Documentation

📘 Commands

ulimits

rfswift ulimits

Manage resource limits (ulimits) for containers to optimize SDR performance.

Synopsis

rfswift ulimits add -c CONTAINER -n NAME -v VALUE
rfswift ulimits rm -c CONTAINER -n NAME
rfswift ulimits list -c CONTAINER

The ulimits command allows you to add, remove, or list resource limits on containers. Ulimits control various kernel-level resource constraints that affect process scheduling, memory locking, and file descriptors. For SDR work for example, proper ulimit configuration can eliminate buffer underruns and improve real-time performance.

Subcommands

Subcommand	Description
`add`	Add or update an ulimit on a container
`rm`	Remove an ulimit from a container
`list`	List all ulimits set on a container

Options

ulimits add

Flag	Description	Required	Example
`-c, --container STRING`	Container ID or name	✅ Yes	`-c my_container`
`-n, --name STRING`	Ulimit name	✅ Yes	`-n rtprio`
`-v, --value STRING`	Ulimit value	✅ Yes	`-v 95`

ulimits rm

Flag	Description	Required	Example
`-c, --container STRING`	Container ID or name	✅ Yes	`-c my_container`
`-n, --name STRING`	Ulimit name to remove	✅ Yes	`-n rtprio`

ulimits list

Flag	Description	Required	Example
`-c, --container STRING`	Container ID or name	✅ Yes	`-c my_container`

Common Ulimit Names

Name	Description	SDR Use Case
`rtprio`	Real-time scheduling priority (0-99)	Enables `chrt` for real-time SDR processing
`memlock`	Max locked memory in bytes (-1 = unlimited)	Prevents sample buffers from being swapped
`nice`	Nice priority range (40 allows nice -20)	Higher process priority
`nofile`	Max open file descriptors	Multiple SDR devices or large file operations
`nproc`	Max number of processes	Parallel processing pipelines

Value Format

Ulimit values can be specified in two formats:

Format	Description	Example
`value`	Sets both soft and hard limit	`95`
`soft:hard`	Sets soft and hard limits separately	`1024:65536`
`-1` or `unlimited`	Unlimited value	`-1`

Examples

Basic Usage

Add rtprio ulimit for real-time scheduling:

rfswift ulimits add -c sdr_work -n rtprio -v 95

Set unlimited memory locking:

rfswift ulimits add -c sdr_work -n memlock -v -1

Set file descriptor limits with soft/hard values:

rfswift ulimits add -c sdr_work -n nofile -v 1024:65536

List current ulimits:

rfswift ulimits list -c sdr_work

Remove an ulimit:

rfswift ulimits rm -c sdr_work -n rtprio

Real-World Scenarios

Optimize container for SDR work:

# Add all SDR-relevant ulimits
rfswift ulimits add -c hackrf_work -n rtprio -v 95
rfswift ulimits add -c hackrf_work -n memlock -v unlimited
rfswift ulimits add -c hackrf_work -n nice -v 40

# Verify configuration
rfswift ulimits list -c hackrf_work

Fix buffer underruns:

# Check current ulimits
rfswift ulimits list -c sdr_container

# Add rtprio if missing
rfswift ulimits add -c sdr_container -n rtprio -v 95

# Verify inside container
rfswift exec -c sdr_container -e "ulimit -r"
# Should output: 95

Use real-time scheduling inside container:

# After setting rtprio ulimit
rfswift exec -c sdr_container

# Inside container, run SDR tool with real-time priority
chrt -f 50 rtl_sdr -f 433920000 -s 2048000 - | ...

# Or with higher nice priority
nice -n -10 gqrx

Troubleshooting

Ulimit Not Taking Effect

Problem: Ulimit set but not working inside container

Solutions:

# Verify ulimit is set
rfswift ulimits list -c container

# Check inside container
rfswift exec -c container -e "ulimit -a"

# For rtprio, also need SYS_NICE capability
rfswift capabilities add -c container -p SYS_NICE

# Or use realtime mode which sets everything
rfswift realtime enable -c container

Permission Denied with chrt

Problem: chrt: failed to set pid 0's policy: Operation not permitted

Solutions:

# Need both rtprio ulimit AND SYS_NICE capability
rfswift ulimits add -c container -n rtprio -v 95
rfswift capabilities add -c container -p SYS_NICE

# Or simply enable realtime mode
rfswift realtime enable -c container

Container Recreation

Note: Modifying ulimits requires container recreation. The container will be stopped, removed, and recreated with the new settings. Your data in mounted volumes is preserved, but uncommitted changes inside the container may be lost.

# Commit important changes before modifying ulimits
rfswift commit -c container -i my_image:backup

# Then modify ulimits
rfswift ulimits add -c container -n rtprio -v 95

Related Commands

realtime - Quick setup for all SDR-related ulimits
capabilities - Manage container capabilities
exec - Test ulimits inside container
run - Create container with ulimits using --ulimits flag

⚡

Quick Setup: For RF work, use rfswift realtime enable -c container instead of setting individual ulimits. It configures rtprio, memlock, nice, and SYS_NICE capability automatically!

Container Restart: Changing ulimits requires recreating the container. Commit your work first with rfswift commit if you have uncommitted changes!

Host Alternative: You can also configure default ulimits in /etc/docker/daemon.json to apply to all containers on the host. See the realtime documentation for details.

Last updated on January 28, 2026

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