realtime

rfswift realtime

Enable or disable realtime mode for optimal SDR performance.

Synopsis

rfswift realtime enable -c CONTAINER
rfswift realtime disable -c CONTAINER
rfswift realtime status -c CONTAINER

The realtime command provides a one-command solution to configure containers for low-latency RF/SDR operations. It automatically sets up the necessary capabilities and ulimits to eliminate buffer underruns and enable real-time scheduling.

What Realtime Mode Configures

When you enable realtime mode, RF Swift automatically configures:

Subcommands

Options

realtime enable / disable / status

Examples

Basic Usage

Enable realtime mode:

rfswift realtime enable -c sdr_work

Check realtime status:

rfswift realtime status -c sdr_work

Disable realtime mode:

rfswift realtime disable -c sdr_work

Real-World Scenarios

Fix SDR buffer underruns:

# Experiencing underruns with HackRF/BladeRF/RTL-SDR?
rfswift realtime enable -c sdr_container

# Verify it's working
rfswift exec -c sdr_container -e "ulimit -r"
# Output: 95

# Now run your SDR application with real-time priority
rfswift exec -c sdr_container
chrt -f 50 hackrf_transfer -r samples.bin -f 433920000 -s 8000000

Create new container with realtime mode:

# Use --realtime flag during creation
rfswift run -n sdr_realtime -i penthertz/rfswift_noble:sdr_full --realtime

# Container is ready for low-latency SDR work
rfswift exec -c sdr_realtime

Professional RF testing setup:

# Create optimized container
rfswift run -n rf_pentest -i penthertz/rfswift_noble:rfid --realtime

# Verify configuration
rfswift realtime status -c rf_pentest

# Run time-critical captures
rfswift exec -c rf_pentest
chrt -f 70 proxmark3 /dev/ttyACM0

Using real-time scheduling inside container:

rfswift exec -c sdr_container

# Run with FIFO real-time scheduling (priority 1-99)
chrt -f 50 rtl_sdr -f 433920000 -s 2048000 output.bin

# Run with round-robin real-time scheduling
chrt -r 50 gqrx

# Run with elevated nice priority
nice -n -15 gnuradio-companion

# Check current process scheduling
chrt -p $$

When to Use Realtime Mode

✅ Enable Realtime Mode For:

• High sample rate captures - Prevents dropped samples at high bandwidths
• Real-time signal processing - GNU Radio flowgraphs, SDR++, GQRX
• Multiple SDR tools - Running several applications simultaneously
• Time-critical protocols - RFID, NFC, automotive keyfobs
• Professional pentesting - When reliability is critical
• Live demonstrations - Avoid embarrassing buffer underruns

❌ Not Necessary For:

• Offline analysis - Inspectrum, signal analysis of recorded files
• Low sample rates - Simple FM reception, slow protocols
• Non-SDR work - General container usage, development

Alternative: Host-Level Configuration

Instead of per-container settings, you can configure Docker daemon defaults:

Edit /etc/docker/daemon.json:

{
  "default-ulimits": {
    "rtprio": { "Name": "rtprio", "Hard": 95, "Soft": 95 },
    "memlock": { "Name": "memlock", "Hard": -1, "Soft": -1 },
    "nice": { "Name": "nice", "Hard": 40, "Soft": 40 }
  }
}

Restart Docker:

sudo systemctl restart docker

This applies realtime ulimits to all containers on the host automatically.

Troubleshooting

Realtime Mode Not Working

Problem: Still experiencing buffer underruns after enabling realtime mode

Solutions:

# Verify realtime mode is enabled
rfswift realtime status -c container

# Check ulimits inside container
rfswift exec -c container -e "ulimit -r && ulimit -l"
# Should show: 95 and unlimited

# Ensure you're actually using real-time scheduling
rfswift exec -c container
chrt -f 50 your_sdr_command  # Must use chrt!

# Check if host kernel supports real-time
uname -a  # Should not be a -virtual or -cloud kernel

Permission Denied

Problem: Operation not permitted when using chrt

Solutions:

# Re-enable realtime mode (recreates container)
rfswift realtime enable -c container

# Verify SYS_NICE capability
rfswift exec -c container -e "grep Cap /proc/self/status"

# If using rootless Docker, also set host ulimits
# Edit /etc/security/limits.conf:
# your_user  -  rtprio  95
# your_user  -  memlock unlimited

Container Won’t Start After Enable

Problem: Container fails to start after enabling realtime mode

Solutions:

# Check Docker logs
docker logs container_name

# Try disabling and re-enabling
rfswift realtime disable -c container
rfswift realtime enable -c container

# If persists, recreate container
rfswift run -n new_container -i image_name --realtime

Technical Details

How It Works

1. Container inspection - RF Swift reads current container configuration
2. Configuration update - Adds SYS_NICE capability and ulimits
3. Container recreation - Stops, removes, and recreates container with new settings
4. Verification - Container starts with realtime configuration

Ulimit Values Explained

Kernel Requirements

Real-time scheduling works best with:

• Standard kernels (not -virtual or -cloud variants)
• PREEMPT_RT patches (optional, for hard real-time)
• Sufficient CPU resources (avoid overcommitting)

Related Commands

• ulimits - Fine-grained ulimit control
• capabilities - Manage container capabilities
• run - Create container with --realtime flag
• exec - Access container to use real-time scheduling