I have recently been working on system to implement Real-Time computer control using Linux 1.2.13 with the POSIX.4 (should that be .1b ?) FIFO scheduler.
When I run my RT progs. I also have the usual system running doing various things, and have not experieneced any unpredictability down to sampling intervals of 10ms. This includes doing things like AD/DA device driver access.
I have also, though less deterministically, had intervals as low as 500us to 600us going. (I have been using multiple processes, but in a single process (threaded maybe) fast, deterministic intervals should go fine?)