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CPU

By default, each container’s access to the host machine’s CPU cycles is unlimited. Docker uses the default Completely Fair Scheduler (CFS) but also supports the Real-time Scheduler.

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CPU Shares

By default, Docker assigns a CPU share of 1024 to each container. CPU shares specify how the host’s total CPU resources are proportionally shared among containers — they are not hard limits. 
docker run --cpu-shares=<amount> <image>

# Example: assign 512 shares (half of the default)
docker run --cpu-shares=512 ubuntu

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CPU Sets

Limits a container to specific CPUs or cores. CPUs are numbered from 0.
  • 0-3 — Use the first four CPUs
  • 1,3 — Use the second and fourth CPUs
docker run --cpuset-cpus=<value> <image>

# Use first and second CPU
docker run --cpuset-cpus=0-1 ubuntu

# Use second and fourth CPU
docker run --cpuset-cpus=1,3 ubuntu

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CPU Count

Without a CPU limit, containers can consume all CPU resources on the host, affecting other containers and the Docker Daemon itself.
Specifies the maximum amount of available CPU a container can use. If the host has 4 CPUs and you set --cpus=2.5, the container can use at most 2.5 CPUs. 
docker run --cpus=<value> <image>

# Example: limit to 2.5 out of 4 CPUs
docker run --cpus=2.5 ubuntu

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Memory

Set the maximum amount of memory a container can use: 
docker run -m <size> <image>
docker run --memory=<size> <image>

# Example: limit to 512MB
docker run -m 512m ubuntu
docker run --memory=512m ubuntu
Supported size suffixes: m (megabytes), g (gigabytes).

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Swap Space

Sum of memory = --memory + swap spaceBy default, Docker allocates the same amount as --memory for swap, effectively doubling the total memory available.
Control swap space with --memory-swap: 
# No swap space (memory-swap equals memory)
docker run --memory=512m --memory-swap=512m ubuntu
# Swap space = 512m - 512m = 0m

# 256MB of swap space
docker run --memory=512m --memory-swap=768m ubuntu
# Swap space = 768m - 512m = 256m

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Additional Options

# Unlimited swap space
docker run --memory=512m --memory-swap=-1 ubuntu

# Soft memory reservation (activated under memory contention)
docker run --memory=512m --memory-swap=-1 --memory-reservation=100m ubuntu
--memory-reservation sets a soft limit that is activated when Docker detects low memory or contention on the host. It must be set lower than --memory and does not guarantee the container stays under the limit — it’s a best-effort reservation.

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