Short Term Scheduling Policies The Issues

1
Short Term Scheduling PoliciesThe Issues

• For any of the Short Term scheduling policies,
  one needs to compute the following
• i) Specify the Gantt Chart ( A chart depicting
  the allocation sequence of each of the processes
  User Process block as well as Kernel Processes
  to the CPU as well as the corresponding CPU
  occupancy time.)
• ii) The average Turn around time for ALL the
  concerned USER Processs CPU bound Chunks.
• iii) The average Waiting time for ALL the
  concerned USER Processs CPU bound Chunks.
• .

2
Short Term Scheduling Policies Common
Assumptions - 1

• Assume Context switching time 1 unit (fixed).
• Dispatcher Execution time 3 units
  (fixed).
• Enqueuer Execution time 2 units
  (fixed).
• For any of the specified Ready Queue Status one
  needs to compare the performances of the adopted
  scheduling policies following non preemptive
  scheduling policies
• (a)First come First Served (FCFS).
• (b) Shortest Job First (SJF) based on the
  Estimated Service Time.
• (c) Dead Line based (starting with job having
  shortest service time in case of Same Deadline
  Limits).
• (d) Pre-Fixed Priority Based with Fixed Number
  of Priority levels , where each of the Processes
  assigned a Pre-Fixed Priority from the set of
  available number of Priorities.

3
Short Term Scheduling Policies Common
Assumptions - 2

• Each of the Process has to be scheduled by the
  Dispatcher.
• Context Switching Dispatcher Time Context
  Switching has to be there between any two
  processes .
• Arrival of any new Process will cause the running
  user process to be pre-empted and invoking of the
  Enqueuer while invoking of Enqueuer will also
  cause the context switch before after the
  Enqueuer.

Context Switch Dispatcher 1 Unit 3 Units
Context Switch 1 Unit
Context Switch Enqueuer 1 Unit
2 Units
Context Switch 1 Unit

4. There cannot be two Successive Context
Switches.
4
Non Pre Emptive Short Term Scheduling Policies

• (a)First come First Served (FCFS).
• (b) Shortest Job First (SJF) based on the
  Estimated Service Time.
• (c) Dead Line based (starting with job having
  shortest service time in case of Same Deadline
  Limits).
• (d) Pre-Fixed Priority Based with Fixed Number of
  Priority levels , where each of the Processes
  assigned a Pre-Fixed Priority from the set of
  available number of Priorities.

5
Case Study - 1

• Consider a typical Ready queue status of a
  computer system having Uni-processor with support
  for multi programming as given below.
• --------------------------------------------------
  ----------------------------------
• Process Arrival Time Service time
  Deadline Limits
• Chunk No. in units in units (Estimated)
  in units
• --------------------------------------------------
  ----------------------------------
• P0 10 200
  560
• P1 5
  400 1100
• P2 13 250
  none
• P3 8
  50 250
• P4 20 300
  650
• --------------------------------------------------
  -----------------------

6
Short Term Scheduling Policies Non Pre-emptive
FCFS - 1

• The Policy
• Form the READY Queue of Processes based on their
  Arrival Time.
• Select Process from the READY Queue Front for
  Dispatching.
• Repeat the previous step till READY Queue is
  Empty.

7
Short Term Scheduling Policies Non Pre-emptive
FCFS - 2

• The READY Queue / List structure
• --------------------------------------------------
  ------------------------
• Process Arrival Time Service time
• --------------------------------------------------
  ------------------------
• P1 5
  400
• P3 8
  50
• P0 10 200
  
• P2 13 250
  
• P4 20 300
  
• --------------------------------------------------
  ------------------------

8
Short Term Scheduling Policies Non Pre-emptive
FCFS - 3

• GANTT Chart

P1
P3
P0
P2
P4
DiS P3
DiS P0
DiS P2
DiS P4
DiS P1
EnQ P1
EnQ P3
EnQ P0
EnQ P2
EnQ P4
DiS P1
5 8 11 14 17 22 25 30
430 435 485 490
690 695 945 950 1250

9
Short Term Scheduling Policies Non Pre-emptive
FCFS - 4

• The Various Time Structure
• --------------------------------------------------
  ------------------------
• Process Arrival Time Service time
  Turnaround Waiting
• 
  Time Time
• --------------------------------------------------
  ------------------------
• P1 5
  400 430- 5 425 25-5
  20
• P3 8
  50 485-8 477 435-8 427
  
• P0 10 200
  690-10 680 490-10 480
• P2 13 250
  945-13932 695-13 687
• P4 20 300
  1250-201230 950-20 930
• --------------------------------------------------
  ---------------------------------------

10
Short Term Scheduling Policies Non Pre-emptive
FCFS - 5

• Total Turn Around Time 4254776809321230
  3744 units
• Average Turn Around Time 3744 / 5 748.8
  units
• Total Waiting Time 20427480682930 2539
  units
• Average Waiting Time 2539 / 5 507.8 units

11
Short Term Scheduling Policies Non Pre-emptive
SJF - 1

• The Policy
• Form the READY Queue of Processes based on their
  Estimated Service Time.
• Select Process from the READY Queue Front for
  Dispatching.
• Repeat the previous step till READY Queue is
  Empty.

12
Short Term Scheduling Policies Non Pre_emptive
SJF - 2

• Ready List / Queue Structure
• --------------------------------------------------
  ------------------------
• Process Arrival Time Service time
  Deadline Limits
• No. in units in units
  in units
• --------------------------------------------------
  ------------------------
• P3 8
  50 250
• P0 10 200
  560
• P2 13 250
  none
• P4 20 300
  650
• P1 5
  400 1100
• --------------------------------------------------
  ------------------------

13
Short Term Scheduling Policies Non Pre-emptive
SJF - 3

• GANTT Chart

P3
P0
P2
P4
P1
DiS P0
DiS P2
DiS P4
DiS P1
DiS P3
EnQ P1
EnQ P3
EnQ P0
EnQ P2
EnQ P4
DiS P3
5 8 11 14 17 22 25 30
80 85 285 290
540 545 845 850 1250

14
Short Term Scheduling Policies Non Pre-emptive
SJF - 4

• The various Time Status
• --------------------------------------------------
  ------------------------
• Process Arrival Time Service time
  Turnaround Waiting
• 
  Time Time
• --------------------------------------------------
  ------------------------
• P3 8
  50 80-8 72 30-8 22
• P0 10 200
  285-10 275 85-10 75
• P2 13 250
  540-13527 290-13 277
• P4 20 300
  845-20825 545-20 525
• P1 5
  400 1250- 5 1245 850-5 845
  
• --------------------------------------------------
  ---------------------------------------

15
Short Term Scheduling Policies Non Pre-emptive
SJF - 5

• Total Turn Around Time 722755278251245
  2944 units
• Average Turn Around Time 2944/ 5 588.8
  units
• Total Waiting Time 2275277525845 1744
  units
• Average Waiting Time 1744 / 5 348.8 units

16
Short Term Scheduling Policies Non Pre-emptive
Deadline Based - 1

• The Policy
• Form the READY Queue of Processes based on their
  pre fixed Deadline.
• Select Process from the READY Queue Front for
  Dispatching.
• Repeat the previous step till READY Queue is
  Empty.

17
Short Term Scheduling Policies Non Pre-emptive
Deadline Based - 2

• Ready List/ Queue Structure
• --------------------------------------------------
  ------------------------
• Process Arrival Time Service time
  Deadline Limits
• No. in units in units
  in units
• --------------------------------------------------
  ------------------------
• P3 8
  50 250
• P0 10 200
  560
• P4 20 300
  650
• P1 5
  400 1100
• P2 13 250
  none
• --------------------------------------------------
  ------------------------

18
Short Term Scheduling Policies Non Pre-emptive
Deadline Based - 3

• GANTT Chart

P3
P0
P4
P1
P2
DiS P0
DiS P4
DiS P1
DiS P2
DiS P3
EnQ P1
EnQ P3
EnQ P0
EnQ P2
EnQ P4
DiS P3
5 8 11 14 17 22 25 30
80 85 285 290
590 595 995 1000 1250

19
Short Term Scheduling Policies Non Pre-emptive
Deadline Based - 4

• The various Time Status
• --------------------------------------------------
  ------------------------
• Process Arrival Time Service time
  Turnaround Waiting
• 
  Time Time
• --------------------------------------------------
  ------------------------
• P3 8
  50 80-8 72 30-8 22
• P0 10 200
  285-10 275 85-10 75
• P4 20 300
  590-20 570 290-20 270
• P1 5
  400 995- 5 990 595-5 590
  
• P2 13 250
  1250-131237 1000-13987
• --------------------------------------------------
  ---------------------------------------

20
Short Term Scheduling Policies Non Pre-emptive
Deadline Based - 5

• Total Turn Around Time 722755709901237
• 
  3144 units
• Average Turn Around Time 3144 / 5 628.8
  units
• Total Waiting Time 2275270590 9871944
  units
• Average Waiting Time 1944 / 5 388.8 units

21
Non Pre-emptive Priority based Scheduling Basic
Features - 1

• Each process is assigned a pre-fixed priority (
  a non ve integer between 1 .. 10) either by the
  System Manager depending on the jobs importance
  OR by the internal policy of the concerned
  operating system.
• Lower priority value ? Higher Priority.
• Higher priority processes are dispatched /
  scheduled first.
• Processes having the same priority are chosen on
  a First Come First Served (FCFS) basis.

22
Non Pre-emptive Priority based Scheduling Basic
Features - 2

• As soon as any new process is brought in the
  memory by the Medium Term Scheduler, the
  following events take place in sequence
• a) The currently running process is brought
  back into Ready
• State from the Running State.
• b) The Enqueuer is brought into execution
  (running state) after
• a context switch which brings the
  newly arrived process in
• the Ready Queue.
• c) Next the Dispatcher is invoked in order
  to select the process
• having the current highest priority in
  the Ready Queue to be
• dispatched next.
• Thus the previously Running Process may
  re-run if that is still having the Highest
  Priority OR alternately a new process may get
  dispatched.

23
Non Pre-emptive Priority Based Scheduling Case
Study- 1

• Consider the following Process mix
• --------------------------------------------------
  ----------------------------------
• Process Arrival Time Service time
  Pre-Assigned
• Chunk No. in units in units
  Priority
• --------------------------------------------------
  ----------------------------------
• P0 80 200
  1
• P1 10 400
  3
• P2 130 250
  1
• P3 60 500
  4
• P4 200 300
  2
• --------------------------------------------------
  -----------------------

24
Non Pre-emptive Priority based Scheduling Case
Study - 2

• Consider the Process mix to be scheduled in order
  of arrival.
• --------------------------------------------------
  ----------------------------------
• Process Arrival Time Service time
  Pre-Assigned
• Chunk No. in units in units
  Priority
• --------------------------------------------------
  ----------------------------------
• P1 10 400
  3
• P3 60 500
  4
• P0 80 200
  1
• P2 130 250
  1
• P4 200 300
  2
• --------------------------------------------------
  -----------------------

25
Non Pre-emptive Priority based Scheduling Case
Study - 3

• Dispatcher Enqueuer Block Structures BUT with
  no two consecutive Context Switches.

Context Switch Dispatcher Context Switch 1 Unit
3 Units 1 Unit
Context Switch Enqueuer Context Switch 1 Unit
2 Units 1 Unit

26
Non Pre-emptive Priority based Scheduling GANTT
Chart -1

EnQ P1 (Pr3)
P1(Pr 3) 42 Done 358 Left
EnQ P3 (Pr4)
DiS P1
P1(Pr 3) 54 Done 346 Left
EnQ P0 (Pr1)
DiS P0
DiS P1
10 13 18
60 63 68
80 83 88
P0(Pr 1) 42 Done 158 Left
EnQ P2 (Pr1)
DiS P0
P0(Pr 1) 104 Done 96 Left
EnQ P4 (Pr2)
DiS P0
88 130 133 138
200 203 208
27
Non Pre-emptive Priority based Scheduling GANTT
Chart - 2

P0(Pr 1) 200 Done COMPLETED
DiS P2
P2(Pr 1) 250 Done COMPLETED
DiS P4
P4(Pr 2) 300 Done COMPLETED
208 304 309
559 564
864
P1(Pr 3) 400 Done 346 was Left
DiS P1
P3(Pr 4) 500 Done COMPLETED
DiS P3
864 869 1215 1220
1720
28
Non Pre-emptive Priority based Scheduling - 8

• The various timing Calculations
• --------------------------------------------------
  ----------------------------------
• Process Arrival Time Turn Around time
  Waiting Time
• Chunk No. in units in units
  in units
• Priority
• --------------------------------------------------
  ----------------------------------
• P0 1 80 304 80
  224 88-80 8
• P1 3 10 1215 10
  1205 18 10 8
• P2 1 130 559 130
  429 309 -130 179
• P3 4 60 1720 60
  1660 1220 60 1160
• P4 2 200 864 200
  664 564 200 364
• --------------------------------------------------
  -----------------------

29
Short Term Scheduling Policies Non Pre-emptive
Priority Based - 9

• Total Turn Around Time 22412054291660664
• 
  4182 units
• Average Turn Around Time 4182 / 5 836.4
  units
• Total Waiting Time 881791160 364 1719
  units
• Average Waiting Time 1719 / 5 343. 8units