Approaches to realtime Linux
Posted Oct 12, 2004 19:29 UTC (Tue) by karim
Parent article: Approaches to realtime Linux
There are a few things that need to be pointed out:
1- I'm not sure what is meant by "kernel-mode" code, but hard-rt deterministic tasks in RTAI DO NOT need be written as kernel modules. In fact, such tasks can be written as normal shell applications that use a
special RTAI syscall vector to access RTAI services, including morphing from
normal Linux processes into RTAI-scheduled tasks, and hence obtaining deterministic scheduling.
2- To put Philippe's words in laymen's terms, the aim of RTAI/fusion is to allow normal Linux processes to be serviced by RTAI transparently without requiring the use of any special API. To this end, normal Linux application calls are transparently "redirected" to RTAI using the Adeos nanokernel. It must be said that while there are a few system calls already successfully diverted in this way, nanosleep() being an example, this is still a work in progress. The ultimate goal being to allow those tasks that use time-sensitive calls to obtain the performance they would obtain had they been running on a real hard-rt RTOS. Of course there are calls that cannot be "hardened". Needless to say that an open() or a read() on a file located in an ext3 partition is unlikely to be deterministic any time soon.
Personally, I believe that this approach to real-time is much more sane than threading the interrupt handlers and introducing yet another level of locks. The fact of the matter is that if we have hard-rt like that, we don't need the threading of int handlers and the likes. But if we have threaded int handlers and co., we still need hard-rt because reducing latency doesn't provide deterministic response times.
NOTE: deterministic hard-rt is not about speed, it's about determinism. While Ingo's work is great at reducing latency, it cannot guarantee response times regardless of the load, kernel configuration, and driver set. RTAI/fusion, and the Adeos interrupt pipeline on a smaller scale, can provide such guarantees.
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