CONTENTS
========
+0. WARNING
1. Overview
1.1 The problem
1.2 The solution
3. Future plans
+0. WARNING
+==========
+
+ Fiddling with these settings can result in an unstable system, the knobs are
+ root only and assumes root knows what he is doing.
+
+Most notable:
+
+ * very small values in sched_rt_period_us can result in an unstable
+ system when the period is smaller than either the available hrtimer
+ resolution, or the time it takes to handle the budget refresh itself.
+
+ * very small values in sched_rt_runtime_us can result in an unstable
+ system when the runtime is so small the system has difficulty making
+ forward progress (NOTE: the migration thread and kstopmachine both
+ are real-time processes).
+
1. Overview
===========
realtime tasks have explicitly allocated the CPU time they need to perform
their tasks, buffer underruns in the graphics or audio can be eliminated.
-NOTE: the above example is not fully implemented as of yet (2.6.25). We still
+NOTE: the above example is not fully implemented yet. We still
lack an EDF scheduler to make non-uniform periods usable.
.o CONFIG_CGROUP_SCHED (aka "Basis for grouping tasks" = "Control groups")
-This uses the /cgroup virtual file system and "/cgroup/<cgroup>/cpu.rt_runtime_us"
-to control the CPU time reserved for each control group instead.
+This uses the /cgroup virtual file system and
+"/cgroup/<cgroup>/cpu.rt_runtime_us" to control the CPU time reserved for each
+control group instead.
For more information on working with control groups, you should read
Documentation/cgroups/cgroups.txt as well.
-Group settings are checked against the following limits in order to keep the configuration
-schedulable:
+Group settings are checked against the following limits in order to keep the
+configuration schedulable:
\Sum_{i} runtime_{i} / global_period <= global_runtime / global_period
Implementing SCHED_EDF might take a while to complete. Priority Inheritance is
the biggest challenge as the current linux PI infrastructure is geared towards
-the limited static priority levels 0-139. With deadline scheduling you need to
+the limited static priority levels 0-99. With deadline scheduling you need to
do deadline inheritance (since priority is inversely proportional to the
-deadline delta (deadline - now).
+deadline delta (deadline - now)).
This means the whole PI machinery will have to be reworked - and that is one of
the most complex pieces of code we have.