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, it’s not really guaranteed to be random

3 Mechanics Lamport’s 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

5 Job Swapping Partially executed swapped-out processes Ready Queue CPU I/O Waiting queues I/O Terminate Swap out Swap in

6 Add Job Swapping to State Transition Diagram Running Blocked Ready Resource becomes available (move to ready queue) Create a process Terminate (call scheduler) Yield (call scheduler) Block for resource (call scheduler) Scheduler dispatch Swap out Swap in Swap

7 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 I’ll give you a million dollars and a new cat. Tell your friends. I wuv you, Bill Gates

10 You Have to Fix It accept new connection; say hello wait for hello; print advertising message wait for goodbye, say goodbye close connection

11 What’s the Simplest Option? While (1) accept new connection fork new process let process handle connection Drawback: lots of process creation/deletion

12 Can We Reduce Forks? At program launch, fork some number While (1) accept wait for hello wait for goodbye close Note: wait = implicit yield

13 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?

14 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)

15 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

16 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

17 Scheduling Considerations Timer granularity Finer timers = more responsive Coarse timers = more efficient Accounting Cheap Accurate Fair – consider I/O versus CPU applications

18 Preemptive Scheduling Running Blocked Ready I/O completion interrupt (move to ready queue) Create Terminate (call scheduler) Yield, Timer Interrupt (call scheduler) Block for resource (call scheduler) Scheduler dispatch

19 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)

20 “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