Real Time in Embedded Linux System

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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 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

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