Chapter 4 Processor Management

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ICS-023 - PC Operating Systems Chapter
4 Processor Management King Fahd University
of Petroleum Minerals
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Processor Management

• A program (job) is an inactive unit, such as a
  file stored on the disk.
• A process (task) is an active entity that
  requires a set of resources, including processor
  and memory.
• Processor also known as CPU is the part of the
  machine that performs calculations and executes
  the programs.

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Processor Management

• Thread of control (thread) a portion of a
  process that can run independently.
• Processor ( CPU, central processing unit) part
  of machine that performs calculations and
  executes programs.

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Processor Management

• Processor Manager performs job scheduling,
  process scheduling, and interrupt management.
• In single-user systems, processor is busy only
  when user is executing a jobat all other times
  it is idle.
• Processor management is simple.
• In multiprogramming environment, processor must
  be allocated to each job in a fair and efficient
  manner.
• Requires scheduling policy and a scheduling
  algorithm.
• Requires that the processor be allocated to
  each job or to each process for a period of time
  and deallocated at an appropriate moment.

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Context Switching

• Disrupting a process during execution and giving
  the CPU to another process is called Context
  Switching.
• Example We need to execute 2 jobs A and B.
• Get the input for job A.
• Identify resources.
• Execute the process.
• Interrupt
• Context switch to job B.
• Get the input for job B.
• Identify resources
• Execute the process.
• Terminate job B.
• Context switch to job A.
• Resume executing interrupted process.
• Terminate job A.

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Processor Management

• Processor Manager has 2 sub-managers
• Job Scheduler
• in charge of job scheduling.
• Process Scheduler
• in charge of process scheduling.

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Job Scheduler

• High-level scheduler.
• Selects jobs from a queue of incoming jobs.
• Places them in process queue, based on each jobs
  characteristics.
• Goal is to put jobs in a sequence that uses all
  systems resources as fully as possible.
• Strives for balanced mix of jobs with large I/O
  interaction and jobs with lots of computation.
• Tries to keep most system components busy most of
  time.

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Process Scheduler

• Low-level scheduler
• Assigns the CPU to execute processes of those
  jobs placed on ready queue by Job Scheduler.
• After a job has been placed on the READY queue by
  Job Scheduler, Process Scheduler takes over.
• Determines which jobs will get CPU, when, and for
  how long.
• Decides when processing should be interrupted.
• Determines queues job should be moved to during
  execution.
• Recognizes when a job has concluded and should be
  terminated.

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CPU Cycles and I/O Cycles

• To schedule CPU, Process Scheduler uses common
  trait among most computer programs they
  alternate between CPU cycles and I/O cycles.

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Processor Manager

• I/O-bound jobs (such as printing a series of
  documents) have many brief CPU cycles and long
  I/O cycles.
• CPU-bound jobs (such as finding the first 300
  prime numbers) have long CPU cycles and shorter
  I/O cycles.
• In a highly interactive environment theres a
  third layer called Middle-level scheduler.
• Removes active jobs from memory to reduce degree
  of multiprogramming and allows jobs to be
  completed faster.

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Job and Process Status

• Job status -- one of the 5 states that a job
  takes as it moves through the system.
• HOLD
• READY
• WAITING
• RUNNING
• FINISHED

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Job and Process Status
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Transition Among Process States

• HOLD to READY Job Scheduler using a predefined
  policy.
• READY to RUNNING Process Scheduler using some
  predefined algorithm
• RUNNING back to READY Process Scheduler
  according to some predefined time limit or other
  criterion.
• RUNNING to WAITING Process Scheduler and is
  initiated by an instruction in the job.
• WAITING to READY Process Scheduler and is
  initiated by signal from I/O device manager that
  I/O request has been satisfied and job can
  continue.
• RUNNING to FINISHED Process Scheduler or Job
  Scheduler.

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Process Control Block (PCB)

• It is a data structure that contains basic info
  about the job
• Process identification
• Process status (HOLD, READY, RUNNING, WAITING)

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Process state

• It contains all the information needed to
  indicate the current status of the job.
• Process Status Word contains the current
  instruction counter and register contents when
  the job isnt running (HOLD, READY, WAITING).
• Register Contents show the contents of the
  register if the job has been interrupted and is
  waiting to resume.
• Main Memory
• Resources
• Process Priority

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Accounting

• It contains information used mainly for billing
  purposes and performance measurement.
• CPU time used.
• Total job time.
• Main storage occupancy.
• System programs used (compilers, editors, and
  utilities).
• No. and types of I/O operations.

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PCBs and Queuing

• PCB of job created when Job Scheduler accepts it
• updated as job goes from beginning to
  termination.
• Queues use PCBs to track jobs.
• PCBs, not jobs, are linked to form queues.
• PCBs for every ready job are linked on READY
  queue all PCBs for jobs just entering system are
  linked on HOLD queue.
• Queues must be managed by process scheduling
  policies and algorithms.

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Process Scheduling Policies

• Before operating system can schedule all jobs in
  a multiprogramming environment, it must resolve
  three limitations of system
• finite number of resources (such as disk drives,
  printers, and tape drives)
• some resources cant be shared once theyre
  allocated (such as printers)
• some resources require operator intervention
  (such as tape drives).

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A Good Scheduling Policy

• Maximize throughput by running as many jobs as
  possible in a given amount of time.
• Maximize CPU efficiency by keeping CPU busy 100
  of time.
• Ensure fairness for all jobs by giving everyone
  an equal amount of CPU and I/O time.
• Minimize response time by quickly turning around
  interactive requests.
• Minimize turnaround time by moving entire jobs
  in/out of system quickly.
• Minimize waiting time by moving jobs out of READY
  queue as quickly as possible.

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Interrupts

• There are instances when a job claims CPU for a
  very long time before issuing an I/O request.
• Builds up READY queue empties I/O queues.
• Creates an unacceptable imbalance in the system.
• Process Scheduler uses a timing mechanism to
  periodically interrupt running processes when a
  predetermined slice of time has expired.
• Suspends all activity on the currently running
  job.

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Preemptive Non-preemptiveScheduling Policies

• Preemptive scheduling policy interrupts
  processing of a job and transfers the CPU to
  another job.
• Non-preemptive scheduling policy functions
  without external interrupts.
• Once a job captures processor and begins
  execution, it remains in RUNNING state
  uninterrupted.
• Until it issues an I/O request (natural wait) or
  until it is finished (exception for infinite
  loops).

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Process Scheduling Algorithms

• First Come First Served (FCFS)
• Shortest Job Next (SJN)
• Priority Scheduling
• Shortest Remaining Time (SRT)
• Round Robin
• Multiple Level Queues

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First Come First Served (FCFS)

• Non-preemptive.
• Handles jobs according to their arrival time --
  the earlier they arrive, the sooner theyre
  served.
• Simple algorithm to implement
• Uses a FIFO queue.
• Good for batch systems not so good for
  interactive ones.
• Turnaround time is unpredictable.

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FCFS Example

• Process CPU cycle (Turnaround Time)
• A 15 msecs, B 2 msecs, C 1 msec.
• If they arrive in order of A, B, and C.
• Turnaround time
• 151718 16.67

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FCFS Example

• Job order C, B, A.
• Turnaround time
• 1831 7.3

Job C
Job B
Job A
0
3
1
18
3
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Shortest Job Next (SJN)

• Non-preemptive.
• Handles jobs based on length of their CPU cycle
  time.
• Use lengths to schedule process with shortest
  time.
• Optimal for batch system when all of jobs are
  available at same time and the CPU estimates are
  available.
• Doesnt work well in interactive systems because
  users dont estimate in advance CPU time required
  to run their jobs.

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SJN Example

• Job A (5 cycles), B (2 cycles), C (6 cycles), D
  (4 cycles)
• Turnaround time
• 261117 9.0

Job B
Job D
Job A
Job C
0
6
2
17
11
4
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Priority Scheduling

• Non-preemptive.
• Gives preferential treatment to important jobs.
• Programs with highest priority are processed
  first.
• Arent interrupted until CPU cycles are completed
  or a natural wait occurs.
• If 2 jobs with equal priority are in READY
  queue, processor is allocated to one that arrived
  first (first come first served within priority).
• Many different methods of assigning priorities by
  system administrator or by Processor Manager.

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Prioritizing Criteria

• Memory Requirements.
• No. and types of devices required.
• Total CPU time.
• Amount of time already spent in the system.

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Shortest Remaining Time (SRT)

• Preemptive version of the SJN algorithm.
• Processor allocated to job closest to completion.
• This job can be preempted if a newer job in READY
  queue has a time to completion that's shorter.
• Cant be implemented in interactive system
  requires advance knowledge of CPU time required
  to finish each job.
• SRT involves more overhead than SJN.
• OS monitors CPU time for all jobs in READY queue
  and performs context switching.

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SRT Example

• 4 jobs coming in quick succession (1 cycle
  apart).
• Job A (6 cycles), B (3 cycles), C (1 cycle), D (4
  cycles)
• Turnaround time for a job completion time -
  arrival time
• Total 14416 6.25

Job B
Job B
Job A
Job C
Job D
Job A
0
14
9
5
3
2
1
4
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Context Switching Is Required by All Preemptive
Algorithms

• When Job A is preempted
• All of its processing information must be saved
  in its
• PCB for later (when Job As execution is
  continued).
• Contents of Job Bs PCB are loaded into
  appropriate registers so it can start running
  again (context switch).
• Later when Job A is once again assigned to
  processor another context switch is performed.
• Info from preempted job is stored in its PCB.
• Contents of Job As PCB are loaded into
  appropriate registers.

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Round Robin

• Preemptive.
• Used extensively in interactive systems because
  its easy to implement.
• Isnt based on job characteristics but on a
  predetermined slice of time thats given to each
  job.
• Ensures CPU is equally shared among all active
  processes and isnt monopolized by any one job.
• Time slice is called a time quantum
• size crucial to system performance (100 ms to 1-2
  secs)

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Example

• Arrival time 0 1 2 3
• Job A B C D
• CPU cycle 8 4 9 5
• Turnaround time
• 2072422 18.25

Job A
Job C
Job D
Job C
Job A
Job D
Job C
Job B
0
8
4
16
12
24
20
26
25
4
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If Jobs CPU Cycle lt Time Quantum

• If the job finishes before the time quantum
  expires, then all resources allocated to it are
  released completed job is returned to user.
• If CPU cycle was interrupted by I/O request, then
  info about the job is saved in its PCB it is
  linked at end of the appropriate I/O queue.
• Later, when I/O request has been satisfied, it is
  returned to end of READY queue to await
  allocation of CPU.

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Cache memory

• It is a quickly accessible memory thats designed
  to alleviate speed differences between a very
  fast CPU and slower main memory.
• Stores copy of frequently used data in an easily
  accessible memory area instead of main memory.

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Cache memory
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Types of Interrupts

• Page interrupts to accommodate job requests.
• Time quantum expiration.
• I/O interrupts when issue READ or WRITE command.
• Internal interrupts (synchronous interrupts)
  result from arithmetic operation or job
  instruction currently being processed.
• Illegal arithmetic operations (e.g., divide by
  0).
• Illegal job instructions (e.g., attempts to
  access protected storage locations).

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Interrupt Handler

• Describe store type of interrupt (passed to
  user as error message).
• Save state of interrupted process (value of
  program counter, mode specification, and contents
  of all registers).
• Process the interrupt
• Send error message state of interrupted process
  to user.
• Halt program execution
• Release any resources allocated to job are
  released
• Job exits the system.
• Processor resumes normal operation.