Process Management
Process Management
Operating System Services
1.
Program execution
2.
I/O operations
3.
File system manipulation
4.
Communication
5.
Error detection
6.
Resource allocation
7.
Protection
Process Management
Operating System Services
Program execution
•
The operating system Loads program into
memory and then:
1.
It Handles the program’s execution
2.
It Provides a mechanism for process
synchronization
3.
It Provides a mechanism for process communication
Process Management
Operating System Services
Program execution
•
The operating system Loads program into
memory and then:
1.
It Handles the program’s execution
2.
It Provides a mechanism for process
synchronization
3.
It Provides a mechanism for process communication
Process Management
Operating System Services
Program execution
•
The operating system Loads program into
memory. It also Executes the program
1.
It Handles the program’s execution
2.
It Provides a mechanism for process
synchronization
3.
It Provides a mechanism for process communication
Process Management
Process
Text section
program code
value of the program counter
Stack
contains temporary data (such as function parameters,
return addresses, and local variables)
data section
contains global variables
Heap
dynamically allocated during process run time
Process Management
Process
A program becomes a
process when an
executable file is loaded
into memory.
Process:
A process is a program at the time of execution.
Process
Program
Process is a dynamic object
Program is a static object
Process is sequence of instruction
execution
Program is a sequence of instructions
Process loaded into main memory
Program loaded into secondary storage
Devices
Time span of process is limited
Time span of program is unlimited
Process is a active entity
Program is a passive entity
Process Management
Process State:
1. The state of a process is defined in part by the
current activity of that process.
Process Management
Process State:
1. The state of a process is defined in part by the
current activity of that process.
2. A process can be in one of the following 5 states
1. New. The process is being created.
2. Running. Instructions are being executed.
3. Waiting. The process is waiting for some event to occur
(such as an I/0 completion or reception of a signal).
4. Ready. The process is waiting to be assigned to a
processor.
5. Terminated. The process has finished execution.
Process Management
Threads
A process is divided into number of light weight
process, each lightweight process is said to be a
Thread.
Components:
1. program counter (Keeps track of which instruction
to execute next)
2. registers (holds its current working variables),
3. stack (execution History)
Thread
States
1. born State : A thread is just created.
2. ready state : The thread is waiting for CPU.
3. running : System assigns the processor to the
thread.
4. sleep : A sleeping thread becomes ready after the
designated sleep time expires.
5. dead : The Execution of the thread finished.
Thread
Examples of Thread:
Word processor
1. Typing,
2. Formatting,
3. Spell check,
4. Saving
are threads.
Differences between Process and Thread
Process
Thread
Process takes more time to create.
Thread takes less time to create.
it takes more time to complete execution &
terminate.
Less time to terminate.
Execution is very slow.
Execution is very fast.
It takes more time to switch b/w two
processes.
It takes less time to switch b/w two
threads.
Communication b/w two processes is difficult Communication b/w two threads is
easy.
Process can’t share the same memory area.
Threads can share same memory area.
System calls are requested to communicate
each other.
System calls are requested to communicate
each other.
Process is loosely coupled.
Threads are tightly coupled.
It requires more resources to execute.
Requires few resources to execute.
Multithreading
The number of Threads within a Process execute at a
time is called Multithreading.
Why Multithreading can give best performance?
Multithreading
The number of Threads within a Process execute at a
time is called Multithreading.
Why Multithreading can give best performance?
If a program, is multithreaded, even when some portion of it is blocked, the
whole program is not blocked. The rest of the program continues working If
multiple CPU’s are available
Multithreading
Multithreading
The number of Threads within a Process execute at a
time is called Multithreading.
Why Multithreading can give best performance?
If a program, is multithreaded, even when some portion of it is blocked, the
whole program is not blocked. The rest of the program continues working If
multiple CPU’s are available
If we have only a single thread, number of CPU’s available, No performance
benefits achieved.
PROCESS SCHEDULING
Scheduling Queues
1. job queue: consists of all processes in the system
2. ready queue: processes that are residing in main
memory and are ready and waiting to execute are
kept on a list called the ready queue.
3. device queue: if a process is present in waiting state
(or) waiting for an i/o event to complete is said to be
in device queue.(or) The processes waiting for a
particular I/O device is called device queue
PROCESS SCHEDULING
Schedulers
There are 3 schedulers:
1. Long term scheduler.
2. Medium term scheduler
3. Short term scheduler
PROCESS SCHEDULING
Schedulers
There are 3 schedulers:
1. Long term scheduler.
2. Medium term scheduler
3. Short term scheduler
Scheduler duties:
1. Maintains the queue.
2. Select the process from queues assign to CPU.
PROCESS SCHEDULING
Context Switch:
Assume, main memory contains more than one
process. If cpu is executing a process, if time expires
or if a high priority process another main memory,
then the scheduler saves information about current
process in the PCB and switches to execute another
process. The concept of moving CPU by scheduler
from one process to other process is known as
context switch.
PROCESS SCHEDULING
Non-Preemptive Scheduling:
CPU is assigned to one process, CPU do not release
until the competition of that process. The CPU will
assigned to some other process only after the
previous process has finished.
PROCESS SCHEDULING
Preemptive scheduling:
Here CPU can release the processes even in the
middle of the execution. CPU received a signal from
process p2. OS compares the priorities of p1 ,p2. If
p1>p2, CPU continues the execution of p1. If p1<p2
CPU preempt p1 and assigned to p2.
PROCESS SCHEDULING
Dispatcher:
The main job of dispatcher is switching the cpu from
one process to another process. Dispatcher connects
the cpu to the process selected by the short term
scheduler.
PROCESS SCHEDULING
Dispatcher latency:
The time it takes by the dispatcher to stop one
process and start another process is known as
dispatcher latency. If the dispatcher latency is
increasing, then the degree of multiprogramming
decreases.
SCHEDULING CRITERIA
1. Throughput: how many jobs are completed by the
cpu with in a time period.
2. Turn around time : The time interval between the
submission of the process and time of the
completion is turn around time.
TAT = Waiting time in ready queue + executing time + waiting
time in waiting queue for I/O.
PROCESS SCHEDULING
Waiting time: The time spent by the process to wait
for cpu to be allocated.
Response time: Time duration between the submission
and first response.
CPU Utilization: CPU is costly device, it must be
kept as busy as possible. Eg: CPU efficiency is 90%
means
CPU Scheduler
CPU Scheduler
Selects from among the processes in memory that are
ready to execute and allocates the CPU to one of
them.
CPU scheduling decisions may take place when a
process:
1. Switches from running to waiting state.
2. Switches from running to ready state.
3. Switches from waiting to ready.
4. Terminates.
Scheduling
under 1 and 4 is nonpreemptive.
Operating System Concepts
Alternating Sequence of CPU And I/O Bursts
PROCESS SCHEDULING
ALGORITHMS
First-Come, First-Served Scheduling:
• the simplest CPU-scheduling algorithm
• Data structure: FIFO queue
• PCB Position: Tail of the queue
Allocated Resource : ??
PROCESS SCHEDULING
ALGORITHMS
First-Come, First-Served Scheduling:
• the simplest CPU-scheduling algorithm
• Data structure: FIFO queue
• PCB Position: Tail of the queue
Allocated Resource : CPU
A period during which a process or program actively
uses the CPU to execute instructions:-CPU burst
PROCESS SCHEDULING
ALGORITHMS
Consider the following set of processes that arrive at
time 0, with the length of the CPU burst given in
milliseconds:
Process
Burst Time
𝑃1
24
𝑃2
3
𝑃3
3
Assumption: the processes arrive in the order 𝑃1 , 𝑃2 , 𝑃3
PROCESS SCHEDULING
ALGORITHMS
Assumption: the processes arrive in the order 𝑃1 , 𝑃2 , 𝑃3
Process
Burst Time
𝑃1
24
𝑃2
3
𝑃3
3
𝑷𝟏
0
Average Waiting Time: 17ms
𝑷𝟐
24
𝑷𝟑
27
30
PROCESS SCHEDULING
ALGORITHMS
Assumption: the processes arrive in the order 𝑃1 , 𝑃2 , 𝑃3
Process
Burst Time
𝑃1
24
𝑃2
3
𝑃3
3
𝑷𝟏
0
𝑷𝟐
24
𝑷𝟑
27
Average Waiting Time: 17ms ->quite long
30
First-Come, First-Served (FCFS) Scheduling
Process Burst Time
P1
24
P2
3
P3
3
P1
0
P2
24
P3
27
30
Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
• Waiting time for P1 = 0; P2 = 24; P3 = 27
• Average waiting time: (0 + 24 + 27)/3 = 17
Operating System Concepts
FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order P2 , P3 , P1 .
The Gantt chart for the schedule is:
P2
0
P3
3
P1
6
• Waiting time for P1 = 6; P2 = 0; P3 = 3
30
• Average waiting time: (6 + 0 + 3)/3 = 3
Much better than previous case.
Convoy effect short process behind long process
Operating System Concepts
PROCESS SCHEDULING
ALGORITHMS
Assumption: the processes arrive in the order 𝑃2 , 𝑃3 , 𝑃1
Process
Burst Time
𝑃1
24
𝑃2
3
𝑃3
3
𝑷𝟐
0
𝑷𝟑
3
𝑷𝟏
6
Average Waiting Time: 3ms ->much better
30
PROCESS SCHEDULING
ALGORITHMS
Assumption: the processes arrive in the order 𝑃2 , 𝑃3 , 𝑃1
Process
Burst Time
𝑃1
24
𝑃2
3
𝑃3
3
𝑷𝟐
0
𝑷𝟑
3
𝑷𝟏
6
Average Waiting Time: 3ms ->much better
30
PROCESS SCHEDULING
ALGORITHMS
FCFS scheduling:
The average waiting time under an FCFS policy is
generally not minimal and may vary substantially if
the processes CPU burst times vary greatly.
Problem:
CPU-bound process and many I/O-bound process
PROCESS SCHEDULING
ALGORITHMS
FCFS scheduling:
Problem: CPU-bound process and many I/O-bound process.
1. The CPU-bound process will get and hold the
CPU.
2. During this time, all the other processes will finish
their I/0 and will move into the ready queue,
waiting for the CPU.
3. While the processes wait in the ready queue, the
I/0 devices are idle.
PROCESS SCHEDULING
ALGORITHMS
FCFS scheduling:
Problem: CPU-bound process and many I/O-bound process.
3. Eventually, the CPU-bound process finishes its CPU
burst and moves to an I/0 device.
4. All the I/O-bound processes, which have short CPU
bursts, execute quickly and move back to the I/0 queues.
5. At this point, the CPU sits idle.
6. The CPU-bound process will then move back to the
ready queue and be allocated the CPU.
PROCESS SCHEDULING
ALGORITHMS
FCFS scheduling:
Problem: CPU-bound process and many I/O-bound process.
7. Again, all the I/0 processes end up waiting in the ready
queue until the CPU-bound process is done.
8. There is a convoy effect as all the other processes wait
for the one big process to get off the CPU.
9. This effect results in lower CPU and device utilization
than might be possible if the shorter processes were
allowed to go first
PROCESS SCHEDULING
ALGORITHMS
FCFS scheduling algorithm:
•
is nonpreemptive.
•
Once the CPU has been allocated to a process, that process
keeps the CPU until it releases the CPU, either by
terminating or by requesting I/0.
•
particularly troublesome for time-sharing systems, where
it is important that each user get a share of the CPU at
regular intervals.
•
It would be disastrous to allow one process to keep the
CPU for an extended period.
Shortest-Job-First (SJF) Scheduling
Associate with each process the length of its next CPU burst.
Use these lengths to schedule the process with the shortest
time.
Two schemes:
•
nonpreemptive – once CPU given to the process it cannot be preempted
until completes its CPU burst.
•
preemptive – if a new process arrives with CPU burst length less than
remaining time of current executing process, preempt. This scheme is
know as the Shortest-Remaining-Time-First (SRTF).
SJF is optimal – gives minimum average waiting time for a
given set of processes.
Operating System Concepts
PROCESS SCHEDULING
ALGORITHMS
Shortest-Job-First Scheduling:
Assigned available CPU to a process with the smallest
CPU burst time.
PROCESS SCHEDULING
ALGORITHMS
Shortest-Job-First Scheduling:
Assigned available CPU to a process with the smallest
CPU burst time.
What happen when two process have the same CPU
burst time.
FCFS scheduling is used to break the tie.
PROCESS SCHEDULING
ALGORITHMS
Shortest-Job-First Scheduling:
Note: a more appropriate term for this scheduling
method would be the shortest-next-CPU-burst
algorithm, because scheduling depends on the length
of the next CPU burst of a process, rather than its
total length.
The term SJF is use because most people and
textbooks use this term to refer to this type of
scheduling.
PROCESS SCHEDULING
ALGORITHMS
Shortest-Job-First Scheduling:
Assigned available CPU to a process with the smallest
CPU burst time.
Process
𝑷𝟏
𝑷𝟐
𝑷𝟑
𝑷𝟒
Burst Time
6
8
7
3
PROCESS SCHEDULING
ALGORITHMS
Shortest-Job-First Scheduling:
Assigned available CPU to a process with the smallest
CPU burst time.
Process
𝑷𝟏
𝑷𝟐
Burst Time
6
8
𝑷𝟑
𝑷𝟒
7
3
𝑷𝟒
0
𝑷𝟏
3
𝑷𝟑
9
𝑷𝟐
16
24
PROCESS SCHEDULING
ALGORITHMS
Shortest-Job-First Scheduling:
Assigned available CPU to a process with the smallest CPU burst time.
Process
Burst Time
𝑷𝟏
6
𝑷𝟐
8
𝑷𝟑
7
𝑷𝟒
3
𝑷𝟒
0
𝑷𝟏
3
𝑷𝟑
9
𝑷𝟐
16
average waiting time :>7ms
24
PROCESS SCHEDULING
ALGORITHMS
Shortest-Job-First Scheduling:
Assigned available CPU to a process with the smallest CPU burst time.
Process
Burst Time
𝑃1
6
𝑃2
8
𝑃3
7
𝑃4
3
𝑷𝟒
0
𝑷𝟏
3
𝑷𝟑
9
𝑷𝟐
16
average waiting time :>7ms
What is the average waiting time for FCFS?
24
PROCESS SCHEDULING
ALGORITHMS
Shortest-Job-First Scheduling:
Assigned available CPU to a process with the smallest CPU burst time.
Process
Burst Time
𝑷𝟏
6
𝑷𝟐
8
𝑷𝟑
7
𝑷𝟒
3
𝑷𝟒
0
𝑷𝟏
3
𝑷𝟑
9
𝑷𝟐
16
average waiting time :>7ms
What is the average waiting time for FCFS? 10.25ms
24
PROCESS SCHEDULING
ALGORITHMS
Shortest-Job-First Scheduling:
. Moving a short process before a long one decreases
the waiting time of the short process more than it
increases the waiting time of the long process.
Consequently, the average waiting time decreases
PROCESS SCHEDULING
ALGORITHMS
Shortest-Job-First Scheduling:
The SJF scheduling algorithm is provably optimal, in
that it gives the minimum average waiting time for a
given set of processes.
Moving a short process before a long one decreases the
waiting time of the short process more than it
increases the waiting time of the long process.
Consequently, the average waiting time decreases
PROCESS SCHEDULING
ALGORITHMS
Shortest-Job-First Scheduling:
The SJF scheduling algorithm is provably optimal, in that it gives the
minimum average waiting time for a given set of processes.
Moving a short process before a long one decreases the waiting time of
the short process more than it increases the waiting time of the long
process. Consequently, the average waiting time decreases
Challenge:The real difficulty with the SJF algorithm is
knowing the length of the next CPU request.
Example of Non-Preemptive SJF
1. At time 0ms, only P1 is there and so it is assigned to CPU.
2. P1 completes execution at 6ms and at that time P2 and P3 have
arrived, but not P4.
3. P3 is assigned to CPU since it has the shortest burst time among
current processes.
4. P3 completes execution at time 10ms.
5. By that time P4 has arrived and so SJF algorithm is run on the
processes P2 and P4.
Hence, we find that the order of execution is P1, P3, P4, P2
Operating System Concepts
Example of Non-Preemptive SJF
1. At time 0ms, only P1 is there and so it is assigned to CPU.
2. P1 completes execution at 6ms and at that time P2 and P3 have
arrived, but not P4.
3. P3 is assigned to CPU since it has the shortest burst time among
current processes.
4. P3 completes execution at time 10ms.
5. By that time P4 has arrived and so SJF algorithm is run on the
processes P2 and P4.
Hence, we find that the order of execution is P1, P3, P4, P2
Operating System Concepts
Example of Non-Preemptive SJF
Operating System Concepts
Example of Non-Preemptive SJF
Operating System Concepts
Example of Non-Preemptive SJF
Operating System Concepts
Example of Preemptive SJF (SRTF)
algorithm
Example of Preemptive SJF (SRTF)
algorithm
Determining Length of Next CPU
Burst
Can only estimate the length.
Can be done by using the length of previous CPU
bursts, using exponential averaging.
1. tn = actual lenght of n CPU burst
th
2.  n +1 = predicted value for the next CPU burst
3.  , 0    1
4. Define :
 n=1 =  tn + (1 −  ) n .
Prediction of the Length of the Next CPU Burst
Operating System Concepts
Examples of 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 -1 + …
+(1 -  )n=1 tn 0
Since both  and (1 - ) are less than or equal to 1, each successive term
has less
Operating
System weight
Concepts than its predecessor.
Priority Scheduling
• A priority number (integer) is associated with each process
• The CPU is allocated to the process with the highest priority
(smallest integer  highest priority).
1. Preemptive
2. nonpreemptive
• SJF is a priority scheduling where priority is the predicted
next CPU burst time.
• Problem  Starvation – low priority processes may never
execute.
• Solution  Aging – as time progresses increase the priority
of the process.
Operating System Concepts
Round Robin (RR)
• Each process gets a small unit of CPU time (time quantum),
usually 10-100 milliseconds. After this time has elapsed,
the process is preempted and added to the end of the ready
queue.
• If there are n processes in the ready queue and the time
quantum is q, then each process gets 1/n of the CPU time in
chunks of at most q time units at once. No process waits
more than (n-1)q time units.
• Performance
1. q large  FIFO
2. q small  q must be large with respect to context switch,
otherwise overhead is too high.
Operating System Concepts
Example of RR with Time Quantum = 20
Process
Burst Time
P1
53
P2
17
P3
68
P4
24
The Gantt chart is:
P1
0
P2
20
P3
37
P4
57
P1
77
P3
97
P4
117
P1
P3
121 134
P3
154 162
Typically, higher average turnaround than SJF, but
better response.
Operating System Concepts
Time Quantum and Context Switch Time
Turnaround Time Varies With The Time Quantum
Multilevel Queue
1. Ready queue is partitioned into separate queues:
• foreground (interactive)
• background (batch)
2. Each queue has its own scheduling algorithm,
•
•
foreground – RR
background – FCFS
3. Scheduling must be done 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
Operating System Concepts
Multilevel Queue Scheduling
Multilevel Feedback Queue
• A process can move between the various queues;
aging can be implemented this way.
• Multilevel-feedback-queue scheduler defined by the
following parameters:
•
•
•
•
•
number of queues
scheduling algorithms for each queue
method used to determine when to upgrade a process
method used to determine when to demote a process
method used to determine which queue a process will
enter when that process needs service
Operating System Concepts
Example of Multilevel Feedback Queue
• Three queues:
o Q0 – time quantum 8 milliseconds
o Q1 – time quantum 16 milliseconds
o Q2 – FCFS
• Scheduling
o 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.
o 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.
Multilevel Feedback Queues
Multiple-Processor Scheduling
• CPU scheduling more complex when multiple CPUs
are available.
• Homogeneous processors within a multiprocessor.
• Load sharing
• Asymmetric multiprocessing – only one processor
accesses the system data structures, alleviating the
need for data sharing.
Real-Time Scheduling
• Hard real-time systems – required to complete a
critical task within a guaranteed amount of time.
• Soft real-time computing – requires that critical
processes receive priority over less fortunate ones.
Dispatch Latency
Algorithm Evaluation
• Deterministic modeling – takes a particular
predetermined workload and defines the
performance of each algorithm for that workload.
• Queueing models
• Implementation
Operating System Concepts
Evaluation of CPU Schedulers by Simulation
Solaris 2 Scheduling
Windows 2000 Priorities