Cost and benefit of separate address spaces in real-time operating systems

The combination of a real-time executive and an off-the-shelf time-sharing operating system has the potential of providing both predictability and the comfort of a large application base. To isolate the real-time section from a significant class of faults in the (ever-growing) time-sharing operating system, address spaces can be used to encapsulate the time-sharing subsystem. However, in practice, designers seldom use address spaces for this purpose, fearing that the extra cost induced limits the system´s predictability. To analyze this cost, we compared in detail two systems with almost identical interfaces-both are a combination of the Linux operating system and a small real-time executive. Our analysis revealed that for interrupt-response times, the delay and jitter caused by address spaces are similar to or even smaller than those caused by caches and blocked interrupts. As a side effect of our analysis, we observed that published figures on predictability must be carefully checked whether or not such hardware features are included in the analysis. This paper is a follow-up of an earlier publication at the 3rd Real-Time Linux workshop. It is different in that we have further optimized our microkernel and examined more hardware.