1. COT 4600 Operating Systems Fall 2009
Dan C. Marinescu
Office: HEC 304
Office hours: Tu-Th 3:00-4:00 PM

2. Lecture 29 – Tuesday November 30, 2010
Final exam – T December 9 4-6:50 PM
Last time:
Stack replacement algorithms
The clock algorithm
Today:
Priority inversion
Multi-level queue scheduling
Comparison of scheduling algorithms
Queuing
Next Time:
Review of the class.
Lecture 29

3. Priority scheduling
Each thread/process has a priority and the one with the highest priority (smallest integer  highest priority) is scheduled next.
Preemptive
Non-preemptive
SJF is a priority scheduling where priority is the predicted next CPU burst time
Problem  Starvation – low priority threads/processes may never execute
Solution to starvation  Aging – as time progresses increase the priority of the thread/process
Priority my be computed dynamically
Lecture 29

4. Priority inversion
A lower priority thread/process prevents a higher priority one from running.
T3 has the highest priority, T1 has the lowest priority; T1 and T3 share a lock.
T1 acquires the lock, then it is suspended when T3 starts.
Eventually T3 requests the lock and it is suspended waiting for T1 to release the lock.
T2 has higher priority than T1 and runs; neither T3 nor T1 can run; T1 due to its low priority, T3 because it needs the lock help by T1.
Allow a low priority thread holding a lock to run with the higher priority of the thread which requests the lock
Lecture 29

5. Estimating the length of next CPU burst
Done using the length of previous CPU bursts, using exponential averaging
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6. Exponential averaging
 =0
n+1 = n
Recent history does not count
 =1
n+1 =  tn
Only the actual last CPU burst counts
If we expand the formula, we get:
n+1 =  tn+(1 - ) tn -1 + …
+(1 -  )j  tn -j + …
+(1 -  )n +1 0
Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor
Lecture 29 6

7. Predicting the length of the next CPU burst
Lecture 29 7

8. Multilevel queue
Ready queue is partitioned into separate queues each with its own scheduling algorithm :
foreground (interactive)  RR
background (batch)  FCFS
Scheduling between the queues
Fixed priority scheduling - (i.e., serve all from foreground then from background). Possibility of starvation.
Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e.,
80% to foreground in RR
20% to background in FCFS
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9. Multilevel Queue Scheduling
Lecture 29 9

10. Multilevel feedback queue
A process can move between the various queues; aging can be implemented this way
Multilevel-feedback-queue scheduler characterized by:
number of queues
scheduling algorithms for each queue
strategy when to upgrade/demote a process
strategy to decide the queue a process will enter when it needs service
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11. Example of a multilevel feedback queue exam
Three queues:
Q0 – RR with time quantum 8 milliseconds
Q1 – RR time quantum 16 milliseconds
Q2 – FCFS
Scheduling
A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.
At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.
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12. Multilevel Feedback Queues
Lecture 29 12

13. Unix scheduler
The higher the number quantifying the priority the lower the actual process priority.
Priority = (recent CPU usage)/2 + base
Recent CPU usage  how often the process has used the CPU since the last time priorities were calculated.
Does this strategy raises or lowers the priority of a CPU-bound processes?
Example:
base = 60
Recent CPU usage: P1 =40, P2 =18, P3 = 10
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14. Comparison of scheduling algorithms
Round Robin
FIFO
MFQ
Multi-Level
Feedback Queue
SFJ
Shortest Job First
SRJN
Shortest Remaining Job Next
Throughput
Response
time
May be low is quantum is too small
Shortest average response
time if quantum chosen correctly
Not emphasized
May be poor
Good for I/O bound but poor for CPU-bound
processes
High
Good for short processes
But maybe poor for longer processes
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15. Shortest Remaining Job Next
Round Robin
FIFO
MFQ
Multi-Level
Feedback Queue
SFJ
Shortest Job First
SRJN
Shortest Remaining Job Next
IO-bound
Infinite postponement
No distinction
between
CPU-bound and
Does not occur
Gets a high priority if CPU-bound processes are present
May occur for CPU bound processes
May occur for processes with long estimated running times
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16. Shortest Remaining Job Next
Round Robin
FIFO
MFQ
Multi-Level
Feedback Queue
SFJ
Shortest Job First
SRJN
Shortest Remaining Job Next
Overhead
CPU-bound
Low
No distinction
between
CPU-bound and
IO-bound
The lowest
Can be high Complex data structures and processing routines
Gets a low priority if IO-bound processes are present
Can be high Routine to find to find the shortest job for each reschedule
Can be high Routine to find to find the minimum remaining time for each reschedule
Lecture 29 16