Linux kernel

The Linux kernel has not been designed to meet real-time constraints however there are some techniques that can help tune the kernel to meeting certain deadlines. Four kernels have been evaluated (some available here):

The following tests will be formed with a vanilla kernel compiled with the PREEMPT option. We will make use of cylictests code and we will also torture the kernel with stressto obtain the maximum latencies.

Torturing the kernel

For this purpose we use the stress Linux command line tool.

While running the autopilot and using:

stress --cpu 8 --io 8 --vm 2 --vm-bytes 128M

We can see that the load of the system raises up to 17 approximately:

root@beaglebone:~# uptime
 03:20:20 up 14 min,  3 users,  load average: 17.07, 7.86, 3.21

Let's push a bit more launching more threads and io operations:

stress --cpu 64 --io 64 --vm 2 --vm-bytes 128M

With this, the uptime command returns:

root@beaglebone:~# uptime
 03:27:10 up 21 min,  3 users,  load average: 129.08, 81.93, 36.63

Running cyclictests

Now that we have the processor stressed we use cyclictests to measure the latencies (us):

root@beaglebone:~# cyclictest -t1 -p 80 -n -i 10000 -l 10000
# /dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 0.47 0.65 0.44 1/98 1312           

T: 0 ( 1307) P:80 I:10000 C:   3892 Min:     19 Act:   26 Avg:   67 Max:   13743