Raspberry Pi. Cycletest : no better results between PREEMPT_RT & PREEMPT kernels

Navio / Edge Flight stack
1

Hello all,

I am trying to compare cycletest’s between RT-patched preemptive kernel and original preemptive kernel. But the results I get are not what is expected. So may be I am doing something wrong. Please help me to figure this out.

On current website I found an article and comparison of linux preemptive and real-time preemptive kernel. So I have used the raspi image with a patched kernel provided in the article and installed it on my raspi B+. I conducted the cycletest with 5 mio. tests:

cyclictest -l5000000 -m -n -a0 -t1 -p99 -i400 -h400 -q >> ctest-uname -r.dat

for 3.12.28+ kernel and the 3.12.26-rt40+ lernel that were both available on the image, by editing the /boot/config.txt file and replacing kernel_rt.img with kernel.img

I have used gnuplot to visualize my results. But since I can not attach images I insert the output of cycletest.

I very appreciate your help!

here are the outputs of both cycletests:

pi@raspberrypi ~ $ cat cycletest-3.12.26-rt40+.txt

Histogram

Total: 004999674

Min Latencies: 00043

Avg Latencies: 00087

Max Latencies: 04195

Histogram Overflows: 00326

And now for the not patched kernel
pi@raspberrypi ~ $ cat cycletest-3.12.28+.txt

Histogram

Total: 004999836

Min Latencies: 00040

Avg Latencies: 00081

Max Latencies: 05159

Histogram Overflows: 00164

2

Hello Sergii,
I am not able to reproduce your results on rt kernel that comes with this image - http://www.emlid.com/raspberry-pi-real-time-kernel/

I’m running:
sudo cyclictest -l5000000 -m -n -a0 -t1 -p99 -i400 -h400 -q

My results with kernel-rt.img (3.12.26-rt40+) are:

Total: 005000000

Min Latencies: 00012

Avg Latencies: 00027

Max Latencies: 00074

Histogram Overflows: 00000

Histogram Overflow at cycle number:

Thread 0:

Do you have any other process running with rt priority?

3

3rd part response,

Total: 005000000

Min Latencies: 00013

Avg Latencies: 00028

Max Latencies: 00078

Histogram Overflows: 00000

Histogram Overflow at cycle number:

Thread 0:

using the same RT linux download as Mikhail

4

Could you release the .config file used to compile your kernel. Sources are available but i’m also interested in which option you turned on.

5

Vincenzo,
You can obtain the config from the running system, to do that use:

  
cat /proc/config.gz | gunzip > running.config

Also, it should be somewhere in the folder with sources /usr/src/

6

here’s my cat /proc/config.gz running.config if you dont want to make your own image and check.