Resource sem issue

Paolo Mantegazza mantegazza at aero.polimi.it
Thu Oct 28 17:08:11 CEST 2004


Steven Seeger wrote:
> I've never used LXRT before. What would I do differently? I thought LXRT was
> doing more scheduling stuff from user space or something.
> 

Once more! LXRT can schedule kernel only applications also. It can do so 
by using Linux kernel threads or RTAI proper tasks. The choice is set by 
setting its macro USE_RTAI_TASKS to 0/1. The default that comes with the 
distribution is 0 (do not use), because RTAI proper tasks have not been 
tested well in mixed user/kernel mode. The difference in performance 
between the two mode is just that you have to pay a bit more overhead, 
in the latter case. Such a overhead is negligible with respect to the 
architectural jitter, at least on ix86.

I already informed RTAI users that, better sooner than later, they will 
be using just LXRT, even if they insmod rtai_up. The reasons are:
- for more than 2 years nobody has used anything but LXRT here at DIAPM
- performancewise there is just the illusion of better performances, as 
said above. At DIAPM we have been likely the only ones to run latency 
calibration in parallel with real lasting applications. The result is 
that, given enough running time, sched_up has the same worst case than LXRT
- so it is simpler to maintain just one thing.

Clearly there is the problem of making LXRT architecture indifferent. 
That is not the same as having user space support for real time and can 
be done quite easily. It is just a matter of finding somebody with the 
good will to interoperate with me on a non ix86 architecture, to make 
LXRT nomore ix86 specific anywhere, it is close to that already.

In your case the behaviour will be likely the same but I asked you to do 
so because I'm willing to care of just LXRT.

Paolo.





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