Round Robin RR Scheduling

1
Round Robin (RR) Scheduling

• Designed for time-sharing systems
• The algorithm
• Each process gets a small unit of CPU time (time
  quantum or time slice), usually 10-100
  milliseconds.
• After the time quantum has elapsed, the process
  is preempted and added to the end of the ready
  queue (an FIFO queue).
• Essentially FCFS scheduling with preemption
• If there are n processes in the ready queue and
  the time quantum is q, then response time is no
  more than (n-1)q
• Smaller q ? smaller response time, but more
  context switches
• Performance of RR depends on size of time quantum
• q large same as FCFS
• q small each of the n processes has the
  impression that it has its own processor running
  at 1/n the speed of the real processor
• q must be large with respect to context switch
  time, otherwise overhead is too high.

2
Example of RR with Time Quantum 20

• Process Burst Time
• P1 53
• P2 17
• P3 68
• P4 24
• The Gantt chart is

3
Time Quantum and Context Switch Time
4
Turnaround Time Varies With The Time Quantum
Rule of thumb 80 of CPU bursts should be
shorter than the time quantum
5
Multilevel Queue Scheduling

• Ready queue is partitioned into separate queues
  each process is permanently assigned to one queue
  based on some property of the process (e.g.,
  process type)
• E.g., separate queues for foreground
  (interactive) processes and background (batch)
  processes
• Each queue has its own scheduling algorithm
• E.g., foreground RR, background FCFS
• Scheduling must be done between the queues.
• Fixed priority preemptive scheduling
• E.g., foreground queue has absolute priority over
  background queue
• Possibility of starvation.
• Time slice between the queues each queue gets a
  certain amount of CPU time
• E.g., 80 CPU time to foreground in RR, 20 to
  background in FCFS

6
Example of Multilevel Queue Scheduling
7
Multilevel Feedback Queue Scheduling

• A process can move between the various queues
• A process will be moved to a lower-priority queue
  if it uses too much CPU time
• A process that waits too long in a lower-priority
  queue may be moved to a higher priority queue ?
  prevent starvation
• Multilevel feedback queue scheduler defined by
  the following parameters
• number of queues
• scheduling algorithm for each queue
• when to upgrade a process to a higher-priority
  queue
• when to demote a process to a lower priority
  queue
• how to select the initial queue for a process
• The most general and most complex scheduling
  algorithm

8
Example of Multilevel Feedback Queue

• Three queues with fixed priority preemptive
  scheduling among them
• Q0 (highest priority) RR, time quantum 8
  milliseconds
• Q1 RR, time quantum 16 milliseconds
• Q2 (lowest priority) FCFS
• Scheduling algorithm
• A new job enters Q0 , which is served FCFS. When
  the job gains CPU, it receives 8 milliseconds.
  If it does not finish in 8 milliseconds, it is
  moved to the tail of Q1.
• At Q1 job is again served FCFS and receives 16
  additional milliseconds. If it does not
  complete, it is preempted and moved to Q2.

9
Example of Multilevel Feedback Queues