Unit2 CPU Scheduling 24252 (sssssss1).ppt

Unit 2: CPU/ Process Scheduling

Basic Concept
 Imagine you're at a busy cafe,
 Orders must be managed efficiently, ensuring prompt
customer service.
 Similarly, Modern operating systems rely on CPU
scheduling,
 optimizing task execution and resource utilization
efficiently.
 This effectively determines CPU task order, balancing
performance, fairness, and responsiveness.

Basic Concept
 CPU Scheduling is:
 Basis of Multi-programmed OS
 Objective of Multi-programming:
 To have some processes running all the time to
maximize CPU Utilization.

Sequence of CPU and I/O Bursts

CPU Scheduling Decisions
1. When process switches from
Running State to Waiting State
(i/o request or wait)
Running to Ready State
(interrupt)
Waiting State to Ready State
(at completion of i/o)
4. When a process terminates
1. Non-preemptive
2. Pre-emptive
3. Pre-emptive
4. Non-Preemptive
(allow)

Scheduling
 Non-Preemptive
 Preemptive

Non-Preemptive
 Once CPU is allocated to process,
 process retains CPU until completion.
 It may switch to a waiting state.
 Example: Windows 3.x, Apple Macintosh systems.
 Process executes till it finishes.
 Example: First In, First Out scheduling.

Preemptive Scheduling
 The running process is interrupted briefly.
 It resumes once priority task finishes.
 CPU or resources are taken away.
 A high-priority process needs immediate
execution.
 Lower-priority processes must wait for turn.
 This ensures critical tasks are completed
efficiently.

Dispatcher
 Dispatcher manages CPU control, enabling process
execution.
 Functions include context switching, user mode
switching.
 Restarts program at proper memory location.
 Dispatcher must operate efficiently, minimizing delays.
 Invoked during every process switch promptly.
 Dispatch latency measures process-switch time
duration.

Scheduling Criteria
 Which algorithm to use in a particular situation
1. CPU Utilization: CPU should be busy to the fullest
2. Throughput: No. of processes completed per unit of
time.
3. Turnaround Time: The time interval from submitting a
process to the time of completion.
Turnaround Time= Time spent to get into memory + waiting
in ready queue + doing I/O + executing on CPU
(It is the amount of time taken to execute a particular process)

Scheduling Criteria
4. Waiting Time: Time a process spends in a ready queue.
Amount of time a process has been waiting in the ready
queue to acquire control on the CPU.
5. Response Time: Time from the submission of a request
to the first response, Not Output
6. Load Average: It is the average number of processes
residing in the ready queue waiting for their turn to get
into the CPU.

Scheduling Algorithm Optimization Criteria
 Max CPU utilization
 Max throughput
 Min turnaround time
 Min waiting time
 Min response time

Formula
Turn around time= Completion Time- Arrival Time
Waiting Time= Turn around Time-Burst Time
OR
Turnaround time = Burst time + Waiting time

First-Come, First-Served (FCFS)
 Processes that request CPU first, are allocated the CPU
first
 It is non-preemptive scheduling algorithm
 FCFS is implemented with FIFO queue.
 A process is allocated the CPU according to their arrival
times.

First-Come, First-Served (FCFS)
 When process enters the ready queue,
 its PCB is attached to the Tail of queue.
 When CPU is free,
 it is allocated to the process selected from Head/Front
of queue.
 FCFS leads to Convoy Effect

First-Come, First-Served (Example)
 Example: Consider three processes arrive in order
P1, P2, and P3.
 P1 burst time: 24
 Draw the Gantt Chart and compute Average Waiting
Time and Average Turn Around Time.
Sol:
19

Process A T BT CT TAT WT
P1 0 2
P2 3 1
P3 5 6
Calculate avg.
CT, TAT, WT

Process A T BT CT TAT WT
P1 0 4
P2 1 3
P3 2 1
P4 3 2
P5 4 5
H.W
Calculate avg.
CT, TAT,WT

(2) Shortest Job First
 Processes with least burst time (BT) are selected.
 CPU assigned to process with less BT.
 SJF has two types:
1. Non-Preemptive.
 CPU stays until process completion only.

(2) Shortest Job First
2. Preemption:
New process enters the scheduling queue.
Scheduler compares execution time with current.
 A shorter process preempts an already running process.
Efficient scheduling ensures minimized waiting time.

Shortest Job First(Preemptive)
Q1. Consider foll. Processes with A.T and B.T
Process A.T B.T
P1 0 4
P2 0 6
P3 0 4
Cal. Completion time, turn around time and avg. waiting time.
SJF(Pre-emptive)-> SRTF

Process A.T B.T
P1 0 9
P2 1 4
P3 2 9

Process A.T B.T
P1 0 5
P2 1 7
P3 3 4

Process A.T B.T
P1 0 5
P2 1 3
P3 2 3
P4 3 1

Shortest Job First (Non-Preemption)
Processes arrived at the same time.
29

Process A.T B.T
P1 1 7
P2 2 5
P3 3 1
P4 4 2
P5 5 8
Practice: Shortest Job First (Non Preemption)

SRTF Example
CPU idle time: (idle time/ total time spent) * 100

SRTF Example
Consider three processes, all arriving at time zero, with total
execution time of 10, 20 and 30 units, respectively. Each
process spends the first 20% of execution time doing I/O, the
next 70% of time doing computation, and the last 10% of time
doing I/O again. The operating system uses a shortest
remaining compute time first scheduling algorithm and
schedules a new process either when the running process gets
blocked on I/O or when the running process finishes its
compute burst. Assume that all I/O operations can be
overlapped as much as possible. For what percentage of time
does the CPU remain idle?
(A) 0% (B) 10.6% (C) 30.0% (D) 89.4%

SRTF Example
Explanation: Let three processes be p0, p1 and p2. Their execution time is 10, 20
and 30 respectively. p0 spends first 2 time units in I/O, 7 units of CPU time and
finally 1 unit in I/O. p1 spends first 4 units in I/O, 14 units of CPU time and finally 2
units in I/O. p2 spends first 6 units in I/O, 21 units of CPU time and finally 3 units in
I/O.
idle p0 p1 p2 idle
0 2 9 23 44 47
Total time spent = 47
Idle time = 2 + 3 = 5
Percentage of idle time = (5/47)*100 = 10.6 %

Priority Scheduling
 Priority is associated with each process.
 CPU is allocated to the process with highest priority.
 If 2 processes have same priority  FCFS
Disadvantage: Starvation (Low priority Processes wait for
long)
Solution of Starvation:
Aging: The priority of a process is gradually increased over
time to prevent it from starving (waiting indefinitely).

Priority Scheduling (Preemptive)
Process Arrival
Time
Priority Burst
Time
Completion
Time
P1 1 5 4
P2 2 7 2
P3 3 4 3
Consider 4 as
Highest and 7 as
Lowest Priority

Process Arrival
Time
Priority Burst
Time
Completion
Time
P1 0 2 10
P2 2 1 5
P3 3 0 2
P4 5 3 20
Consider 0 as
Lowest and 3
as Highest
Priority

Process Arrival
Time
Priority Burst
Time
Completion
Time
P1 0 2 10
P2 2 1 5
P3 3 0 2
P4 5 3 20
Consider 3 as
Lowest and 0
as Highest
Priority

Round Robin Scheduling
 A Time Quantum is associated to all processes
 Time Quantum: Maximum amount of time for which
process can run once it is scheduled.
 RR scheduling is always Pre-emptive.

Round Robin
Process Arrival
Time
Burst
Time
Completion
Time
P1 0 5
P2 1 7
P3 2 1
TQ: 2
Process Arrival
Time
Burst
Time
Completion
Time
P1 0 3
P2 3 4
P3 4 6

Round Robin
Process Arrival
Time
Burst
Time
Completion
Time
P1 0 4
P2 1 5
P3 2 2
P4 3 1
P5 4 6
P6 6 3
TQ: 2

Multilevel Queue Scheduling
Partition the ready queue into several separate queues.
For Example: a multilevel queue scheduling algorithm with
five queue:
1. System processes
2. Interactive processes
3. Interactive editing processes
4. Batch processes
5. Student/ user processes

Multilevel Queue
 A process moves between various queues.
 Multilevel Queue Scheduler defined by parameters.
 Number of queues is defined here.
 Scheduling algorithms for each queue specified.
 Method determines when to upgrade process.
 Method decides which queue gets service.

Multilevel Queue
1. System Processes: These are programs that belong to OS
(System Files)
2. Interactive Processes: Real-Time Processes e.g. playing
games online, listening to music online, etc.
3. Batch Processes: Lots of processes are pooled and one
process at a time is selected for execution. I/O by Punch Cards
4. Student/User Processes

Multilevel Queue
 No process in the batch queue could run unless the other
queues for the system were empty.
 If an interactive editing process entered the ready queue
while a batch process was running, the batch process would
be preempted.

Multilevel Queue
 Processes can be :
 Foreground Process: processes that are running
currently
 Background Process: Processes that are running in the
background but their effects are not visible to the user.
 Processes are permanently assigned to one queue on some
properties:
 memory size, process priority, process type.
 Each queue has its own scheduling algorithm

Multilevel Queue
 Different types of processes exist, So
 Cannot apply same scheduling algorithm.
Disadvantages:
1. Until high priority queue is not empty,
2. No process from lower priority queues will be selected.
3. Starvation for lower priority processes.
Advantage:
Can apply separate scheduling algorithm for each queue.

Multilevel Feedback Queue
 Solution is: Multilevel Feedback Queue
 If a process is taking too long to execute..
 Pre-empt it send it to low priority queue.
 Don’t allow a low priority process to wait for long.
 After some time move the least priority process to
high priority queue  Aging

Multi-processor Scheduling
Concerns:
• Multiple CPUs enable load sharing.
• Focus is on homogeneous processors' systems.
• Use any processor for any process.
• Various types of limitations exist.
• Systems with I/O devices have restrictions.
• Devices must run on specific processors.

Unit2 CPU Scheduling 24252 (sssssss1).ppt

Approaches to Multiple-Processor Scheduling
1. Asymmetric multiprocessing
•Scheduling decisions, I/O processing handled by server.
•Other processors execute only user code.
•One processor accesses system data structures.
•Reduces the need for data sharing.
•Single processor controls system activities efficiently.
•Improves data management, reducing conflicts.

Approaches to Multiple-Processor Scheduling
 2. Symmetric multiprocessing (SMP)
 Each processor is self-scheduling and efficient.
 All processes may share a common queue.
 Each processor may have a private queue.
 Schedulers examine queues and select processes.
 Modern operating systems widely support SMP.
 Like, Windows, Linux, Mac OS X included.

Issues concerning SMP systems
 1. Processor Affinity
(a process has an affinity for the processor on which it is currently
running.)
 Consider what happens to cache memory when a process
has been running on a specific processor.
The data most recently accessed by the process populate
the cache for the processor.
As a result, successive memory accesses by the process are
often in cache memory.

1. Processor Affinity
 If the process migrates to another processor.
 The contents of cache memory must be invalidated for the
first processor,
and the cache for the second processor must be repopulated.

1. Processor Affinity
High cost of invalidating and repopulating caches.
Most SMP systems avoid process migration entirely.
They attempt to keep processes running consistently.
This concept is called processor affinity generally.
A process has affinity for its processor.
It runs on the same processor continuously.

Forms of Processor Affinity
1. Soft affinity
• An OS tries keeping processes on the same processor.
• This is called soft affinity policy.
2. Hard affinity
• The OS tries to keep processes stable.
• Processes may migrate between different processors.

2. Load Balancing
•On SMP systems, workload balance is important.
•All processors should be fully utilized efficiently.
•Load balancing prevents some processors from idling.
•High workloads may burden specific processors excessively.
•Idle processors waste system resources and efficiency.
•Balanced workloads maximize the system's overall
performance.

2. Load Balancing
Two approaches to load balancing: push migration and pull
migration.
1. Push migration: a specific task periodically checks the
load on each processor and—if it finds an imbalance—evenly
distributes the load by moving (or pushing) processes from
overloaded to idle or less-busy processors.
2. Pull migration: occurs when an idle processor pulls a
waiting task from a busy processor.

3. Multicore Processors
•A multi-core processor has independent cores.
•It reads and executes program instructions.
•A CPU may contain multiple cores.
•A core includes registers and cache.
•Core handles processor tasks, not entire.
•It's essential for efficient task performance.

3. Multicore Processors
Multicore processors may complicate scheduling issues:
When a processor accesses memory, it spends a significant
amount of time waiting for the data to become available. This
situation, known as a memory stall.
Memory Stall may occur due to a cache miss (accessing
data that are not in cache memory).

Memory Stall
the processor can spend up to 50 percent of its time waiting for
data to become available from memory.

Real Time Scheduling
Analysis and testing of the scheduler system and the
algorithms used in real-time applications
a) soft real-time systems b) hard real-time systems
a)Soft real-time systems provide no guarantee as to when
a critical real-time process will be scheduled.
b)Hard real-time systems have stricter requirements. A
task must be serviced by its deadline;

Rate-Monotonic Scheduling
• Rate-monotonic scheduling assigns tasks' priorities statically.
• It uses a preemption-based scheduling policy efficiently.
• RMS operates in real-time operating systems seamlessly.
• Static priorities depend on task cycle duration.
• Shorter cycles lead to higher job priority.
• This ensures periodic tasks are scheduled optimally.

Rate-Monotonic Scheduling
rate-monotonic scheduling assumes that the processing time of
a periodic process is the same for each CPU burst.
For example
Consider P1, P2 with time period 50,100 resp. and B.T 20,35
resp.
Cal. CPU utilization of each process and total CPU
utilization.
Sol:
CPU utilization=(Burst time/Time period)= (Ti/Pi)
For P1: (20/50)=0.40 i.e 40%
For P2: (35/100)=0.35 i.e 35%
Total CPU utilization is 75%

Earliest Deadline First Scheduling
Earliest-deadline-first (EDF) scheduling dynamically assigns
priorities according to deadline.
The earlier the deadline, the higher the priority the later the
deadline, the lower the priority.
Under the EDF policy, when a process becomes runnable, it
must announce its deadline requirements to the system.

Unit2 CPU Scheduling 24252 (sssssss1).ppt

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