ENVS 1800 Lecture 16: ENVS 1800 Lecture 16 Notes
14 May 2018
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ENVS 1800 Lecture 16 Notes – High-Level Scheduler
Introduction
• As a way to optimize system performance, the CPU scheduling task is separated into two
different phases.
• The high-level, or long-term, scheduler is responsible for admitting processes to the
system.
• The dispatcher provides short-term scheduling, specifically, the instant-by-instant
decision as to which one of the processes that are ready should be given CPU execution
time.
• The dispatcher also performs context switching.
• Some systems also include a third, middle-level scheduler, which monitors system
performance.
• When present, the middle-level scheduler can suspend, or swap out, a process by
removing it from memory temporarily and replace it with another waiting process.
• This operation is known as swapping. Swapping is done to improve overall system
performance.
• It would be used if a particular process were hogging a resource in such a way as to
prevent other processes from executing.
• The high-level scheduler determines which processes are to be admitted to the system.
• The role of the high-level scheduler is minimal for processes created in an interactive
environment.
• Such processes are usually admitted to the system automatically.
• If a user requests a service that requires the creation of a new process, the high-level
scheduler will attempt to do so unless the system is seriously overloaded.
• To refuse the user in the middle of her or his work would be undesirable.
• The high-level scheduler will refuse a login process, however, if it appears that doing so
would overload the system.
• Long-term, scheduler is responsible for admitting processes to the system.
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