Scheduling
COMP755 Advanced OS 1
Scheduling
COMP450 Operating Systems
Goals of Scheduling
• User Oriented
– minimize response time (interactive)
– minimize Turnaround time (batch jobs)
– Meet deadlines
• System Oriented
– Predictable
– maximize Throughput
– Processor Utilization
–Fair
– Enforce priorities
– Balance resources
– Balance user groups
Levels of Scheduling
• Long Term scheduling
• Medium Term scheduling
• Short Term scheduling
• I/O scheduling
Long-term scheduling
• Only relevant for systems that have a
backlog of batch jobs to run
• Deals with creating a new process
• Controls the degree of multiprogramming
• Interactive systems tend to accept users
unless the system is swamped
• The more processes, the smaller
percentage of time each process is
executed
This is all we are going to say about long-term scheduling
Medium-term scheduling
• The decision to add to the number of
processes that are partially or fully in main
memory
• Deals with swapping processes in and out
• A swapped out process cannot be executed
until it is swapped back into RAM
Medium-term scheduling will be discussed when we talk about
memory management.
Short-Term Scheduling
• Determines which process is going to execute
next
• The short term scheduler is known as the
dispatcher
• Is invoked on an event that may lead to choosing
another process for execution:
– clock interrupts
– I/O interrupts
– operating system calls and traps
– signals
Short Term Scheduling is the subject of this chapter
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