RT Linux is a hard real time system, which provides an excellent interrupt latency of 30ns (worst case). but it has to be used with a standard linux kernel since it doesnt have the primitive OS functions like Memory and File system management.
RTLinux is an extension of Linux to a Real-time operating system, which was originally developed by V. Yodaiken at the New Mexico Institute of Mining and Technology. Today, RTLinux is available as a community supported free version as well as a commercial version from FSMLabs.
RTLinux supports hard real-time (deterministic) operation through interrupt control between the hardware and the operating system. Interrupts needed for deterministic processing are processed by the real-time core, while other interrupts are forwarded to the non-real time operating system. The operating system (Linux) runs as a low priority thread. First-In-First-Out pipes(FIFOs) or shared memory can be used to share data between the operating system and the real-time core.