CPU Scheduling
Basic Concept
Maximum CPU utilization obtained with multiprogramming.
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.
The CPU scheduling decisions may take place when a process:
- Switches from running to waiting
- Switches from running to ready (time slot)
- Switches from waiting to ready
- Process terminates
Dispatcher module gives control of the CPU to he process selected by the short-term scheduler, involving:
- Switching context
- Switching to user mode
- Jumping to the proper location in the user program to restart that program
Dispatch latency is the time it takes for the dispatcher to stop one process and start another running.
Scheduling Criteria
CPU utilization: keep the CPU as busy as possible.
Throughput: number of processes that complete their execution per time unit.
Turnaround: amount of time to execute a particular process.
Waiting time: amount of time a process has been waiting in the ready
Scheduling Algorithms
First-Come First-Server
Shortest Job First
Shortest Remaining Time First
Priority Scheduling
A priority number is associated with each process. The CPU is allocated to the process with the highest priority.
And there also can be two ways:
- Preemptive
- Nonpreemptive
The shortest job first scheduling is also a priority scheduling where priority is the predicted next CPU burst time.
But the problem is the low priority process may be never execute. We can age every process to make low priority process can be executed after a long time, though calculating priority every time may waste resources.
Round Robin
Multilevel Queue
Ready queue is partitioned into two queues:
- Foreground queue:
FCFS - Background queue:
RR