Demo Resource Limits

In this lesson, you’ll learn how to manage memory and CPU usage in Docker containers. Proper resource constraints help ensure predictable performance and protect the host from runaway containers.

Table of Contents

• Memory Limits
  • Default Behavior
  • Hard Memory Limit
  • Unlimited Swap
  • Soft Memory Reservation
  • Memory Flags Summary
• CPU Limits
  • Limiting CPU Count
  • Pinning to Specific Cores
  • CPU Flags Summary

Memory Limits

Docker containers run without memory constraints by default. You can view and modify these settings using docker run flags.

Default Behavior

Run a container named testone without any memory restrictions:

docker container run -itd --name=testone ubuntu

Inspect its memory settings:

docker container inspect testone | grep -i mem

Output:

"Memory": 0,
"KernelMemory": 0,
"KernelMemoryTCP": 0,
"MemoryReservation": 0,
"MemorySwap": 0,
"MemorySwappiness": null

A value of 0 indicates “no limit.”

Hard Memory Limit

Set a hard cap on RAM usage with --memory. The following command limits the container to 200 MiB:

docker container run -itd --name=testtwo --memory=200m ubuntu

Verify:

docker container inspect testtwo | grep -i mem

Expected:

"Memory": 209715200,
"KernelMemory": 0,
"KernelMemoryTCP": 0,
"MemoryReservation": 0,
"MemorySwap": 0,
"MemorySwappiness": null

Unlimited Swap

By default, swap is limited to the same size as --memory. To allow unlimited swap, use --memory-swap=-1:

docker container run -itd \
  --name=testthree \
  --memory=200m \
  --memory-swap=-1 \
  ubuntu

Inspect:

docker container inspect testthree | grep -i mem

Result:

"Memory": 209715200,
"KernelMemory": 0,
"KernelMemoryTCP": 0,
"MemoryReservation": 0,
"MemorySwap": -1,
"MemorySwappiness": null

Soft Memory Reservation

With --memory-reservation, you set a soft limit that Docker tries to enforce under memory contention:

docker container run -itd \
  --name=testfour \
  --memory=200m \
  --memory-reservation=100m \
  --memory-swap=-1 \
  ubuntu

Inspect:

docker container inspect testfour | grep -i mem

Output:

"Memory": 209715200,
"KernelMemory": 0,
"KernelMemoryTCP": 0,
"MemoryReservation": 104857600,
"MemorySwap": -1,
"MemorySwappiness": null

This configuration guarantees between 100 MiB and 200 MiB of RAM, with unlimited swap.

Memory Flags Summary

CPU Limits

Docker lets you restrict CPU usage by setting the number of CPUs or pinning containers to specific cores.

Limiting CPU Count

First, check your host’s CPU count:

nproc

Assuming 2 cores, limit the container to 1 CPU:

docker container run -itd --name=testcpu --cpus=1 ubuntu

Inspect:

docker container inspect testcpu | grep -i nano

You’ll see:

"NanoCpus": 1000000000,

NanoCpus: 1000000000 represents 1 full CPU core.

Pinning to Specific Cores

Use --cpuset-cpus to bind a container to specific cores. For example, pin to core 1:

docker container run -itd --name=testcpus --cpuset-cpus="1" ubuntu

Inspect:

docker container inspect testcpus | grep -i cpuset

Output:

"CpusetCpus": "1",

This container will only run on CPU core 1.

CPU Flags Summary

Links and References

• Docker Run Reference
• Docker Resource Management
• Kubernetes Limits and Requests