Part 2: Real-time on Raspberry pi

raspberry-pi-logo
RaspberryPi

As promised, here’s my Part Two blog posting regarding the raspberrry pi.

The default kernel in Raspbian Jesse, the latest release of the raspberry pi’s official OS, could use a tune-up before being fit for duty running time-sensitive machine controls. On a vanilla Linux kernel program, latencies depend on everything running on the system making consistent and punctual tasks difficult to guarantee. The RT PREEMPT patch is a popular fix for this problem. The patch converts Linux into a fully preempt-able RTOS. If you want to control physical processes with your pi, as I intend to in DISTek’s training curriculum update, this is the way to go. But how big of a difference does it make on a pi?

To answer this, I turned to the high resolution test program cyclictest. Cyclictest measures a system’s latency response by running at regular intervals and measuring the time in between them. This simple algorithm accounts for everything that can delay a task and contribute to latency. Below is how you run this on the Raspberry pi:

pi@distekpi:~ $ git clone git://git.kernel.org/pub/scm/utils/rt-tests.git

pi@distekpi:~ $ cd rt-tests

pi@distekpi:~ $ make all

pi@distekpi:~ $ cp ./cyclictest /usr/bin/

pi@distekpi:~ $ sudo cyclictest -m -t1 -p 80 -n -i 500 -l 100000

Note: on the newer multi-core raspberry pi’s,  the -t1 option should be -tn, where n is the number of cores.

Here are my before and after results on DISTek’s Raspberry pi B+:

rp

All of those times are in μs. With this performance, 1ms tasks should be reliable and safe. For instructions on how to build and install a patched kernel to your pi, follow this tutorial on the real time Linux wiki.

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