cpu scheduling - uni department of computer …multilevel feedback queues multilevel feedback queues...
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CPU Scheduling
Sarah Diesburg Operating Systems CS 3430
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CPU Scheduler
A CPU scheduler is responsible for Removal of running process from the CPU Selection of the next running process
Based on a particular strategy
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Goals for a Scheduler
Maximize CPU utilization: keep the CPU as busy as
possible Throughput: the number of processes
completed per unit time
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Goals for a Scheduler
Minimize Response time: the time of submission to the
time the first response is produced Wait time: total time spent waiting in the ready
queue Turnaround time: the time of submission to the
time of completion
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Goals for a Scheduler
Suppose we have processes A, B, and C, submitted at time 0
We want to know the response time, waiting time, and turnaround time of process A
A B C A B C A C A C Time
response time = 0 + + wait time
turnaround time
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Goals for a Scheduler
Suppose we have processes A, B, and C, submitted at time 0
We want to know the response time, waiting time, and turnaround time of process B
A B C A B C A C A C Time
response time + wait time
turnaround time
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Goals for a Scheduler
Suppose we have processes A, B, and C, submitted at time 0
We want to know the response time, waiting time, and turnaround time of process C
A B C A B C A C A C Time
response time + + + wait time
turnaround time
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Goals for a Scheduler
Achieve fairness There are tensions among these goals
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Assumptions
Each user runs one process Each process is single threaded Processes are independent
They are not realistic assumptions; they
serve to simplify analyses
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Scheduling Policies
FIFO (first in, first out) Round robin SJF (shortest job first) Multilevel feedback queues Lottery scheduling
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FIFO
FIFO: assigns the CPU based on the order of requests Nonpreemptive: A process keeps running on a
CPU until it is blocked or terminated Also known as FCFS (first come, first serve) + Simple - Short jobs can get stuck behind long jobs
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Round Robin
Round Robin (RR) periodically releases the CPU from long-running jobs Based on timer interrupts so short jobs can get a
fair share of CPU time Preemptive: a process can be forced to leave its
running state and replaced by another running process
Time slice: interval between timer interrupts
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More on Round Robin
If time slice is too long Scheduling degrades to FIFO
If time slice is too short Throughput suffers Context switching cost dominates
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More on Round Robin
Round robin based on FIFO Gives time slice to process that has waited
longest to run Used to choose next process to run
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More on Round Robin
Suppose we have three jobs of equal length
Which job goes next?
A B C A Time
Round Robin
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A B C A Time
Round Robin
B
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FIFO vs. Round Robin
With zero-cost context switch, is RR always better than FIFO?
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FIFO vs. Round Robin
Suppose we have three jobs of equal length
A B C A B C Time A B C turnaround time of A turnaround time of B turnaround time of C
Round Robin
A B Time C turnaround time of A turnaround time of B turnaround time of C
FIFO
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FIFO vs. Round Robin
Round Robin + Shorter response time + Fair sharing of CPU - Not all jobs are preemptive - Not good for jobs of the same length
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Shortest Job First (SJF)
SJF runs whatever job puts the least demand on the CPU, also known as STCF (shortest time to completion first) + Provably optimal + Great for short jobs + Small degradation for long jobs
Real life example: supermarket express checkouts
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SJF Illustrated
A B Time C
turnaround time of A turnaround time of B turnaround time of C
Shortest Job First
response time of A = 0 response time of B response time of C
wait time of A = 0 wait time of B wait time of C
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Shortest Remaining Time First (SRTF) SRTF: a preemptive version of SJF If a job arrives with a shorter time to completion,
SRTF preempts the CPU for the new job Also known as SRTCF (shortest remaining time
to completion first) Generally used as the base case for comparisons
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SJF and SRTF vs. FIFO and Round Robin If all jobs are the same length, SJF FIFO FIFO is the best you can do
If jobs have varying length Short jobs do not get stuck behind long jobs under
SRTF
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A More Complicated Scenario (Arrival Times = 0) Process A (6 units of CPU request) 100% CPU 0% I/O
Process B (6 units of CPU request) 100% CPU 0% I/O
Process C (infinite loop) 33% CPU 67% I/O
A
Time
B
Time
C
C Time
C
C
C
C
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A More Complicated Scenario
FIFO CPU I/O
Round Robin with time slice = 3 units CPU I/O
A B C
C
Time Poor response and wait time for process C
A B C
C
A B C
C
Time Disk utilization: 29% (2 out of 7 units)
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A More Complicated Scenario
Round Robin with time slice = 1 unit CPU I/O
SRTF CPU I/O
Time
A B A B C
C
C A B
C
C A B
C
C A B
C
C A B
C
Disk utilization 66% (2 out of 3 units)
Time
C
C A
C
C A C
C
A C
C
B C
C
B
Disk utilization: 66% (2 out of 3 units)
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Drawbacks of Shortest Job First
- Starvation: constant arrivals of short jobs can keep long ones from running
- There is no way to know the completion time of jobs (most of the time) Some solutions
Ask the user, who may not know any better If a user cheats, the job is killed
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Priority Scheduling (Multilevel Queues) Priority scheduling: The process with the
highest priority runs first Priority 0: Priority 1: Priority 2: Assume that low numbers represent high
priority
A
B
C
A B Time C
Priority Scheduling
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Priority Scheduling
+ Generalization of SJF With SJF, priority = 1/requested_CPU_time
- Starvation
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Multilevel Feedback Queues
Multilevel feedback queues use multiple queues with different priorities Round robin at each priority level Run highest priority jobs first Once those finish, run next highest priority, etc Jobs start in the highest priority queue If time slice expires, drop the job by one level If time slice does not expire, push the job up by
one level
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Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
A B C
time = 0
Time
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Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
B C
time = 1
A
A Time
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Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
C
time = 2
A B
A B Time
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Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
C
time = 3
A B
A B C Time
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Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
C
time = 3
A B
A B C Time
suppose process A is blocked on an I/O
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Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
C
time = 3
A
B
A B C Time
suppose process A is blocked on an I/O
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Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 5
B A B C Time
C
A
suppose process A is returned from an I/O
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Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 6
B A B C Time
C
A
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Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 8
B A B C Time A C
C
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Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 9
B A B C Time A C C
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Multilevel Feedback Queues
Approximates SRTF A CPU-bound job drops like a rock I/O-bound jobs stay near the top Still unfair for long running jobs Counter-measure: Aging
Increase the priority of long running jobs if they are not serviced for a period of time
Tricky to tune aging
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Lottery Scheduling
Lottery scheduling is an adaptive scheduling approach to address the fairness problem Each process owns some tickets On each time slice, a ticket is randomly picked On average, the allocated CPU time is
proportional to the number of tickets given to each job
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Lottery Scheduling
To approximate SJF, short jobs get more tickets
To avoid starvation, each job gets at least one ticket
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Lottery Scheduling Example
short jobs: 10 tickets each long jobs: 1 ticket each # short jobs/# long jobs
% of CPU for each short job
% of CPU for each long job
1/1 91% 9% 0/2 0% 50% 2/0 50% 0%
10/1 10% 1% 1/10 50% 5%
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