Existing variants of Real-Time Linux: RTAI

The initial RT-Linux release from NMT only provided a one-shot timer. This timer required a lot of overhead for keeping the clock time and for reprogramming the 8254 timer on every interrupt. This original release did furthermore not provide in a number of fundamental real-time functionalities, such as semaphores, timing functions and mailboxes.

For both these reasons, the need for a periodic timer and the fact that important functions were missing from the NMT RT-Linux approach, researchers at the "Dipartimento di Ingegneria Aerospaziale" of the "Politecnico di Milano" developed their own approach to real-time Linux. Their version is most often labelled RTAI (for "Real-Time Application Interface").

Since then, a periodic clock, semaphores and mailboxes have been added to NMT RT-Linux, and on the other hand a one-shot timer was added to RTAI. Although, at first sight, the functionalities now seem to have converged for both families of real-time Linux, their development efforts nevertheless continue to diverge. According to the developers of RTAI, their implementation of the one-shot timer is more efficient than the one in RT-Linux because it is based on the "Time Stamp Clock" (TSC) of the Pentium processor. They furthermore justify their decision to continue developing in parallel a second approach to real-time Linux by their claim that a bug-free FPU support has been missing for a certain period in RT-Linux.

Version 0.1 of RTAI was made available in May 1999 and was based on Linux kernel 2.2.6. Version 1.0, based on kernel 2.2.12 was released in January 2000. Version 1.5, based on kernel 2.2.16, is available since October 11th, 2000.