The IRMOS realtime scheduler

“In the context of the IRMOS European Project (Interactive
Real-Time Applications on Service-Oriented Infrastructures), a new
realtime scheduler for Linux has been developed by the Real-Time
Systems Laboratory of Scuola Superiore Sant’Anna in Pisa. The
purpose of this article is to provide a general overview of this
new scheduler, describe its features and how it can be practically
used, provide a few details about the implemented algorithms, and
gathering feedback by the community about possible
improvements.

“The IRMOS realtime scheduler (a.k.a., EDF throttling or
realtime throttling), allows the administrator to reserve a “slice”
of the processing capability of a system for a group of Linux
tasks. It is based on a direct modification of the POSIX realtime
scheduling class within the Linux kernel, and in particular, to the
throttling mechanism already built into the kernel for realtime
tasks. Basically, the realtime throttling mechanism is changed from
a mechanism that exclusively limits the computation power granted
to groups of realtime tasks, to one that provides them with both a
limit and precise scheduling guarantees (in terms of a guaranteed
runtime every period, on each of the available CPUs). Also, it has
been designed from scratch with SMP support in mind, and it
implements a hierarchical scheduling policy based on both deadlines
and priorities. Specifically, POSIX fixed priority (FP) realtime
scheduling is nested inside EDF (Earliest Deadline First)
scheduling.”

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