Real-Time DSP Engine & Zero-Drop Scheduling

Kernel TuningPre-configured

Overview & Purpose

Software Defined Radio applications such as GNU Radio, srsRAN, and OpenAirInterface require real-time CPU scheduling and unconstrained physical memory allocation. Under high sample rates (e.g. 30.72–61.44 MSps for LTE and 5G NR), standard time-sharing kernels preempt radio processing threads, causing packet buffer underruns and corrupting air interface frames. TelcoChisel pre-configures real-time SCHED_RR scheduling and unlimited memory locking (RLIMIT_MEMLOCK), ensuring zero sample drops.

Verification Command

Run the following to confirm this configuration is active on your system:

Real-Time DSP Engine & Zero-Drop Scheduling — Verification
ulimit -r -l && chrt -m

Configuration Details & Steps

TelcoChisel applies these settings automatically at ISO build time. The following steps document each configuration item:

  1. 1
    Verify that real-time scheduling limits and memory locking are active in your user session: ulimit -r -l
  2. 2
    Inspect the maximum real-time priority supported by the running kernel scheduler: chrt -m
  3. 3
    Check the real-time scheduling policy of active SDR processes: chrt -p $(pgrep -f 'srsenb|openair|gnuradio' 2>/dev/null || echo $$)
  4. 4
    Monitor thread-level CPU affinity and scheduling jitter during live RF operations: top -H -p $(pgrep -f srsenb 2>/dev/null || echo $$)

Troubleshooting

Common Issues
If your SDR application reports 'SCHED_RR: Operation not permitted', verify that your session limits return `99` for `ulimit -r` and `unlimited` for `ulimit -l`. In virtualized environments or containers, ensure the hypervisor or container runtime delegates real-time capabilities (CAP_SYS_NICE).

Frequently Asked Questions

Why does TelcoChisel use SCHED_RR priority instead of running tools as root?

Running complex DSP stacks like GNU Radio as root introduces unnecessary attack surface. Granting real-time limits to unprivileged user sessions adheres to the principle of least privilege while providing the exact timing guarantees needed for RF streaming.

What is RLIMIT_MEMLOCK and why is it set to unlimited?

RLIMIT_MEMLOCK controls the amount of memory a process can pin to physical RAM, preventing the OS from swapping it to disk. SDR buffers must remain pinned in RAM to guarantee low-latency Direct Memory Access (DMA) transfers to and from the transceiver.

How does real-time scheduling prevent sample drops?

SCHED_RR ensures that critical DSP and baseband signal processing threads preempt lower-priority user-space tasks immediately whenever new RF samples arrive from the transceiver.

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