View source for Real Time in Embedded Linux System

== Real-time - Timers and scheduling latency ==

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

The purpose of this article is measure scheduling latency in different conditions.

== Using high-resolution timers ==

Have a look at the [http://downloads.isee.biz/pub/files/examples/rttest.c rttest.c] source file

Now compile this program:

 # gcc -o rttest rttest.c -lrt

Execute the program on the board

=== Testing the voluntary preemption kernel ===

Test done with linux-2.6.35.y branch from  git://git.igep.es/pub/scm/linux-omap-2.6.git

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 ===

Test done with linux-2.6.35.y branch from  git://git.igep.es/pub/scm/linux-omap-2.6.git

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 ===

Test done with linux-2.6.35.y-rt branch from  http://git.isee.biz/?p=pub/scm/linux-omap-2.6.git;a=shortlog;h=refs/heads/linux-2.6.35.y-rt

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

 # ./rttest
 Clock resolution (ns): 1
 Measurement, please wait 1 minute...
 Samples: 327833
 Min latency: 73 us
 Max latency: 9702 us
 Average latency: 80 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: 320301
 Min latency: 63 us
 Max latency: 10006 us
 Average latency: 85 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: 453705
 Min latency: 18 us
 Max latency: 395 us
 Average latency: 30 us

== 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
** http://elinux.org/Realtime_Testing_Best_Practices
* Other resources
** http://www.osadl.org/Realtime-Linux.projects-realtime-linux.0.html
** https://rt.wiki.kernel.org/index.php/Main_Page
** https://www.osadl.org/fileadmin/dam/rtlws/12/Brown.pdf
** http://embedded.communities.intel.com/servlet/JiveServlet/download/4435-1248/xeno_intel.pdf


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