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2017-07-22 - 04:36

Dates and Events:

Breaking News:

2016-11-12 12:00

Raspberry Pi and real-time Linux

Let's have a look at the OSADL QA Farm data


2016-09-17 12:00

Preemption latency of real-time Linux systems

How to measure it – and how to fix it, if it's too high?


2014-11-18 00:00

Linux real-time: New stable release available -

but next one depends on more support from industrial users



OSADL HOWTOs

HOWTO: Create a latency plot from cyclictest histogram data

Quick description with inlined code

1. Redirect the output of cyclictest to a file, for example

cyclictest -l100000000 -m -Sp90 -i200 -h400 -q >output 

(Please note that this will take 5 hours and 33 minutes.)

2. Get maximum latency

max=`grep "Max Latencies" output | tr " " "\n" | sort -n | tail -1 | sed s/^0*//`

3. Grep data lines, remove empty lines and create a common field separator

grep -v -e "^#" -e "^$" output | tr " " "\t" >histogram 

4. Set the number of cores, for example

cores=4

(If the script is used on a variety of systems with a different number of cores, this can, of course, be determined from the system.)

5. Create two-column data sets with latency classes and frequency values for each core

for i in `seq 1 $cores`
do
column=`expr $i + 1`
cut -f1,$column histogram >histogram$i
done

6. Create plot command header

echo -n -e "set title \"Latency plot\"\n\
set terminal png\n\
set xlabel \"Latency (us), max $max us\"\n\
set logscale y\n\
set xrange [0:400]\n\
set yrange [0.8:*]\n\
set ylabel \"Number of latency samples\"\n\
set output \"plot.png\"\n\
plot " >plotcmd

7. Append plot command data references

for i in `seq 1 $cores`
do
if test $i != 1
then
echo -n ", " >>plotcmd
fi
cpuno=`expr $i - 1`
if test $cpuno -lt 10
then
title=" CPU$cpuno"
else
title="CPU$cpuno"
fi
echo -n "\"histogram$i\" using 1:2 title \"$title\" with histeps" >>plotcmd
done

8. Execute plot command

gnuplot -persist <plotcmd

9. Enjoy

firefox plot.png

Download the script

mklatencyplot.bash

Bash script (must be run as root)

1.2 K

Example plot

And this is how the resulting image may look like.

Example latency plot (click to enlarge)