WP1 - RTOS State of the Art Analysis: Deliverable D1.1 - RTOS Analysis
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Possible Solutions

Two different approaches can be used to reduce the size of kernel non-preemptable sections: the one used by the Low-Latency patches (Ingo Molnar and Andrew Morton)[LowLat], and the one used by the kernel preemptability patch (MontaVista, TimeSys)[kpreem, TimeSys].

Low-Latency Linux

This approach ``corrects'' the monolithic structure by inserting explicit rescheduling points (that effectively are preemption points) inside the kernel. In this approach, when a thread is executing inside the kernel it can explicitly decide to yield the CPU to some other thread. In this way, the size of non-preemptable sections is reduced, thus decreasing the latency. In a low-latency kernel, the consistency of kernel data is enforced by using cooperative scheduling (instead of non-preemptive scheduling) when the execution flow enters the kernel.

This approach is also used by some real-time versions of Linux, such as RED Linux. In a low-latency kernel, the maximum latency decreases to the maximum time between two rescheduling points. Since the low-latency patch has been carefully hand-tuned for quite a long time, it performs surprisingly well.

Preemptable Linux

The preemptable approach, used in most real-time systems, removes the constraint of a single execution flow inside the kernel. Thus it is not necessary to disable preemption when an execution flow enters the kernel. To support full kernel preemptability, kernel data must be explicitly protected using mutexes or spinlocks. The Linux preemptable kernel patch, sponsored by MontaVista, uses this approach and makes the kernel fully preemptable. Kernel preemption is disabled only when a spinlock is held.

A similar approach is used by TimeSys, that uses mutexes instead of spinlocks, and provide priority inheritance. While the MontaVista patch disables preemption when a spinlock is acquired, the TimeSys one is based on blocking synchronization.

In a preemptable kernel, the maximum latency is determined by the maximum amount of time for which a spinlock is held inside the kernel. Again, it is important to note that BHs are serialized using a spinlock, thus they contribute to the latency.

An additional patch (lock-breaking) merges some of the low-latency rescheduling points into the preemptable kernel, for decreasing the amount of time for which spinlocks are held.

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