Traditional OSes with Soft Real-Time Scheduling

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Traditional OSes with Soft Real-Time Scheduling

• Module 3.3
• For a good summary, visit
• http//www.cs.uah.edu/weisskop/osnotes_html/M8.ht
  ml

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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.

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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.

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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.

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UNIX SVR4 Scheduling

• Highest preference to real-time processes
• Next-highest to kernel-mode processes
• Lowest preference to other user-mode processes

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SVR4 Dispatch Queues
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Windows 2000 Scheduling

• Priorities organized into two bands or classes
• Real-time
• Variable
• Priority-driven preemptive scheduler

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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.