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From: http://www.xenomai.org/documentation/xenomai-2.3/pdf/Life-with-Adeos-rev-B.pdf

The above behaviour is to be opposed to what happens with RTAI/LXRT for instance, where threads migrating to the Linux space actually lose their real­- time priority in the same move, by inheriting the lowest priority defined by the RTAI scheduler.

There must be some reason behind this design decision. I wish to understand what kind of advantages can it offer - I mean when the real time thread is shifted to userspace, it gets a low priority (and therefore can be preempted by a high priority real time thread running in kernel space). Are there some assumptions behind this design decision?

Aquarius_Girl
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1 Answers1

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Two assumptions:

  1. Threads don't spend much time in kernel space.

  2. Thread in kernel space tend to hold resources that need to be released as quickly as possible.

David Schwartz
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  • thanks for replying. Actually, Xenomai is quite proud of having the opposite feature- their threads maintain the high priority in user space. Therefore I am confused. Are there any real advantages too of maintaining the thread priority in user space? – Aquarius_Girl Apr 19 '12 at 05:38
  • If those two assumptions don't apply. For example, if the user space code acts as the "kernel" for other, lower-priority user-space code or where the user-space code doesn't do much of the "real work". – David Schwartz Apr 19 '12 at 05:42
  • David, but when we are talking about the hard real time, does it make sense to think that those two assumptions don't apply? AHH, can it be the reason that Xenomai doesn't care much about the speed? – Aquarius_Girl Apr 19 '12 at 05:46