Preemptive Scheduling
Vivek Pai / Kai Li
Princeton University
Overview for Today
Wrap up regular scheduling
Move on to pre-emptive scheduling
Discuss scheduling project
Reading assignments missing
Will be added to web page soon
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Rethinking Your Server
Server: Accept connection
User: Send request
Server: Send (possibly long) response
User: Receive response
Both: Disconnect
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Drawbacks…
Server: Accept connection
User: Send request
Server: Send (possibly long) response
No control over time
User: Receive response
Both: Disconnect
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What’s 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
Memory
overlap with computation
CPU
Interrupt
Interrupt
Between instructions
Within an instruction
Enable and disable
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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)
Scheduler
dispatch
Running
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?
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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
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