1. Concurrency and Threading: CPU Scheduling
Execution State of a Thread or Process
Schedulers in the OS
Structure of CPU Schedulers
Criteria for Scheduling
Scheduling Algorithms: FIFO, SJF, FP, MLFQ
Multiprocessor Scheduling
Concurrency and Threading: Scheduling

2. State of a Thread/Process
CSCE 410/611 : Operating Systems
State of a Thread/Process
starting
starting
suspended
ready
suspended
ready
ready
ready
running
running
terminated
suspended
blocked
suspended
blocked
blocked
blocked
Concurrency and Threading: Scheduling

3. CSCE 410/611 : Operating Systems
Schedulers
starting
long-term (admission) scheduler
short-term (CPU) scheduler
suspended
ready
ready
running
terminated
suspended
blocked
blocked
medium-term (memory) scheduler
Concurrency and Threading: Scheduling

4. CSCE 410/611 : Operating Systems
Multiprogramming
starting
starting
starting
starting
ready
ready
ready
ready
running
running
running
blocked
blocked
Concurrency and Threading: Scheduling

5. CPU Bursts vs. I/O Bursts
CSCE 410/611 : Operating Systems
CPU Bursts vs. I/O Bursts
starting
short-term (CPU) scheduler
CPU Burst
ready
ready
running
running
I/O “Burst”
blocked
blocked
Concurrency and Threading: Scheduling

6. CSCE 410/611 : Operating Systems
CPU Bursts and Jobs
thread starts
thread terminates
blocked
blocked
CPU burst
CPU burst
CPU burst
Concurrency and Threading: Scheduling

7. CSCE 410/611 : Operating Systems
CPU Bursts and Jobs
thread starts
thread terminates
blocked
blocked
CPU burst
CPU burst
CPU burst
Concurrency and Threading: Scheduling

8. CSCE 410/611 : Operating Systems
CPU Bursts and Jobs
job “starts”
job “terminates”
“Job”
thread starts
thread terminates
blocked
blocked
CPU burst
CPU burst
CPU burst
Concurrency and Threading: Scheduling

9. Scheduling Decisions Decision: “Who is going to use the CPU next?!”4 2
ready
running 3 1
non-preemptive
blocked
preemptive
Scheduling decision points:
1. The running thread changes from running to blocked (current CPU burst of that thread is over).
2. The running thread terminates.
3. A blocked thread becomes ready (new CPU burst of that thread begins).
4. The current thread changes from running to ready .
Concurrency and Threading: Scheduling

10. Structure of a Scheduling Subsystem
CSCE 410/611 : Operating Systems
Structure of a Scheduling Subsystem
ready threads
TCB
ready queue
scheduling subsystem
CPU
Scheduler
Dispatcher
select thread
start new thread ?
Concurrency and Threading: Scheduling

11. What Is a Good Scheduler? Criteria
CSCE 410/611 : Operating Systems
What Is a Good Scheduler? Criteria
User oriented:
Response time : time interval from start of job to first response
Normalized response time: ratio of response time to execution time
Waiting time : sum of periods spent waiting in ready queue
System oriented:
CPU utilization : percentage of time CPU is busy
Throughput : number of jobs completed per time unit
Concurrency and Threading: Scheduling

12. What Is a Good Scheduler? Criteria
CSCE 410/611 : Operating Systems
What Is a Good Scheduler? Criteria
Any good scheduler should:
maximize CPU utilization and throughput
minimize waiting time, response time
Huh?
maximum/minimum values?
average values?
variance?
Concurrency and Threading: Scheduling

13. Scheduling Algorithms
CSCE 410/611 : Operating Systems
Scheduling Algorithms
FCFS : First-come-first-served
SPN: Shortest Process Next
Priority scheduling
RR : Round-Robin
MLFQ: Multilevel Feedback Queue Scheduling
Multiprocessor scheduling
Concurrency and Threading: Scheduling

14. Common Structure of a Scheduler
CSCE 410/611 : Operating Systems
Common Structure of a Scheduler
ready queue
“priority list”
new jobs
CPU
Concurrency and Threading: Scheduling

15. First-Come-First-Served (FCFS/FIFO)
CSCE 410/611 : Operating Systems
First-Come-First-Served (FCFS/FIFO)
ready queue
TCB
head
tail
new jobs
CPU
CPU
Cons:
long average and worst-case waiting times
poor dynamic behavior (convoy effect)
Pros:
very simple
Concurrency and Threading: Scheduling

16. CSCE 410/611 : Operating Systems
Shortest-Job-First
ready queue
new jobs
longest jobs
shortest jobs
CPU
CPU
TCB
Whenever CPU is idle, pick thread with shortest next CPU burst.
Pros:
minimizes average waiting times
Cons:
Starvation of threads with long CPU bursts.
How to determine length of next CPU burst?!
Concurrency and Threading: Scheduling

17. SJF Minimizes Average Waiting Time
CSCE 410/611 : Operating Systems
SJF Minimizes Average Waiting Time
Provably optimal! Proof: swapping of jobs
Plong
Plong
Pshort
Pshort
Pshort
dW = tshort - tlong < 0
Plong
Example: 6 8 12 4
W = = 50 12 8 6 8 12 4 12 4
W = = 46 6 8 12 4 8 4
W = = 38 6 8 12 4 6 4
W = = 34 6 8 12 4
W = = 32
Concurrency and Threading: Scheduling

18. Fixed-Priority Scheduler
CSCE 410/611 : Operating Systems
Fixed-Priority Scheduler
ready queue
new jobs
low priority
high priority
CPU
CPU
TCB
Whenever CPU is idle, pick thread with highest priority.
Cons:
Starvation of low-priority threads
Priority:
thread/process class
urgency
Concurrency and Threading: Scheduling

19. Fixed-Priority Scheduler
CSCE 410/611 : Operating Systems
Fixed-Priority Scheduler
ready queue
new jobs
low priority
high priority
CPU
CPU
TCB
Whenever CPU is idle, pick thread with highest priority.
Cons:
Starvation of low-priority threads
Priority:
thread/process class
urgency
Solution:
Aging: increase priority over time
Concurrency and Threading: Scheduling

20. Fixed Priority Scheduling (implementation)
CSCE 410/611 : Operating Systems
Fixed Priority Scheduling (implementation)
Conceptually
Priority Queues
q=f(p)
priority queue
priority
low priority
CPU
low priority
high priority
CPU
(compare priorities)
Selector
(compare priorities)
Selector
Array of FIFO queues!
high priority
Concurrency and Threading: Scheduling

21. CSCE 410/611 : Operating Systems
Round-Robin
go back to
end of queue!
FIFO with preemption after time quantum
Popular method for time sharing
Choice of time quantum:
large: FCFS
small: “Processor sharing”
Time quantum also defines context-switching overhead
FIFO queue
time quantum
CPU
CPU
terminate
Concurrency and Threading: Scheduling

22. Multilevel Feedback Queue Scheduling
CSCE 410/611 : Operating Systems
(conceptually!)
low priority
RR (quantum=2ms)
RR (quantum=4ms)
RR (quantum=8ms)
FIFO (RR with quantum infinity)
FIFO
FIFO
aging
demotion
(compare priorities)
Selector
FIFO
FIFO
high priority
CPU
Concurrency and Threading: Scheduling

23. Multilevel Feedback Queue Scheduling
CSCE 410/611 : Operating Systems
(conceptually!)
low priority
RR (quantum=2ms)
RR (quantum=4ms)
RR (quantum=8ms)
FIFO (RR with quantum infinity)
FIFO
FIFO
aging
(compare priorities)
Selector
FIFO
FIFO
high priority
CPU
CPU
demotion!
Concurrency and Threading: Scheduling

24. Dynamic Multiprocessor Scheduling
CSCE 410/611 : Operating Systems
Dynamic Multiprocessor Scheduling
global scheduler
CPU
ready queue
CPU
new jobs
CPU
CPU
CPU
Concurrency and Threading: Scheduling

25. Static Multiprocessor Scheduling
CSCE 410/611 : Operating Systems
Static Multiprocessor Scheduling
local scheduler
Partition
ready queue
CPU
ready queue
thread
partitioning
CPU
new threads
ready queue
CPU
ready queue
CPU
Concurrency and Threading: Scheduling

26. CPU Scheduling: Summary
Execution State of a Thread or Process
Schedulers in the OS
Structure of Schedulers
Criteria for Scheduling
Scheduling Algorithms: FIFO, SJF, FP, MLFQ
Multiprocessor Scheduling
Concurrency and Threading: Scheduling