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CPU Scheduling
• Basic Concepts• Scheduling Criteria
• Scheduling Algorithms
• Multiple-Processor Scheduling
• Real-Time Scheduling
• Algorithm Evaluation
Basic Concepts
• Maximum CPU utilization obtained with
multiprogramming
• CPU–I/O Burst Cycle – Process execution
consists of a cycle of CPU execution and I/O
wait.
• CPU burst distribution
CPU Scheduler
• Selects from among the processes in memory
that are ready to execute, and allocates the
CPU to one of them.
• CPU scheduling decisions may take place when
a process:
1. Switches from running to waiting state.
2. Switches from running to ready state.
3. Switches from waiting to ready.
4. Terminates.
• Scheduling under 1 and 4 is nonpreemptive.
• All other scheduling is preemptive.
Dispatcher
• Dispatcher module gives control of the CPU to
the process selected by the short-term
scheduler; this involves:
– switching context
– switching to user mode
– jumping to the proper location in the user program
to restart that program
• Dispatch latency – time it takes for the
dispatcher to stop one process and start
another running.
Scheduling Criteria
• CPU utilization – keep the CPU as busy aspossible
• Throughput – # of processes that complete
their execution per time unit
• Turnaround time – amount of time to execute
a particular process
• Waiting time – amount of time a process has
been waiting in the ready queue
• Response time – amount of time it takes from
when a request was submitted until the first
response is produced, not output (for timesharing
environment)
Optimization Criteria
• Max CPU utilization• Max throughput
• Min turnaround time
• Min waiting time
• Min response time
Scheduling Criteria
• CPU utilization – keep the CPU as busy as possible
• Throughput – # of processes that complete their
execution per time unit
• Turnaround time – amount of time to execute a
particular process
• Waiting time – amount of time a process has been
waiting in the ready queue
• Response time – amount of time it takes from when a
request was submitted until the first response is
produced, not output (for time-sharing environment)
Short-term scheduler is invoked very frequently
(milliseconds) (must be fast).
Long-term scheduler is invoked very infrequently
(seconds, minutes) (may be slow).
The long-term scheduler controls the degree of
multiprogramming.
Processes can be described as either:
I/O-bound process – spends more time doing I/O than
computations, many short CPU bursts.
CPU-bound process – spends more time doing
computations; few very long CPU bursts.
Context Switch
When CPU switches to another process, the
system must save the state of the old process
and load the saved state for the new process.
Context-switch time is overhead; the system
does no useful work while switching.
Time dependent on hardware support.
Process Creation
• Parent process create children processes, which, in
turn create other processes, forming a tree of
processes.
• Resource sharing
– Parent and children share all resources.
– Children share subset of parent’s resources.
– Parent and child share no resources.
• Execution
– Parent and children execute concurrently.
– Parent waits until children terminate.
• Address space
– Child duplicate of parent.
– Child has a program loaded into it.
• UNIX examples
– fork system call creates new process
– exec system call used after a fork to replace the
process’ memory space with a new program.