[LITMUS^RT] Question about the overhead measurement in LITMUS and potential memory leak?

[Meng Xu]

Hi,

I'm Meng Xu, a PhD student at the PRECISE lab at the University of
Pennsylvania.

I'm trying to measure the context switch overhead with the feather trace
tool on Freescale IMX6 ARM board. The board has 1GB RAM and 4 cores.
I followed the description at https://wiki.litmus-rt.org/litmus/Tracing.
I measured the context switch overhead with the command:

ST_TRACE_PATH=/dev/shm
ST_TRACE_NAME=GSN-EDF
OH_EVENTS="CXS_START CXS_END"
ftcat /dev/litmus/ft_cpu_trace0 ${OH_EVENTS} >
${ST_TRACE_PATH}/oh-${ST_TRACE_NAME}-0.bin &
ftcat /dev/litmus/ft_cpu_trace1 ${OH_EVENTS} >
${ST_TRACE_PATH}/oh-${ST_TRACE_NAME}-1.bin &
ftcat /dev/litmus/ft_cpu_trace2 ${OH_EVENTS} >
${ST_TRACE_PATH}/oh-${ST_TRACE_NAME}-2.bin &
ftcat /dev/litmus/ft_cpu_trace3 ${OH_EVENTS} >
${ST_TRACE_PATH}/oh-${ST_TRACE_NAME}-3.bin &

I run a rtspin RT task under GSN-EDF scheduler for 20 seconds and then
combine the bin files as one "oh-GSN-EDF-all.bin ".
When I use ft2csv command to parse the bin file, it shows no complete
events.

# ft2csv CXS oh-GSN-EDF-all.bin
Total       :      11936
Skipped     :          1
Avoided     :          0
Complete    :          0
Incomplete  :       4941
Non RT      :          0
Interleaved :          0
Interrupted :       1033

​I tried to increase the buffer size of the overhead trace to a larger
value, but it cannot be larger than 1024MB since RAM size is only 1024MB.
/dev/shm has size 512MB.

I also observe that there may be some memory leaking in the feather trace?
When I boot the system, there is about 800MB free memory.
After I run the feather trace, the free memory becomes 538MB. (ftcat has
been killed already at this time.)

​The commit point I used is 7f051e3fa168eb60386b0e8d970551c06696befb, which
is committed by Bjorn on Jun 12th, 2014.​

*My question is:*
Did I configure anything wrong that cause no complete event is recorded?
Do you have any suggestion/advice on how I can get the complete events?

I have tried to set the ftcat to highest priority under FIFO scheduler, it
didn't solve the problem.
I was suspecting it is because the buffer is not consumed by ftcat fast
enough so that the old events are overwritten by new events. However, I
only run on rtspin task which leave at least 3 full cores idle to consume
the buffer.

Thank you very much for your time and help in this question!

best,

Meng​

-----------
Meng Xu
PhD Student in Computer and Information Science
University of Pennsylvania
http://www.cis.upenn.edu/~mengxu/
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