CPU Scheduling: Max CPU Utilization, Three-Level Scheduling, and Algorithms, Slides of Operating Systems

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Lecture 9

Chapter 5: CPU Scheduling (cont)

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 - Alternating Sequence of CPU and I/O Bursts
• Histogram of CPU-burst Times

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
  • Processes keep CPU until it releases either by terminating or I/O wait.
• All other scheduling is preemptive
  • Interrupts

 cooperative scheduling  let a process run until it blocks on I/O, terminates or voluntarily releases the CPU (system call) Ready Running Blocke d

Ready Running Blocke d

 preemptive scheduling  follow clock interrupts (ex: 50Hz) to forcibly switch processes (demote the “Running” to “Ready”)

Two kinds of CPU-scheduling algorithms

Scheduling Criteria

• CPU utilization – keep the CPU as busy as possible - Typically between 40% to 90%
• Throughput – # of processes that complete their execution per time unit - Depends on the length of process
• Turnaround time – amount of time to execute a particular process - Sum of wait for memory, ready queue, execution, and I/O.
• Waiting time – amount of time a processDocsity.com

Scheduling Algorithm Optimization Criteria

• Max CPU utilization
• Max throughput
• Min turnaround time
• Min waiting time
• Min response time
• In most cases, systems optimize average measure

Shortest-Job-First (SJF) Scheduling

• Associate with each process the length of its next CPU burst. - Use these lengths to schedule the process with the shortest time - shortest-next-CPU-burst
• SJF is optimal
  • Gives minimum average waiting time for a given set of processes
  • The difficulty is knowing the length of the next CPU request

A B C D E

Arrival times

Shortest JobFirst (SJF)

SJF^ A^ B^ C^ D^ E^ Mean

Determining Length of Next CPU

Burst

• Can only estimate the length
• Can be done by using the length of previous CPU bursts - using exponential averaging 4. Define : 3. , 0 1 2. predictedvaluefor thenextCPU burst

1. actual lengthof CPU burst

≤ ≤

=

=

α α

τ (^) n 1

tn nth

τ (^) n = 1 =α tn + ( 1 −α) τ (^) n.

 choose the process whose remaining run time is shortestExample of SJF (preemtive)

 allows new short jobs to get good service

A B C D E Mean

A B C D E

Arrival times

Priority Scheduling

• A priority number (integer) is associated with each process
• The CPU is allocated to the process with the highest priority (smallest integer ≡ highest priority) - Preemptive - Nonpreemptive
• SJF is a priority scheduling where priority is

Round Robin (RR)

• Each process gets a small unit of CPU time ( time quantum ), usually 10-100 milliseconds. - After this time has elapsed, the process is preempted and added to the end of the ready queue.
• If there are n processes in the ready queue and the time quantum is q , then each process gets 1/ n of the CPU time in chunks of at most q time units at once. - No process waits more than ( n - 1) q time units.Docsity.com

Examples of RR

A B C D E

Arrival times

A B C D E Mean

A B C D E Mean

Turnaround Time Varies With The Time Quantum

Multilevel Queue

• Ready queue is partitioned into separate queues: - foreground (interactive) - background (batch)
• Each queue has its own scheduling algorithm
  • foreground – RR
  • background – FCFS
• Scheduling must be done between the queues
  • Fixed priority scheduling;