Preemptive Scheduling

1
Preemptive Scheduling

• Vivek Pai / Kai Li
• Princeton University

2
Errata

• while loop example from last time
• Going from randVal 9 to randVal 0 may take
  loop overhead
• So, its not really guaranteed to be random

3
Mechanics

• Lamports paper added to readings
• Project 4 stripped down from last year
• People are hiring good news

4
Overview for Today

• Wrap up swapping
• Move on to pre-emptive scheduling

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Job Swapping
Swap in
Swap out
Partially executed swapped-out processes
Ready Queue
CPU
Terminate
I/O Waiting queues
I/O
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Add Job Swapping toState Transition Diagram
Swap out
Swap
Terminate (call scheduler)
Running
Scheduler dispatch
Block for resource (call scheduler)
Swap in
Yield (call scheduler)
Create a process
Ready
Blocked
Resource becomes available (move to ready queue)
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Think About Swapping

• Is swapping
• Necessary
• Desirable
• Good
• Ideal

• Things to consider
• Performance
• Complexity
• Efficiency

8
The Hello/Goodbye Server

• Server Hello
• User Hello
• Server Drink Slurm! (ka-ching!)
• User Goodbye
• Server Goodbye

9
Life is Simple Until

• Hello,
• My name is Bill Gates. Please try out my new
  Hello/Goodbye Server and Ill give you a
  million dollars and a new cat. Tell your friends.
• I wuv you,
• Bill Gates

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You Have to Fix It

• accept new connection say hello
• wait for hello print advertising message
• wait for goodbye, say goodbye
• close connection

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Whats the Simplest Option?

• While (1)
• accept new connection
• fork new process
• let process handle connection
• Drawback lots of process creation/deletion

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Can We Reduce Forks?

• At program launch, fork some number
• While (1)
• accept
• wait for hello
• wait for goodbye
• close
• Note wait implicit yield

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But What If We Do More

• While (1)
• accept
• wait for hello
• perform possibly unbounded calculation
• wait for goodbye
• close
• Problem when do we yield?

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Signals and Interrupts

• Both are asynchronous
• Used to notify system of event occurrence
• Signal delivered by OS to process
• Interrupt delivered by hardware to OS
• Some overlap/interaction? Definitely
• Examples?
• Code tries executing divide-by-zero
• User disconnects (hangs up)

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I/O and Timer Interrupts

• Why
• Timer interrupt to do CPU management
• Asynchronous I/O to overlap with computation
• Interrupt
• Between instructions
• Within an instruction
• Enable and disable

CPU
Memory
Interrupt
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Using Interrupts For Scheduling

• Timer interrupt
• Generated by hardware
• Assume changing requires privilege
• Delivered to the OS
• Main idea
• Before moving process to running, set timer
• If process yields/blocks, clear timer
• Timer expires? Go to scheduler

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Scheduling Considerations

• Timer granularity
• Finer timers more responsive
• Coarse timers more efficient
• Accounting
• Cheap
• Accurate
• Fair consider I/O versus CPU applications

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Preemptive Scheduling
Terminate (call scheduler)
Running
Scheduler dispatch
Block for resource (call scheduler)
Yield, Timer Interrupt (call scheduler)
Create
Ready
Blocked
I/O completion interrupt (move to ready queue)
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No Control Over Yielding

• Reasons for yielding
• Timer goes off
• Higher-priority interrupt occurs
• Higher-priority process becomes ready
• Some unintentional block (e.g. page fault)

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Atomic Pieces of Code

• Example bank transaction
• Read account balance
• Add/subtract money
• Write account balance
• Problem what happens when two transactions are
  being posted to the same account?

21
Next Time

• Atomic pieces known as critical sections
• Very common in concurrent/parallel programming
• Must share memory
• Possible via forked processes
• Default via threads
• Cover some scheduling policies