Memory limits and reservations in Docker - Step-by-Step Execution

Process Flow - Memory limits and reservations

Start Container

↓

Check Memory Reservation

↓

Reserve Memory for Container

↓

Check Memory Usage

↓

If Usage < Limit

↓

If Usage > Limit

↓

Container Stops or Continues

Docker starts a container, reserves specified memory, monitors usage, and enforces limits by killing the container if it exceeds the limit.

Execution Sample

Docker

docker run --memory=500m --memory-reservation=200m alpine sleep 60

Runs an Alpine container with 500MB max memory and 200MB reserved memory, then sleeps for 60 seconds.

Process Table

Step	Action	Memory Reserved	Memory Limit	Memory Usage	Result
1	Start container	200MB	500MB	0MB	Container starts with reserved memory set
2	Container runs workload	200MB	500MB	150MB	Usage below reservation, normal operation
3	Container runs heavier workload	200MB	500MB	450MB	Usage below limit, container continues
4	Container tries to exceed limit	200MB	500MB	520MB	Usage exceeds limit, container killed
5	Container is killed	200MB	500MB	N/A	Container stopped

💡 Container is killed when memory usage exceeds the 500MB limit

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Memory Reserved	200MB	200MB	200MB	200MB	200MB
Memory Limit	500MB	500MB	500MB	500MB	500MB
Memory Usage	0MB	150MB	450MB	520MB	N/A

Key Moments - 3 Insights

Why does the container continue running when memory usage is above the reservation but below the limit?

Because memory reservation is a soft guarantee, the container can use more memory up to the limit without being stopped, as shown in step 3 of the execution table.

What happens when memory usage exceeds the memory limit?

The container is killed to prevent it from using more memory than allowed, as shown in step 4 of the execution table.

Is the reserved memory immediately allocated to the container at start?

No, reserved memory is a soft guarantee, not a hard allocation. The container can start with less usage and grow up to the reservation without issues, as seen in step 1 and 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the memory usage at step 3?

A 150MB

B 520MB

C 450MB

D 200MB

💡 Hint

Check the 'Memory Usage' column in row for step 3 in the execution table.

At which step does the container exceed its memory limit?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look for when 'Memory Usage' surpasses 'Memory Limit' in the execution table.

If the memory reservation was increased to 300MB, how would step 2's memory reserved value change?

A It would change to 300MB

B It would stay 200MB

C It would change to 500MB

D It would become 0MB

💡 Hint

Refer to the 'Memory Reserved' column in the variable tracker and relate it to reservation settings.

Concept Snapshot

Docker memory limits and reservations:
- --memory sets the max memory a container can use (hard limit).
- --memory-reservation sets a soft memory guarantee.
- Container can use memory between reservation and limit but risks killing if exceeding limit.
- Reservation helps Docker manage resources without immediate hard allocation.
- Exceeding limit triggers container termination.

Full Transcript

This visual execution shows how Docker manages memory for containers using memory limits and reservations. When a container starts, Docker reserves a soft memory amount and sets a hard memory limit. The container can use memory up to the reservation without issues. If usage grows beyond reservation but stays below the limit, the container continues running normally. If memory usage exceeds the limit, Docker kills the container to protect the host system. The execution table tracks memory reserved, limit, and usage step-by-step, showing container behavior at each stage. Key moments clarify common confusions about soft reservation versus hard limits. The quiz tests understanding of memory usage at different steps and the effect of changing reservation values.