throttling starts, CPU request values, and effects of removing limits.