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

Abstract

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.