Measuring the Latency Time of Real-Time Unix-like Operating Systems

With the advent of continuous-media applications, real-time operating systems, once confined to process control and other specialized applications, are coming to the desktop. The popularity of UNIX made this operating system the first choice for use with such real-time desktop applications. However, since UNIX kernel does not provide real-time responsiveness, some software developers have been trying to adapt it to respond to this new requirements, while others have been proposing its total redesign. Though the evaluation of the performance of a real-time operating system depends on many factors, a predictable small latency time in responding to external events is always essential. In this paper, after a discussion about the probable sources of latency, it is presented a method for collecting information about context-switching and interrupt-acknowledge times in UNIX-like operating systems without requiring external measuring tools. It is also proposed, a form of presentation of these data aimed at facilitating the comparison with previously collected data obtained from the same or from other systems. The paper is illustrated with actual results obtained by the application of the method to TROPIX, a real-time UNIX-like operating system, running on a Motorola 68010-based computer. The impact of kernel preemption and some practical measurement interference considerations due to dynamic memory refresh, DMA operation and disk multiblock access are also discussed.

ICSI Technical Report TR-92-037

Technical Report