Scheduling

1
Scheduling
2
Alternating Sequence of CPU And I/O Bursts
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Simple Categories of Processes

• CPU-bound process is one that has more and larger
  CPU bursts (spends most of its time computing).
• An I/O-bound process spends most of its time
  waiting for I/O.

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Histogram of CPU-burst Times
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Computing Environments

• Batch
• E.g., supercomputing centers, mainframes/workstati
  ons for business computing.

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Job Owner Job Name Queue State Nds Time Used Time Max Time
427130 kensong A3i dque ON HOLD 24 0 240000
427226 kensong A3i dque ON HOLD 24 0 240000
460157 jmehring SCEC_CyberShake_PAS_2 dque Queued 144 0 240000
460233 stolbov cocuphGgga long RUNNING 8 024253 3 960000
460234 stolbov cocuphGgga long ON HOLD 8 0 960000
462375 stolbov cocuphMgga long ON HOLD 8 0 960000
462376 stolbov cocuphMgga long ON HOLD 8 0 960000
464535 zhaol SCEC_LAB_TOMO dque RUNNING 256 020507 9 240000
470342 yujiewu q192a_1 dque RUNNING 64 024306 11 240000
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• Interactive Systems (e.g., Gandalf).
• Real-Time systems.

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CPU Scheduler

• Selects from among the processes in memory (on
  ready queue), 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.
• All other scheduling is preemptive.

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Dispatcher

• Dispatcher module gives control of the CPU to the
  process selected by the short-term scheduler
  this involves
• switching context
• switching to user mode
• jumping to the proper location in the user
  program to restart that program
• Dispatch latency time it takes for the
  dispatcher to stop one process and start another
  running.

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Possible Scheduling Goals

• CPU utilization keep the CPU as busy as
  possible
• Throughput of processes that complete their
  execution per time unit
• Turnaround time amount of time to execute a
  particular process

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Possible Scheduling Goals (continued)

• Waiting time amount of time a process has been
  waiting in the ready queue
• Response time amount of time it takes from
  issuing a command and getting response
  (interactive systems, PCs).

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Optimization Criteria

• Max CPU utilization
• Max throughput
• Min turnaround time
• Min waiting time
• Min response time

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Optimization Criteria Conflicting

• Maximize throughput
• Execute all shortest jobs first.
• Minimize turnaround time
• Turnaround time is increased significantly if
  long jobs are never executed.

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Goals of Scheduling Algorithms

• All systems
• Fairness
• Balance (keep all parts of the system busy).
• Batch Systems
• Maximize throughput.
• Minimize turnaround time
• CPU utilization.
• Interactive systems
• Response time.

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• Real-time systems
• Meeting deadlines

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First-Come, First-Served (FCFS) Scheduling

• Allocate CPU to processes based on arrival order.
• Non-preemptive. Process executes until completes
  or blocks on I/O or other system resource.
• Not a good idea for a timesharing system!

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First-Come, First-Served (FCFS) Scheduling

• Process Burst Time
• P1 24
• P2 3
• P3 3
• Suppose that the processes arrive in the order
  P1 , P2 , P3

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• The Gantt Chart for the schedule is
• Waiting time for P1 0 P2 24 P3 27
• Average waiting time (0 24 27)/3 17

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FCFS Scheduling (Cont.)

• Suppose that the processes arrive in the order
• P2 , P3 , P1 .
• The Gantt chart for the schedule is
• Waiting time for P1 6 P2 0 P3 3
• Average waiting time (6 0 3)/3 3
• Much better than previous case.

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• One problem with FCFS is that average waiting
  time can be quite large.
• Another problem is the Convoy effect.
• Consider
• 4 processes
• 1 CPU-bound with CPU burst of 20 units followed
  by an I/O request requiring 10 units.
• 3 I/O bound processes with 1 unit of CPU burst
  followed by 10 units of I/O.

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0 19 20 21 22
P0 P1 P2 P3
22
0 19 20 21 22
P0 P1 P2 P3
I/O 1 I/O 2 I/O 3
I/O 4
P0 7 P1 8 P2 9 P3 10
23
I/O Devices Idle CPU Idle
0 19 20 21 22 29
P0 P1 P2 P3
P0
I/O 1 I/O 2 I/O 3
I/O 4
P1 1 P2 2 P3 3
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I/O Devices Idle CPU Idle
I/O Devices Idle
0 19 20 21 22
29 32 49
P0 P1 P2 P3
P0
I/O 1 I/O 2 I/O 3
I/O 4
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FCFS Scheduling (Cont.)

• Convoy effect short I/O bound processes behind
  long CPU-bound process.
• CPU-bound process executes, I/O processes wait in
  Ready Queue.
• CPU-bound process completes execution burst and
  waits on I/O device.
• IO-Bound processes quickly complete CPU burst and
  block on I/O device.
• CPU is idle until I/O completed for CPU-bound
  process.
• CPU-bound process resumes, I/O-bound processes
  complete I/O request and move to RQ.
• I/O devices idle while CPU-bound process
  monopolizes CPU.

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Shortest-Job-First (SJR) 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.

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Shortest-Job-First (SJR) Scheduling

• 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.

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Example of Non-Preemptive SJF

• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P4 5.0 4

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Example of Non-Preemptive SJF

• Average waiting time (0 6 3 7)/4 4

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Preemptive Shortest Job First

• If a job arrives at the queue with a burst time
  less than that of the running process, the
  running process is preempted.
• Decision only made when a new process enters the
  queue.

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Preemptive SJF

• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P4 5.0 4

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• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P4 5.0 4

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• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P2 Remainder 4.
• P1 Remainder 5.
• Result P1 preempted at time 2.

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• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P1 5
• P2 2 P2 Preempted. P3 completes at time 5.
• P3 1

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• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P4 5.0 4
• P1 5
• P2 2
• P4 4

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P2 Completes at time 7. P1 Remaining time of
5. P4 Remaining time of 4.
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• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P4 5.0 4

P1 Waits from time 2 to time 11 9 P2 Waits
from time 4 to time 5 1 P3 No waiting
0 P4 Waits from time 5 to time 7
2 Average wait 12/4 3.
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Determining Length of Next CPU Burst

• Can only estimate the length.
• Estimate made based on some sort of statistic of
  historical behavior.
• Assume BL2 BL1 (Next same as last).
• Take mean of last n burst lengths.
• Exponential average of previous bursts.

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Homework 2

• Please answer the following question from Chapter
  5.
• 5.2, 5.4, 5.5, 5.6, 5.7, 5.8, 5.10, and 5.13. For
  problem 5.8, it should read as follows.
• ... Implementing a multi-level feedback queue
  ........

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Scheduling in Batch Systems

• Three level scheduling

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Memory Scheduler

• Decisions based on for example
• Time since swapped out.
• Amount of CPU time allocated so far.
• How large.
• How important.

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Admission Scheduler

• Based on degree of multiprogramming
• Process mix.

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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).
• Preemptive
• nonpreemptive
• SJF is a priority scheduling where priority is
  the predicted next CPU burst time.
• Problem ? Starvation low priority processes may
  never execute.

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Priority Scheduling

• Solution ? Aging as time progresses increase
  the priority of the process.
• Unix has mechanism for user to lower their
  priority through the nice system call.

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Scheduling for Interactive Systems 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.

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Scheduling for Interactive Systems Round Robin
(RR)

• Performance
• q large ? FIFO
• q small ? High overhead Must be large with
  respect to context switch, otherwise overhead is
  too high.

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Scheduling in Interactive Systems

• Round Robin Scheduling
• a) list of runnable processes
• b) list of runnable processes after B uses up its
  quantum

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• How long should the quantum be?
• quantum too short
• Assume switch time 5ms and quantum 20ms
• Wasted time 5/(520) 20
• quantum too long
• e.g., switch time 5ms, quantum 200ms
• Wasted time 5/(5200) approx. 2
• but if have 100 processes, response time for
  200th is pretty bad. This is the quantum Linux
  uses.

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Time Quantum and Context Switch Time
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Multilevel Queue Scheduling
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Multilevel Queue

• Ready queue is partitioned into separate queues
  (e.g.,)
• foreground (interactive)
• background (batch)
• Each queue has its own scheduling algorithm,
  foreground RRbackground FCFS
• Processes do not change queues

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Multilevel Queue

• Scheduling must also 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 Feedback Queue

• A process can move between the various queues
  aging can be implemented this way.
• Higher priority queues can preempt lower priority
  queues.
• Multilevel-feedback-queue scheduler defined by
  the following parameters
• number of queues
• scheduling algorithms for each queue

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Multilevel Feedback 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

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Example of Multilevel Feedback Queue

• Scheduling
• A new job enters queue Q0 which is served RR.
  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 RR 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