SCHEDULING Uni Processor Scheduling

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SCHEDULING / Uni Processor Scheduling

• Objectives Criteria.
• Various types of Scheduling.
• Typical Scheduler Structure
• Mechanisms.
• CPU Scheduling Policies.

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Scheduling Objectives Criteria I(System
Oriented Performance Related)

• Maximize CPU utilization / Minimize CPU Idle
  time.
• Maximize throughput i.e. the no. of process /
  threads completed per unit time.
• Optimize service time / process t serv
  The amount of time a process needs to be in the
  running state before it is completed.

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Scheduling Objectives Criteria II

• Minimize the following (User Oriented
  Parameters)
• a) Process / Thread turn around time Tturn
  The time interval between the point when the
  concerned process is brought in the system and
  the moment it exits out of the system after
  proper completion .
• b) Waiting time Twait The time spent by
  the process/ thread in the ready state before
  being taken up for execution I.e. the time
  interval between the concerned processs arrival
  in the ready state (from several other states)
  and the moment it is allocated the CPU.
• c) Response time T response Time interval
  between job submission by the user and the very
  first time it is taken up for execution.
• d) Deadlines T max. When process
  completion deadlines are specified (especially
  in real time systems) then the scheduling
  criteria should aim to maximize the percentage of
  deadlines met.

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Scheduling Objectives Criteria - III

• User Oriented Parameter Contd.
• e) Predictability A given job should take
  about the same time and use the same set of
  resources / (same cost) regardless of the current
  system load. Wide variation of turnaround time
  and / or response time should be prevented.

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Scheduling Objectives Criteria IVSystem
oriented criteria

• Fairness All processes should be treated in the
  same manner unless specified system / user
  directive (normally in the form of priority) .
  No one process should dominate any resource
  thereby causing any other process to starve for
  that resource.
• Enforcing priorities When processes are
  assigned priority the scheduling policy should
  favor the higher priority processes.
• Balancing Resources Optimal use of system
  resources should be aimed at. Processes that
  will under use stressed resources should be
  favored.

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Scheduling Objectives Criteria VSystem
oriented criteria (contd.)

• Disciplined use of Resources The scheduling
  policy must ensure that modality of sharing non
  shareable resources like printers must be proper
  without sacrificing on fairness predictability.
  This is also dictated by the following.
• I/O Scheduling The decision as to which
  processs pending I/O request shall be handled by
  an available I/O peripheral device. Process
  Synchronization Issue.

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Various types of Scheduling

• Long term Scheduling The decision to choose
  some jobs from the job pool on Disk to be
  converted into processes as well as creating the
  set of processes in the virtual space which
  involves creation of PCBs, PMTs etc. done with
  the help of Virtual Space / File Manager .
• Medium term Scheduling The task of add /
  removal to / from the number of processes/
  working set of the processes lying in the
  physical memory to the Disk Swap Space or
  Virtual Space and vice versa.. This sometimes
  creates a Ready Process ( a candidate for the
  Ready Queue). This task is primarily performed by
  the Swap Space Manager along with the Memory
  Management Unit (MMU).
• Short term Scheduling This creates
  maintains the Ready Queue as well as dictates
  which of the processes in the Ready Queue will
  be allocated the processor next.This can be
  partitioned into three main sub parts , the
  Enqueuer, the process scheduler/ dispatcher
  module and the context switcher. Our present
  area of interest

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Tasks performed by a Long Term Scheduler

• Determines which job will be admitted to the
  system i.e. copied from the job pool on disk into
  the main memory ( Job Selection).
• It creates a process from the job description in
  the virtual space i.e. it creates the Process
  Control Block (PCB), the Segment Descriptor
  Table, Page Map Table (PMT) index as well as the
  Page Map Table(PMT).
• Once created the process waits to be brought
  in the physical memory by the medium term
  scheduler.
• The decision about when to create a new process
  is generally decided by the standard process
  creation methods as illustrated earlier.
• Employs file manager.

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Medium term Scheduler Functions

• The swap space Management MMU Function comes
  into operation whenever any process or a part of
  it need to be swapped out of the physical memory
  to make way for other process to be admitted.
• Swapping In / Out Decision is based on the
  following aspects
• The need to manage the degree of
  multiprogramming.
• The priority of the concerned processes the
  pre_emption policy.
• The swap space requirement of the swapped out
  process.
• The MMU structure support available.
• The disk swap space size management strategy.

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Short term Scheduler Functions

• Creates maintains Ready Queue employing
  Enqueuer Module.
• Makes the time grained decision of which process
  / thread from the ready queue will be executed
  next.
• Involves in dispatching of selected process from
  the Ready State to the Running state using
  Dispatcher Context switcher modules.
• Executes most frequently.
• Invoked when any one of the following events
  occur for the currently running process/thread.
• It gets terminated.
• It goes to wait / Blocked state or alternatively
  scheduled for I/O operation.
• It comes back to ready state due to some
  pre-emption policy.
• Performs the primary functions of the process
  manager.

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Typical Process State Diagram
New
Admit
(Pre_Empt. 1) High Priority Job enters
Admit
Dispatch
No allocated Memory
Activate
Release
Ready Suspend
Ready
Running
Pre_Empt2
Exit
Time Out
Event Occurs
Swap out Disk
Event Wait (I/O wait)
I/O Complete
Wait / Blocked
Activate
Event Occurs
Blocked Suspended
Swap out to Disk
No allocated Memory
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States Transitions vs. Scheduler Role -1

• NEW gtCreate Process Id PCB
• ( done by Long Term Scheduler) .
• NEW ? Ready or ? Ready / Suspend
• gt Process is ready to execute waiting for
  CPU allocation OR Process is waiting only for
  main memory.
• Ready / Suspend ? Ready gt Process is ready
  to run waiting to be scheduled/ dispatched .
• ( Brought in to / out of Physical Memory by
  Medium Term Scheduler. Put in Ready Queue by
  Short Term Scheduler)

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States Transitions vs. Scheduler Role - 2

• Blocked ? Blocked/ Suspend gt used to make
  available more memory to the Ready processes
  and/ or to the Running Process. ( done by Medium
  Term Scheduler) .
• Blocked/Suspend ? Ready Suspend gt Process
  waiting for event completion is over but no main
  memory is currently available. ( primarily
  invoked by I/O Scheduler, completed by Medium
  Term Scheduler) .
• Ready ? Ready/ Suspend adopted if that is the
  only way to make available more memory to a high
  priority / currently running process. (Brought
  out of Ready Queue by Short Term Scheduler
• Brought out of Physical Memory by Medium Term
  Scheduler).

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States Transitions vs. Scheduler Role - 3

• Blocked /Suspend ? Blocked used to bring a high
  priority waiting process into memory. ( done by
  Medium Term Scheduler)
• Running ? Ready /Suspend implies completion of
  time quantum but no memory is currently
  available since a high priority process has
  either moved into the blocked state or has become
  unblocked. (Brought out of Running State by Short
  Term Scheduler .Brought out of Physical Memory
  by Medium Term Scheduler).

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States Transitions vs. Scheduler Role - 4

• Running ? Blocked Queue used for any I/O
  requesting process / parent with unfinished child
  process ( after Executing Wait) .
• ( done by Short Term Scheduler
  Peripheral Manager)
• Running ? Ready implies pre emption either due
  to the completion of time quantum or due to the
  prefixed scheduling policy (done using Enqueuer
  module of the Short Term Scheduler).
• Blocked ? Ready implies completion of the
  requisite event for some waiting process /
  releasing of a Resource initiated through some
  Device interrupt completed due to the Resource
  Release (after Executing Signal ) done through
  the Enqueuer.