Difference between revisions of "Real Time in Embedded Linux System"

Revision as of 18:16, 8 November 2011

Real-time - Timers and scheduling latency

Based on free training materials from Free Electrons covering real-time (http://free-electrons.com/docs/realtime/)

Learn how to handle real-time processes and practice with the different real-time modes. Measure scheduling latency.

Setup

Using high-resolution timers

Have a look at the rttest.c source file

Now compile this program:

# gcc -o rttest rttest.c -lrt

Execute the program on the board

Testing the non-preemption kernel

Testing the voluntary preemption kernel

Recompile your kernel with CONFIG_PREEMPT_VOLUNTARY enabled and test the program with nothing special

Test the program with nothing special

# ./rttest 
Clock resolution (ns): 1
Measurement, please wait 1 minute...
Samples: 153046
Min latency: 113 us
Max latency: 7712 us
Average latency: 292 us

Test your program and at the same time, add some workload to the board, by running

# stress --cpu 1 --io 2 --vm 2 --vm-bytes 128M --timeout 120s &
# ./rttest 
Clock resolution (ns): 1
Measurement, please wait 1 minute...
Samples: 90233
Min latency: 83 us
Max latency: 28586 us
Average latency: 558 us

Test your program again with the workload, but by running the program in the SCHED_FIFO scheduling class at priority 99

# stress --cpu 1 --io 2 --vm 2 --vm-bytes 128M --timeout 120s &
# chrt -f -r 99 ./rttest
Clock resolution (ns): 1
Measurement, please wait 1 minute...
Samples: 210637
Min latency: 52 us
Max latency: 2219 us
Average latency: 181 us

Testing the preemptible kernel

Recompile your kernel with CONFIG_PREEMPT enabled and test the program with nothing special

# ./rttest
Clock resolution (ns): 1
Measurement, please wait 1 minute...
Samples: 154210
Min latency: 113 us
Max latency: 9085 us
Average latency: 288 us

Test your program and at the same time, add some workload to the board, by running

# stress --cpu 1 --io 2 --vm 2 --vm-bytes 128M --timeout 120s &
# ./rttest 
Clock resolution (ns): 1
Measurement, please wait 1 minute...
Samples: 113158
Min latency: 83 us
Max latency: 15921 us
Average latency: 421 us

Test your program again with the workload, but by running the program in the SCHED_FIFO scheduling class at priority 99

# stress --cpu 1 --io 2 --vm 2 --vm-bytes 128M --timeout 120s &
# chrt -f -r 99 ./rttest
Clock resolution (ns): 1
Measurement, please wait 1 minute...
Samples: 235302
Min latency: 83 us
Max latency: 296 us
Average latency: 150 us

Testing the Xenomai scheduling latency

 $ wget http://download.gna.org/xenomai/stable/xenomai-2.6.0.tar.bz2

rt-tests

$ git clone git://github.com/clrkwllms/rt-tests.git

Links

• Tools to ensure a system's realtime capabilities
  • https://www.osadl.org/Single-View.111+M52212cb1379.0.html
  • https://www.osadl.org/Realtime-Preempt-Kernel.kernel-rt.0.html
  • https://www.osadl.org/Driver-to-Program-Signal-Tester.driver-to-program-signal-tester.0.html
  • https://www.osadl.org/Latency-generator.latency-generator.0.html
  • https://www.osadl.org/Visualize-the-temporal-relationship-of-L.taks-visualizer.0.html
• Other resources
  • https://www.osadl.org/fileadmin/dam/rtlws/12/Brown.pdf
  • http://embedded.communities.intel.com/servlet/JiveServlet/download/4435-1248/xeno_intel.pdf