Limiting Container Resources

It’s often useful to limit the resources that are consumed by a container, e.g. to allow the container to only use 1 CPU, or 50% of the RAM on the system. Although on HPC clusters it’s common to launch containers with a job scheduler that can set limits per job, the following scenarios are examples where more direct control is useful:

  • When running multiple containerized applications inside an HPC job, where each container in the job should have different resource limits.

  • When testing HPC code on a workstation, to avoid excessive CPU / RAM usage bringing the desktop environment and other applications to a halt.

  • When benchmarking code that doesn’t provide internal means to limit the number of CPUs it uses.

There are three ways to apply limits to a container that is run with Apptainer:

  • Using per-resource command line limit flags described below.

  • Using the --apply-cgroups flag to apply a cgroups.toml file that defines the resource limits.

  • Using external tools such as systemd-run tool to apply limits, and then call apptainer.

Requirements - Linux Cgroups

Resource limits are applied to containers using functionality in the Linux kernel known as control groups or cgroups. There are two versions of cgroups:

cgroups v1 has a more complex structure, and allows only the root user to safely apply limits to applications. If your system is using cgroups v1 then you can only use the CLI resource limit flags or --apply-cgroups when running containers as the root user.

cgroups v2 has a simplified structure, and is designed in a way that permits delegation of cgroups control to standard users. This delegation is usually accomplished via systemd.

Generally, to apply resource limits to a container as a non-root user your system must:

  • Be using cgroups v2, in the unified hierarchy mode.

  • Have a Linux kernel version >= 4.15.

  • Have systemd version >= 224.

  • Have systemd cgroups enabled in apptainer.conf (this is the default).

  • Have systemd configured to delegate cgroups controllers to non-root users.

Recent distributions such as Ubuntu 22.04, Debian 11, Fedora 31, and newer, satisfy these criteria by default. On older distributions support can often be enabled. Consult the admin documentation or speak to your system administrator about this.

Command Line Limit Flags

Apptainer has a number of simple command line flags that you can use with shell/run/exec and the instance commands to directly apply resource limits to a container when you run it.

The flags detailed below are compatible with those used by the docker CLI, except that the short forms are not supported. For example, you cannot use -c instead of --cpu-shares because -c is used by Apptainer for another purpose.

Not all limits provided by other runtimes are currently supported by Apptainer. Specifically, the --device- flags supported by the docker CLI are not yet available.

CPU Limits

--cpus sets the number of CPUs, or fractional CPUs, that the container can use. The minimum is 0.01 or one tenth of a physical CPU. The maximum is the number of CPU cores on your system.

# Limit container to 3.5 CPUs
$ apptainer run --cpus 3.5 myfirstapp.sif

--cpu-shares sets a relative weight for a container’s access to the system’s CPUs, versus other containers that also have a --cpus-shares value set. If container A has 1024 cpu shares, and container B has 512 cpu shares, then container A will receive twice as much CPU time than container B, but only when there is contention for CPUs, i.e. the containers are able to consume more CPU time than is available.

# Container A - twice as much CPU priority as container B
$ apptainer run --cpu-shares 1024 myfirstapp.sif

# Container A - half as much CPU priority as container A
$ apptainer run --cpu-shares 512 mysecondapp.sif

--cpu-set-cpus specifies a list of physical CPU IDs on which a container can run. For example, on a dual CPU system you might pin one container to the first 12 cores on CPU 1, and a second container to the second 12 cores on CPU 2.

--cpu-set-mems specifies a list of memory nodes the container can use. It should generally be set to the same value as --cpu-set-cpus.

# Container A - first CPU
$ apptainer run --cpu-set-cpus 0-11 --cpu-set-mems 0-11 myfirstapp.sif

# Container B - second CPU
$ apptainer run --cpu-set-cpus 12-23 --cpu-set-mems 12-23 myfirstapp.sif

Memory Limits

--memory sets the maximum amount of RAM that a container can use, in bytes. You can use suffixes such as M or G to specify megabytes or gigabytes. If the container tries to use more memory than its limit, the system will kill it.

# Run a program that will use 10GB of RAM, with a 100MB limit
$ apptainer exec --memory 100M memhog.sif memhog 10G
.................................................Killed

--memory-reservation sets a soft limit, which should be lower than the hard limit set with --memory. When there is contention for memory, the system will attempt to make sure the container doesn’t exceed the soft limit.

# Kill my program if it exceeds 10G, but aim for 8G if there is contention
$ apptainer exec --memory 10G --memory-reservation 8G myfirstapp.sif

--memory-swap sets the total amount of memory and swap space that a container can use. You must set --memory along with --memory-swap. A value of -1 means unlimited swap. If --memory-swap is not set or is 0, then the container can use an amount of swap up to the value of --memory. It’s easier to understand this flag with examples:

# Run a container that can use up to 1G RAM, or swap if it is swapped out
$ apptainer run --memory 1G myfirstapp.sif

# Run a container that can use up to 1G RAM, and no swap space
$ apptainer run --memory 1G --memory-swap 1G myfirstapp.sif

# Run a container that can use up to 1G RAM, and unlimited swap space
$ apptainer run --memory 1G --memory-swap -1 myfirstapp.sif

# Run a container that can use up to 1G RAM, and 1G swap space
$ apptainer run --memory 1G --memory-swap 2G myfirstapp.sif

IO Limits

Note

Requires the cfq or bfq IO scheduler to be configured for block IO on the system. This is common on modern distributions, but not universal. Ask your system administrator if IO limits are not working as expected.

--blkio-weight sets a relative weight for the container when performing block I/O, e.g. reading/writing to/from disk. The weight should be between 10 and 1000, and will control how much I/O access a container recieves when there is contention for I/O with other containers. It may be useful to give high priority to a container that needs infrequent but time sensitive data access, running alongside an application that is continuously performing bulk reads.

# Container A - ten times as much block IO priority as container B
$ apptainer run --blkio-weight 1000 myfirstapp.sif

# Container A - ten times less block IO priority as container A
$ apptainer run --blkio-weight 100 mysecondapp.sif

--blkio-weight-device sets a relative weight for the container when performing block I/O on a specific device. Specify the device and weight as <device path>:weight:

# Container A - ten times as much block IO priority as container B on disk /dev/sda
$ apptainer run --blkio-weight-device /dev/sda:1000 myfirstapp.sif

# Container A - ten times less block IO priority as container A on disk /dev/sda
$ apptainer run --blkio-weight-device /dev/sda:100 mysecondapp.sif

Applying Resource Limits From a TOML file

Apptainer can directly apply resource limitations to systems configured for both cgroups v1 and the v2 unified hierarchy, using the --apply-cgroups flag. Resource limits are specified using a TOML file that represents the resources section of the OCI runtime-spec: https://github.com/opencontainers/runtime-spec/blob/master/config-linux.md#control-groups

On a cgroups v1 system the resources configuration is applied directly. On a cgroups v2 system the configuration is translated and applied to the unified hierarchy.

Under cgroups v1, access restrictions for device nodes are managed directly. Under cgroups v2, the restrictions are applied by attaching eBPF programs that implement the requested access controls.

To apply resource limits to a container, using the --apply-cgroups flag, which takes a path to a TOML file specifying the cgroups configuration to be applied:

$ apptainer shell --apply-cgroups /path/to/cgroups.toml my_container.sif

Note

Using --apply-cgroups as a non-root user requires a cgroups v2 system, configured to use the systemd cgroups manager in apptainer.conf.

CPU Limits

CPU usage can be limited using different strategies, with limits specified in the [cpu] section of the TOML file.

shares

This corresponds to a ratio versus other cgroups with cpu shares. Usually the default value is 1024. That means if you want to allow to use 50% of a single CPU, you will set 512 as value.

[cpu]
    shares = 512

A cgroup can get more than its share of CPU if there are enough idle CPU cycles available in the system, due to the work conserving nature of the scheduler, so a contained process can consume all CPU cycles even with a ratio of 50%. The ratio is only applied when two or more processes conflicts with their needs of CPU cycles.

quota/period

You can enforce hard limits on the CPU cycles a cgroup can consume, so contained processes can’t use more than the amount of CPU time set for the cgroup. quota allows you to configure the amount of CPU time that a cgroup can use per period. The default is 100ms (100000us). So if you want to limit amount of CPU time to 20ms during period of 100ms:

[cpu]
    period = 100000
    quota = 20000

cpus/mems

You can also restrict access to specific CPUs (cores) and associated memory nodes by using cpus/mems fields:

[cpu]
    cpus = "0-1"
    mems = "0-1"

Where the container has limited access to CPU 0 and CPU 1.

Note

It’s important to set identical values for both cpus and mems.

Memory Limits

To limit the amount of memory that your container uses to 500MB (524288000 bytes), set a limit value inside the [memory] section of your cgroups TOML file:

[memory]
    limit = 524288000

Start your container, applying the toml file, e.g.:

$ apptainer run --apply-cgroups path/to/cgroups.toml library://alpine

After that, you can verify that the container is only using 500MB of memory. This example assumes that there is only one running container. If you are running multiple containers you will find multiple cgroups trees under the apptainer directory.

# cgroups v1
$ cat /sys/fs/cgroup/memory/apptainer/*/memory.limit_in_bytes
  524288000

# cgroups v2 - note translation of memory.limit_in_bytes -> memory.max
$ cat /sys/fs/cgroup/apptainer/*/memory.max
524288000

IO Limits

To control block device I/O, applying limits to competing container, use the [blockIO] section of the TOML file:

[blockIO]
    weight = 1000
    leafWeight = 1000

weight and leafWeight accept values between 10 and 1000.

weight is the default weight of the group on all the devices until and unless overridden by a per device rule.

leafWeight relates to weight for the purpose of deciding how heavily to weigh tasks in the given cgroup while competing with the cgroup’s child cgroups.

To apply limits to specific block devices, you must set configuration for specific device major/minor numbers. For example, to override weight/leafWeight for /dev/loop0 and /dev/loop1 block devices, set limits for device major 7, minor 0 and 1:

[blockIO]
    [[blockIO.weightDevice]]
        major = 7
        minor = 0
        weight = 100
        leafWeight = 50
    [[blockIO.weightDevice]]
        major = 7
        minor = 1
        weight = 100
        leafWeight = 50

You can also limit the IO read/write rate to a specific absolute value, e.g. 16MB per second for the /dev/loop0 block device. The rate is specified in bytes per second.

[blockIO]
    [[blockIO.throttleReadBpsDevice]]
        major = 7
        minor = 0
        rate = 16777216
    [[blockIO.throttleWriteBpsDevice]]
        major = 7
        minor = 0
        rate = 16777216

Device Limits

Note

Device limits can only be applied when running as the root user, and will be ignored as a non-root user.

You can limit read (r), write (w), or creation (c) of devices by a container. Like applying I/O limits to devices, you must use device node major and minor numbers to create rules for specific devices or classes of device.

In this example, a container is configured to only be able to read from or write to /dev/null:

[[devices]]
    access = "rwm"
    allow = false
[[devices]]
    access = "rw"
    allow = true
    major = 1
    minor = 3
    type = "c"

Other limits

Apptainer can apply all resource limits that are valid in the OCI runtime-spec resources section. If you use cgroups v1 limits on a cgroups v2 system they will be translated at runtime. You may also specify native cgroups v2 limits under the unified key.

See the runtime spec for information about the available limits. Note that Apptainer uses TOML format for the configuration file, rather than JSON.

Applying Resource Limits With External Tools

Because Apptainer starts a container as a simple process, rather than using a daemon, you can limit resource usage by running the apptainer command inside an existing cgroup. This is convenient where, for example, a job scheduler uses cgroups to control job limits. By running apptainer inside your batch script, your container will respect the limits set by the scheduler on the job’s cgroup.

systemd-run

As well as schedulers you can use tools such as systemd-run to create a cgroup, and run Apptainer inside of it. This is convenient on modern cgroups v2 systems, where the creation of cgroups can be delegated to users through systemd. Without this delegation root privileges are required to create a cgroup.

For example, assuming your system is configured correctly for unprivileged cgroup creation via systemd, you can limit the number of CPUs a container run is allowed to use:

$ systemd-run --user --scope -p AllowedCPUs=1,2 -- apptainer run mycontainer.sif
  • --user instructs systemd that we want to run as our own user account.

  • --scope will run our command in an interactive scope that inherits from our environment. By default the command would run as a service, in the background.

  • -p AllowedCPUs=1,2 sets a property on our scope, so that in this case systemd will then setup a cgroup limiting our command to using CPU 1 and 2 only.

  • The double hyphen -- separates options for systemd-run from the actual command we wish to execute. This is important so that systemd-run doesn’t capture any flags we might need to pass to apptainer.

You can read more about how systemd can control resources uses at the link below, which details the properties you can set using systemd-run.

https://www.freedesktop.org/software/systemd/man/systemd.resource-control.html