Scheduling Example 3 (1)

1
Scheduling Example 3 (1)

• Assume
• FIFO Job Scheduling
• 100 K Main Memory
• Processor Sharing Process Scheduling
• (Cont)

2
Scheduling Example 3 (2)
Job Arrives Run Time Memory 1 10.0
0.3 10 2 10.2 0.5 60
3 10.4 0.1 50 4 10.5
0.4 10 5 10.8 0.1 30
HOLDQ Ready / Running
3
Example 3 (Cont)
Time Event Jobs Headway MM
Free Time Left 10.0 1 A,S 90 1 0.3 10.2 2
A,S 1 0.2 30 1 0.1 2 0.5 10.4 1
F 2 0.1 40 2 0.4 3 A,H 10.5 4
A,S 1 0.1 30 2 0.3 4 0.4 10.8 5
A,S 2 0.15 0 2 0.15 4 0.25 5 0.1 1
1.1 5 F 3 0.1 30 2 0.05 4 0.15 11.2 2
F 2 0.05 90 4 0.1 3 S 40 3 0.1 11.4 3
F 2 0.1 50 4 F 100
Must be memory size
Must be 10.0 sum of all Ri
4
T and W for Example 3

• Job Run Arrival Finish Ti Wi
• 0.3 10.0 10.4 0.4 1.33
• 0.5 10.2 11.2 1.0 2.0
• 0.1 10.4 11.4 1.0 10.0
• 0.4 10.5 11.4 0.9 2.25
• 0.1 10.8 11.1 0.3 3.0
• 3.6 18.58
• T 0.72
• W 3.716

5
Scheduling Example 4 (1)

• Assume
• FIFO Job Scheduling
• 100 K Main Memory
• 5 Devices
• Processor Sharing Process Scheduling
• (Cont)

6
Scheduling Example 4 (2)
Job Arrives Run Time Memory Devices 1
10.0 0.3 10 2 2 10.2
0.5 60 1 3 10.4 0.1
50 4 4 10.5 0.4 10 2 5
10.8 0.1 30 3
HOLDQ Ready / Running
7
Example 4 (Cont)
Time Event Jobs Headway MM
Free Devices Time Left 10.0 1 A,S 90 3 1
0.3 10.2 2 A,S 1 0.2 30
2 1 0.1 2 0.5 10.4 1
F 2 0.1 40 4 2 0.4 3
A,H 10.5 4 A,S 1 0.1 30 2 2
0.3 4 0.4 10.8 5 A,H 2 0.15 30
2 2 0.15 4 0.25 11.1 2
F 2 0.15 90 3 4 0.1 5 S 60
0 5 0.1 11.3 5 F 2 0.1 90 3 3
0.1 4 F 100 5 3 S 50 1 11.4 3
F 1 0.1 100
8
T and W for Example 4

• Job Run Start Finish Ti Wi
• 0.3 10.0 10.4 0.4 1.33
• 0.5 10.2 11.1 0.9 1.8
• 0.1 10.4 11.4 1.0 10.0
• 0.4 10.5 11.3 0.8 2.0
• 0.1 10.8 11.3 0.5 5.0
• 3.6 20.13
• T 0.72
• W 4.026

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Scheduling Example 5 (1)

• Assume
• FIFO Job Scheduling
• 100 K Main Memory
• 5 Devices
• Processor Sharing Process Scheduling
• (Cont)

10
Scheduling Example 5 (2)
Job Arrives Run Time Memory Devices 1
1.0 0.5 30 2 2 1.2 1.0
50 1 3 1.3 1.5 50 1 4
1.4 2.0 20 2 5 1.7 0.5
30 3 6 2.1 1.0 30 2
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Example 5 (Cont) (1)
Time Event Jobs Headway MM
Free Devices Time Left 1.0 1 A,S 70 3 1
0.5 1.2 2 A,S 1 0.2 20 2 1
0.3 2 1.0 1.3 3 A,H 2 0.05 20
2 1 0.25 2 0.95 1.4 4
A,S 2 0.05 0 0 1 0.2 2
0.9 4 2.0 1.7 5 A,H 3 0.1 0
0 1 0.1 2 0.8 4
1.9 2.0 1 F 3 0.1 30 2 2
0.7 4 1.8 2.1 6 A,S 2 0.05 0
0 2 0.65 4 1.75 6
1.0
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Example 5 (Cont) (2)
Time Event Jobs Headway MM
Free Devices Time Left 4.05 2 F 3 0.65 50
1 4 1.1 3 S 0 0 6
0.35 3 1.5 5.1 6 F 3 0.35 30
2 4 0.75 3 1.15 6.6 4
F 2 0.75 50 4 3 0.4 5 S 20 1
5 0.5 7.4 3 F 2 0.4 70 2 5
0.1 7.5 5 F 1 0.1 100 5
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T and W for Example 5

• Job Run Arrival Finish Ti Wi
• 0.5 1.0 2.0 1.0 2.0
• 1.0 1.2 4.05 2.85 2.85
• 1.5 1.3 7.4 6.1 4.06
• 2.0 1.4 6.6 5.2 2.6
• 0.5 1.7 7.5 5.8 11.6
• 1 2.1 5.1 3.0 3.0
• 23.95 26.11
• T 3.99
• W 4.35

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Job Scheduling
Also known as FCFS first come, first
served Scheduling based on arrival
time Non-preemptible discipline Fair no job is
given preferential treatment, and every arriving
job eventually runs Whey is this policy bad for
interactive users
Run
Hold
Ready
Wait
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Process Management
A multiprogramming OS must interleave the
execution of multiple jobs The OS must decide
when each process gets to use each resource
(CPU,disk, etc.) The scheduler decides when
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Time Quantums
How does the OS interleave execution of many
processes on the CPU? Ans Time slicing Quantum
Amount of time given to jobs when time sharing
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Process States

• Running
• Process currently has CPU
• Ready
• Process could use CPU, if CPU were free
• Blocked
• Process is waiting for some event to occur
• I/O Completion
• Buffer available from another process
• Data arrives from network
• (More states will be discussed later.)

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Process State Transitions
Running
Dispatch
Block
Timeout
Ready
Blocked
Wakeup
Is there a maximum number of processes in any
state?
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Suspend and Resume
Wakeup
Blocked
Ready
Timeout
Dispatch
Running
Suspend
Resume
Suspend
Resume
Suspend
Suspended Ready
Suspended blocked
Event (e.g., I/O) completion
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Process Control Block (PCB)

• One PCB per process, containing all information
  about that process
• Process Identifier (pid)
• Parents pid
• State (e.g., Running, Blocked in ReadyQ)
• Priority
• Time at which its execution started
• Amount of CPU time consumed so far
• Copy of all register contents when process was
  last suspended
• Main memory used by process (e.g., base and
  bound registers, page table pointer)
• Accounting information
• Room for pointers to PCB into a queue
• File descriptor table

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Context Switch
A Context Switch occurs when a process exchange
is made between the ready and run
queues Must Save the state of the running
process AND Restore the state of the ready
process
Run
Ready
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What happens on a Context Switch? (1)

• Hardware
• Resets Program Counter (PC to that of the
  interrupt handler (IH), which is an address in
  the OS kernel
• Switches from user to supervisor mode
• Kernel
• Copies As state from CPU registers to As PCB
• Sets As state to Ready or Blocked
• Inserts As PCB on ReadyQ or BlockedQ
• Scheduler selects a new process to run (B), based
  on its scheduling discipline
• (Contd)

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What happens on a Context Switch? (2)

• 4. Kernel
• Sets Bs state to Running
• Copies Bs state from Bs PCB to CPU registers
• Kernel transfers control to B and thereby
  switches from kernel back to user context
• What machine instructions can achieve number 5?

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Preemption

• Two classes of scheduling disciplines
• Premptive
• Scheduler takes CPU away from running job and
  gives it to another job
• Preempt upon arrival of higher priority job
• Non-premptive

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Process Scheduling Algorithms (1)

• 1. Round Robin (RR)
• Each process runs either until
• Its time quantum expires or
• It blocks to perform I/O
• 2. Processor Sharing (PS)
• Limit of RR as time quantum goes to zero (Like
  giving each CPU cycle to a different process, in
  round robin fashion)
• N processes scheduled by PS each job runs on
  dedicated N-fold slower CPU.
• Thus, READY RUNNING

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Process Scheduling Algorithms (2)
3. Priority Each process is statically assigned a
priority run high before low priority 4. Dynamic
Priority Same as 3, except priority level of
each process can change dynamically 5. Inverse of
the Remainder of the Quantum Position in ready
queue is determined by the amount of time
remaining in the time slice (e.g., if ¾ of time
slice is left, the job moves ¼ ahead in the ready
queue)
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Process Scheduling Algorithms (3)
6. Multiple-Level Feedback Variant on the Round
Robin Current processes are forced to wait until
new jobs catch up in time, then all RR 7.
System Balance Balance system between I/O bound
and CPU bound jobs 8. Preference to Interactive
jobs Interactive jobs have higher priority