Wed, 12 Jan 2000 Ivan Martinez wrote:
> > Hi Ivan,
> >
> > > As I explained when I talked about my project, the aim is to communicate
> > > two RT tasks:
> >
> > What do you mean for RT task: task modules in RTL or RTAI, or user
> > processes?
>
> I am not sure. I understood there are two kinds of tasks:
> - real time tasks, scheduled by the real time scheduler, unable to make
> system calls. Are these task modules?
Well, yes, they are kernel modules containing the code for one or more RTL
threads. They cannot make system calls the normal way, as the interface library
isn't available in kernel space. In general, calling the standard kernel space
API functions from RT context is unsafe, as RT context can preempt Linux even if
Linux has disabled the interrupts, and hence break spinlock protection and
similar stuff. (That is, Linux uses a layer that lets RTL get in between Linux
and the IRQ hardware.)
> - normal linux tasks, scheduled by the linux scheduler. Are these user
> processes?.
Yep.
(Note, however, that user space processes can still be RT processes [SCHED_RR
or SCHED_FIFO] - but that's not the same thing as RTL! On kernels prior to the
late 2.3.x and 2.2.x + lowlatency patch, it is *soft* real time, ie there is
an unbounded maximum scheduling latency. You have a typical jitter of a
fraction of a ms, but may get occasional peaks of hundreds of ms.)
//David
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