Violations of Real Time Communication Constraints caused by Memory Transfers exceeding CPU Cache Limits in RTAI and RTnet

Standard Linux is not able to assure real-time guarantees for applications. Several extensions, which permit the use of Linux in real-time systems, have been proposed in the past. The paper focuses on RTAI, which runs standard Linux as low-priority task within a separate real-time kernel. The open source protocol stack RTnet provides deterministic communication over non-deterministic media like Ethernet. While measurements proved that RTAI and RTnet are able to meet deadlines under high CPU loads, memory interactions invalidate real-time constraints when more data than physical CPU caches can handle is transferred at once.