Timing Effects of DDR Memory Systems in Hard Real-Time Multicore Architectures: Issues and Solutions

Multicore processors are an effective solution to cope with the performance requirements of real-time embedded systems due to their good performance-per-watt ratio and high performance capabilities. Unfortunately, their use in integrated architectures such as IMA or AUTOSAR is limited by the fact that multicores do not guarantee a time composable behavior for the applications: the WCET of a task depends on inter-task interferences introduced by other tasks running simultaneously. This article focuses on the off-chip memory system: the hardware shared resource with the highest impact on the WCET and hence the main impediment for the use of multicores in integrated architectures. We present an analytical model that computes the worst-case delay, also known as Upper Bound Delay (UBD), that a memory request can suffer due to memory interferences generated by other co-running tasks. By considering the UBD in the WCET analysis, the resulting WCET estimation is independent from the other tasks, hence ensuring the time composability property and enabling the use of multicores in integrated architectures. We propose a memory controller for hard real-time multicores compliant with the analytical model that implements extra hardware features to deal with refresh operations and interferences generated by co-running non hard real-time tasks.