Day 24
Uniprocessor scheduling
Processor scheduling



Schedule processes on the processor to
meet system objectives.
Scheduling involves managing queues to
minimize queuing delay.
Four types:




Long-term
Medium-term
Short-term
I/O scheduling (not this chapter)
Long-Term Scheduling



Determines which programs are admitted to
the system for processing
Controls the degree of multiprogramming
More processes, smaller percentage of time
each process is executed
Medium-Term Scheduling



Part of the swapping function
Which processes to suspend
Based on the need to manage the degree of
multiprogramming
Short-Term Scheduling



Known as the dispatcher
Executes most frequently
Invoked when an event occurs





Clock interrupts
I/O interrupts
Operating system calls
Signals (from semaphores)
New process enters the system
Short-Term Scheduling Criteria

User-oriented



Response time (interactive process)
 Elapsed time between the submission of a request until
there is output
Turnaround time (batch process)
 Elapsed time between submission of a job and its
completion.
System-oriented

Effective and efficient utilization of the processor
 e.g. throughput, processor utilization
Short-Term Scheduling Criteria

Performance-related




Quantitative
Measurable
e.g. response time and throughput
Not performance related



Qualitative
Can’t measure easily
e.g. predictability, fairness

The scheduling criteria are inter-dependent

e.g. a good response time may require a
scheduling algorithm that switches processes
frequently. This would mean a lot of overhead and
hence a lower throughput.
Priorities



Scheduler will always choose a process of
higher priority over one of lower priority
Have multiple ready queues to represent
each level of priority
Lower-priority may suffer starvation

allow a process to change its priority based on its
age or execution history
Algorithm: Selection function




Selection function – which process among
ready processes to select.
w – time spent in system, waiting and
executing, so far
e – time spent in execution, so far
s- total service time required by the process

Service time – the time that the process actually
runs (includes e)
Algorithm: Decision Mode


Non-pre-emptive
 Once a process is in the running state, it will continue until it
terminates or blocks itself for I/O
 Selection function is used only when a process blocks or
terminates
Pre-emptive
 Currently running process may be forced to release the
processor and moved to the Ready state by the operating system
 Allows for better service since any one process cannot
monopolize the processor for very long
 More switching overhead
 Selection function is used when a process blocks or terminates
or when a new process enters the system or when a time slice
expires.