[LITMUS^RT] Question about the scheduling overhead of GSN-EDF scheduler in LITMUS

[Meng Xu]

Hi,

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

We are measuring the scheduling overhead (SCHED and SCHED2 event) with the
feather trace tool in LITMUS. When we randomly generate 450 tasks (rtspin)
as a taskset and release them with arbitrary offset, we found that the
worst-case scheduling overhead of GSN-EDF plugin is less than 40us.
The hardware we use is Freescale IMX6 ARM board, which has 4 cores.
(We did generate multiple such tasksets and vary the number of tasks from
50 to 450 as Dr. Brandenburg did in his RTSS09 paper[1], the scheduling
overhead is from 12us to 20us when task number increases from 50 to 450.)

However, the data in Dr. Brandenburg's RTSS09 paper [1] shows that the
worst-case scheduling overhead of GSN-EDF is at least 80us (Fig. 6 on page
13 of [1]) when task number is 450.

*My general question is:*
Is the scheduling overhead we measured reasonable?

*My specific question is:*
1) Do we have to release tasks at the same time to reproduce the similar
overhead value for GSN-EDF in [1]?
2) Is it possible that LITMUS^RT has made some improvement since RTSS09
paper. The improvement reduces the scheduling overhead and therefore the
smaller overhead value we measured is reasonable?
3) The platform we use only has 4 cores and therefore the lock contention
among cores may be (much) smaller, compared to the experiment Dr.
Brandenburg did in 2009? (We are not quite sure how many cores are used in
the RTSS09 paper's overhead experiment.) Is it possible that we may observe
much smaller overhead value like 20us due to less lock contention?
4) Do we have to use other RT tasks instead of rtspin to run the overhead
measurement in order to reproduce the result in [1]?

[1] https://www.mpi-sws.org/~bbb/papers/pdf/rtss09_long.pdf

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

Best regards,

Meng​


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