Rfrtos - Linux-based Qos Real-time Operating System

Real Time operating system is inherently different from general-purpose operating system in that the correctness of system's behaviors depends not only on what can be generated, but on when the result is produced, i.e., timely correctness. Those traditional real time systems are generally hard real time systems, which are required to not only generate correct results but meet all time constraints as well. So they are very specially designed and only adapted to such fields as medical equipment, nuclear plant controlling, aerospace etc. On the other hand, with the developing of computer technique and the increasingly available peripheral resources, those functions required only on general-purpose system platform are needed by embedded system as well. At the same time, some systems originally designed for desktop application are now demanded to satisfy real time constraints of some applications, for which a new filed of real time, soft real time, is created.Soft real time applications are not so rigid as hard real time ones in timely correctness, allowing for some cases of missing deadline, but they may require varieties of system services, including file system, network, database etc. But comparing to hard real time system, soft real time ones apply so loose rules in resource management that sometimes the real time property an application requires cannot be guaranteed at all. For the purpose of meeting the needs of industry embedded systems, multi-media applications etc, this paper brings forward the concept of OoS oriented real time system on the bases of research of both correspondent real time system and the inherent characteristics of Linux.QoS oriented real time system not only provides as various system services as general-purpose one, but also presents more comfortable developing platforms than traditional real time system, though the accuracy of system service such as timer management, interrupt management and schedule etc are close to the later. RFRTOS, Linux based QoS real time system, is developed by following the designing philosophy.I'm responsible for three parts of tasks during the development of RFRTOS: Improving Kernel Prememptability. QoS real time system provide full system services whose timing response may not be so predictable from the view point of traditional real time and thus restricted to be used by the later. In this case, kernel preemptability becomes one of the dominant factors for system service granularity, which is one of the key concerns ofRFRTOS. RFRTOS provides kernel preemptability at interrupt level by employing following methods: Tasked kernel services. Linux is designed on the marcro kernel philosophy; in which interrupt service, bottom half and signal processing are all kernel procedures of highest priority and without any task spaces. Though macro kernel can present more high responsive property, which is one of main concern of many general-purpose systems, applications with time constraints may be delayed for an unpredictable duration of time because of the coarse macro kernel services' granularity. While tasked kernel services make the background system jobs visible to the scheduler, as a result of which the behaviors of system are more controllable and thus the system responsive property is improved. Semaphore and priority inheritance protocol(PIP) or priority ceiling protocol(PCP) supporting. Sometimes mutual exclusion can give rise to priority inversion, which in turn may cause unpredictable delay of high priority tasks. As a solution, semaphore and PIP or PCP based on this mutex mechanism must be implemented. Scheduling entry point at hardware interrupts service level. Task scheduling only occurs at kernel service level in Linux, whose designing philosophy conforms to macro kernel, in which schedule won't be checked when system runs in kernel services. On the basis of tasked kernel services, RFRTOS, on the other hand, increases the granulites of schedule by checking the need for schedule once interrupt services...