REAL-TIME OPERATING SYSTEMS AND STANDARDS - Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications - page 435

Xenomai also provides a set of “skins” that implement various APIs offered by com-
mon RTOSs (e.g., VxWorks), as well as a POSIX API and a “native” API. This feature
increases portability of application code from other platforms to Xenomai.

With respect to RTAI, Xenomai offers the support for a whole set of embedded ar-
chitectures, including ARM, Blackfin, Altera Nios2, and PowerPC. Xenomai devel-
opers are currently putting considerable effort into integrating its code with the PRE-
EMPT RT patch, in order to have the benefits of both patches on the same system.

12.3.4 PREEMPT RT

PREEMPT RT 7 is a kernel patch to make a Linux system more predictable and deter-
ministic. This is done through several optimizations. The patch makes almost all ker-
nel code preemptable, except for the most critical kernel routines, so reducing the max-
imum latency experienced by a task (at the cost of a slightly higher average latency).
This feature is achieved by replacing almost every spinlock in the kernel code with
preemptable mutexes with Priority Inheritance (see Section 7.6). A numerical evalua-
tion of the PREEMPT RT patch has been carried out by Lipari and Scordino [LS06].

The patch also provides “threaded interrupts” by converting interrupt handlers into
preemptable kernel threads that are managed in “process context” by the regular Linux
scheduler. Whenever some parts of the PREEMPT RT patch are considered to be
stable enough (e.g., the priority inheritance algorithm), they are merged in the official
Linux kernel distribution.

12.3.5 SCHED DEADLINE

SCHED DEADLINE 8 is a Linux kernel patch developed by Evidence s.r.l. in the
context of the ACTORS European project. 9 It adds a deadline-based scheduler with
resource reservations in the standard Linux kernel. The implemented algorithm is
a global Constant Bandwidth Server (see Section 6.9), implemented as a partitioned
algorithm with migration. Migration can be disabled, thus it can also work as a parti-
tioned algorithm.

7 PREEMPT RT: http://rt.wiki.kernel.org
8 SCHED DEADLINE: http://gitorious.org/sched deadline/pages/Home
9 ACTORS: http://www.actors-project.eu