TrainingSphere - 4 - Installing Realtime Raspbian

3 Jul 2015

It's time to start doing something serious!

So, according to the plan, I will install the realtime version of the Raspbian. I will use Raspberry Pi Model B. I will install this Raspbian version with real-time kernel for Raspberry Pi 2 Model B. It is based on 2015-02-16-raspbian-wheezy with default kernel replaced to 3.18.9-rt5-v7+ kernel and a few additional tunings.

Default Raspbian kernel 3.18 is configured with PREEMPT option and provides worst case latency around hundreds of microseconds. Real-time demanding applications require lower latencies than that. Real-time patch lowers the worst case latency to tens of microseconds, improving performance of real-time applications such as autopilots to be run on Linux.

Emlid (the company that provides this Raspbian version) provides also some useful performance comparisons

By the way, the following interrupt latency data (obtained by running command)

sudo cyclictest -l1000000 -m -n -a0 -t1 -p99 -i400 -h400 -q

gives another idea of the performance improvements between Raspberry Pi and Raspberry Pi 2

Interrupt Latency	Raspberry Pi		Raspberry Pi 2
Interrupt Latency	No RT	RT	No RT	RT
Min	13 us	11 us	13 us	11 us
Avg	23 us	23 us	21 us	18 us
Max	1153 us	66 us	307 us	58 us