Time Quantum and Context Switch Time

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Time Quantum and Context Switch Time
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Turnaround Time Varies With The Time Quantum
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Turnaroun time when q5
5 8 9
14 15 17
Avg turnaround time (all jobs arrived at t0)
(158917)/4 (3217)/4 49/4 12.25 Time
quantum context switch time ? practical figures
are given in text (P185 3rd paragraph)
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Multilevel Queue

• Ready queue is partitioned into separate
  queuesforeground (interactive)background
  (batch)
• Each queue has its own scheduling algorithm
• foreground RR
• background FCFS
• 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

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Multilevel Queue Scheduling
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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

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Example of Multilevel Feedback Queue

• Three queues
• Q0 time quantum 8 milliseconds
• Q1 time quantum 16 milliseconds
• Q2 FCFS
• Scheduling
• 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.
• 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.

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Multilevel Feedback Queues
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Multiple-Processor Scheduling

• CPU scheduling more complex when multiple CPUs
  are available
• Homogeneous processors within a multiprocessor
  system. (Symmetric multiprocessing SMP)
• Load sharing
• Asymmetric multiprocessing only one processor
  (master processor) accesses the system data
  structures, alleviating the need for data sharing
• ? master processor do I/O, assign job to each sub
  processor
• I/O is important enough for the master processor
• (for example, human eye, ear, mouth ?? brain
  )

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Real-Time Scheduling

• Hard real-time systems required to complete a
  critical task within a guaranteed amount of time
• ? such a guarantee made under resource
  reservation.
• ? No secondary storage virtual memory
  (explained later)
• ? general OS cant be used.
• ? embedded controller OS are generally
  used.
• Soft real-time computing requires that critical
  processes receive priority over less fortunate
  ones

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Solaris 2 Scheduling
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Solaris(SUN) CPU scheduling

• Priority the higher number, the higher
  priority (0-59)- time quantum Time quantum for
  the associated priority. Lower has larger
• Time quantum expired priority get lower.
• Return from sleep priority get higher.
• (look Fig. 6.11)

- The selected thread runs on the CPU until it
(1) blocks, (2) use its time slice, (3) is
preempted by a higher-priority thread. - If there
are multiple threads with the same priority, the
scheduler uses a round-robin queue.
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Windows XP Priorities

• priority based, preemptive scheduling.
• XP scheduler ensures that the highest priority
  thread will always run.
• 32 level priority scheme.
• variable class thread (?? ??? 1-15), real-time
  class(16-32)

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Window XP

• each priority class has its base priority.
• for example,
• REALTIME_PRIORITY_CLASS 24 refer text
• NORMAL_PRIORITY_CLASS 8
• IDLE_PRIORITY_CLASS 4
• when a threads time quantum runs out, the
  thread is interrupted. If the thread is in
  variable class, its priority is lowered. The
  priority is never lowered below the base
  priority.
• When a variable-priority thread is released from
  a wait operation, the dispatcher boosts the
  priority. ? a thread waiting for keyboard I/O (or
  mouse I/O) would get a higher priority ? enhance
  its response time.
• Distinguish between the foreground
  process(currently selected on the screen)
  background processes ? by priority 3.

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Linux Scheduling (X)

• Two algorithms time-sharing and real-time
• Time-sharing
• Prioritized credit-based process with most
  credits is scheduled next
• Credit subtracted when timer interrupt occurs
• When credit 0, another process chosen
• When all processes have credit 0, recrediting
  occurs
• Based on factors including priority and history
• Real-time
• Soft real-time
• Posix.1b compliant two classes
• FCFS and RR
• Highest priority process always runs first

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Java Thread Scheduling

• JVM Uses a Preemptive, Priority-Based Scheduling
  Algorithm
• FIFO Queue is Used if There Are Multiple Threads
  With the Same Priority

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Java Thread Scheduling (cont)

• JVM Schedules a Thread to Run When
• The Currently Running Thread Exits the Runnable
  State
• A Higher Priority Thread Enters the Runnable
  State
• Note the JVM Does Not Specify Whether Threads
  are Time-Sliced or Not

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Time-Slicing

• Since the JVM Doesnt Ensure Time-Slicing, the
  yield() Method
• May Be Used
• while (true)
• // perform CPU-intensive task
• . . .
• Thread.yield()
• This Yields Control to Another Thread of Equal
  Priority

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Thread Priorities

• Priority Comment
• Thread.MIN_PRIORITY Minimum Thread Priority
• Thread.MAX_PRIORITY Maximum Thread Priority
• Thread.NORM_PRIORITY Default Thread Priority
• Priorities May Be Set Using setPriority() method
• setPriority(Thread.NORM_PRIORITY 2)

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Algorithm Evaluation

• Deterministic modeling takes a particular
  predetermined workload and defines the
  performance of each algorithm for that workload.
  (Use gantt chart)
• Queueing models
• Simulation
• Implementation the most difficult one

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Evaluation of CPU Schedulers by Simulation