Although UNIX’s descendants or derivatives are not hard real-time operating systems, some have support for soft real-time through allowing to assign to userland processes higher priorities normally reserved to the system, sometimes coupled with preemption of kernel tasks, making them suitable as a fundation of soft real-time system. POSIX standardized its first real-time extensions in 1993 in a document usually referred to as POSIX.1b. At that time, however, some operating systems already had support for soft real-time in the form of ad-hoc APIs, such as System V Release 4 (SVR4) with its priocntl(2) system call and HP/UX with its rtprio(2) one. FreeBSD first adopted its own rtprio(2) system call in 1994, largely based on HP/UX’s with extensions such as idle processes. POSIX.1b extensions concerning processes were implemented later, in 1998, and some preliminary thread support added the next year. Since then, these APIs have been present in the system for applications to use.
In this paper, we provide a thorough description of both FreeBSD’s rtprio(2) and POSIX.1b’s scheduling interfaces and embark on a journey around FreeBSD’s implementation of scheduling priorities. It started with a desire to fix a few apparently simple bugs of rtprio(2) and to add some reasonable features and, one thing leading to another, became an almost complete rewrite of this system call and the POSIX.1b’s interfaces’ implementations. We will expose the many problems that the current implementation has, in terms of POSIX compliance, security and consistency and how we are fixing them. As of this writing, this project is still a work in progress, and we will report about its status during the conference presentation.