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.

Practice

1. What does setting a memory limit for a Docker container do?

easy

A. It sets the maximum memory the container can use.

B. It sets the minimum guaranteed memory for the container.

C. It disables memory usage for the container.

D. It sets the CPU usage limit for the container.

Solution

1. Step 1: Understand memory limit concept

   A memory limit restricts the maximum RAM a container can consume to avoid overuse.

2. Step 2: Compare with other options

   Memory reservation is the soft minimum, CPU limit is unrelated, disabling memory is invalid.

3. Final Answer:

   It sets the maximum memory the container can use. -> Option A

4. Quick Check:

   Memory limit = max memory allowed [OK]

Hint: Memory limit caps max RAM usage, reservation sets minimum [OK]

Common Mistakes:

• Confusing memory limit with memory reservation
• Thinking memory limit affects CPU
• Assuming memory limit disables memory

2. Which of the following is the correct Docker run command syntax to set a memory limit of 500MB?

easy

A. docker run --mem=500MB myimage

B. docker run --memory=500m myimage

C. docker run --memory-limit=500m myimage

D. docker run --mem-limit=500MB myimage

Solution

1. Step 1: Recall Docker memory limit flag

   The correct flag is --memory followed by the size with unit, e.g., 500m for 500MB.

2. Step 2: Check syntax correctness

   Options A, B, and C use incorrect flag names or units.

3. Final Answer:

   docker run --memory=500m myimage -> Option B

4. Quick Check:

   Use --memory=500m for 500MB limit [OK]

Hint: Use --memory=500m to set 500MB limit correctly [OK]

Common Mistakes:

• Using wrong flag names like --mem-limit
• Using uppercase units like MB instead of m
• Omitting the equal sign

3. Given the command docker run --memory=300m --memory-reservation=200m myapp, what happens if the container tries to use 350MB of memory?

medium

A. The container is killed because it exceeded the 300MB limit.

B. The container is allowed to use 350MB without restrictions.

C. The container is throttled but not killed immediately.

D. The container automatically increases its memory reservation.

Solution

1. Step 1: Understand memory limit vs reservation

   Memory limit (300MB) is the hard max; reservation (200MB) is soft minimum.

2. Step 2: Analyze container memory usage

   Using 350MB exceeds the 300MB limit, so Docker kills the container.

3. Final Answer:

   The container is killed because it exceeded the 300MB limit. -> Option A

4. Quick Check:

   Memory limit exceeded = container killed [OK]

Hint: Memory limit is max; exceeding it kills container [OK]

Common Mistakes:

• Confusing reservation with limit
• Thinking container is throttled instead of killed
• Assuming reservation auto-increases

4. You set --memory-reservation=400m but your container keeps getting killed when using 350MB. What is the likely cause?

medium

A. The container is using less memory than reservation, so it is killed.

B. Memory reservation kills container if usage is below it.

C. Docker does not support memory reservation on your system.

D. You forgot to set a memory limit, so default limit is too low.

Solution

1. Step 1: Understand memory reservation role

   Memory reservation is a soft minimum, it does not kill containers.

2. Step 2: Identify missing memory limit effect

   Without a memory limit, Docker may apply a default low limit causing kills.

3. Final Answer:

   You forgot to set a memory limit, so default limit is too low. -> Option D

4. Quick Check:

   Missing memory limit causes unexpected kills [OK]

Hint: Always set memory limit; reservation alone won't prevent kills [OK]

Common Mistakes:

• Thinking reservation kills containers
• Ignoring default memory limits
• Assuming reservation enforces max memory

5. You want to run a container that should always have at least 1GB of memory available but never use more than 2GB. Which Docker run options correctly enforce this?

hard

A. docker run --memory=1g --memory-reservation=2g mycontainer

B. docker run --memory-reservation=1g --memory=2g mycontainer

C. docker run --memory=2g --memory-reservation=1g mycontainer

D. docker run --memory-reservation=2g --memory=1g mycontainer

Solution

1. Step 1: Identify memory reservation and limit roles

   Memory reservation sets soft minimum (1GB), memory limit sets hard max (2GB).

2. Step 2: Match options to roles

   docker run --memory=2g --memory-reservation=1g mycontainer correctly sets --memory=2g (max) and --memory-reservation=1g (min).

3. Final Answer:

   docker run --memory=2g --memory-reservation=1g mycontainer -> Option C

4. Quick Check:

   Limit=2g, Reservation=1g for min and max [OK]

Hint: Memory limit is max, reservation is min; order matters [OK]

Common Mistakes:

• Swapping limit and reservation values
• Setting reservation higher than limit
• Confusing units or flags