1 Traditional OSes with Soft Real- Time Scheduling Module 3.3 For a good summary, visit:

2 General Purpose Operating Systems with Soft Real-Time Capability A number of modern operating systems include features that support soft real time applications such as multimedia, interactive graphics, any other kind of process that needs very quick attention. As a result, other applications may suffer degraded performance and even starvation, but the benefits usually make such side effects acceptable. The characteristics of these systems are similar to those in hard RTS, but the requirements are less stringent.

3 Characteristics Scheduling must be based on priorities, even if this results in starvation for other processes in the system. Real time processes must not be subject to any kind of aging. Once a real time process has become ready to run, it must be dispatched as quickly as possible; i.e., minimize dispatch latency: the delay between the time a process is ready to run, and the time that it's actually dispatched.

4 Characteristics To achieve quick response, the kernel of the operating system should be preemptible. In nonpreemptible kernels, such as traditional UNIX, the operating system is allowed to finish any operation it begins. This is a way for providing mutual exclusion to system data structures, but it does not satisfy the rapid response criteria. Preemptible kernels contribute to determinism in the form of reduced dispatch latency. Systems may put preemption points into kernel routines at places where system variables are in a consistent state ( i.e., outside of critical sections). Or, the entire kernel may be made preemptible. (e.g., SVR4 and Solaris, respectively.) Priority inversion presents problems: a high priority process (P1) can't run because a lower priority process (P2) has locked a needed resource. At the same time, P2 can't run because P3, a process with priority lower than P1 but higher than P3 wishes to run. Thus P2 is blocking P1, even tho P1 has higher priority than P3.

5 UNIX SVR4 Scheduling Highest preference to real-time processes Next-highest to kernel-mode processes Lowest preference to other user-mode processes

SVR4 Dispatch Queues

7 Windows 2000 Scheduling Priorities organized into two bands or classes –Real-time –Variable Priority-driven preemptive scheduler

10 Linux Scheduling Scheduling classes –SCHED_FIFO: First-in-first-out real-time threads –SCHED_RR: Round-robin real-time threads –SCHED_OTHER: Other, non-real-time threads See handout.