Process Scheduling

CSC501 Operating Systems Principles

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Last Lecture

q Process Lifecyclesq Context Switching

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Process Scheduling

q Key stepsQ Examine processes eligible for executionQ Select one based on a certain scheduling policyQ Switch CPU to selected item

q Two-level scheduling possibleQ Select ProcessQ Select thread within Process

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Scheduling Policy

q Fundamental OS policyq Determines when process selected for executionq May depend onQ Process priorityQ Time process waitsQ Process owner (user)

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Process Scheduling Queues

q Ready queueQ Set of processes residing in main memory,

ready, and waiting to executeq Job queueQ Set of all processes in the system

q Device queuesQ Set of processes waiting for an I/O device

q Process migration between the various queues

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Ready Queue And Various I/O Device Queues

Representation of Process Scheduling

Schedulers

q Long-term scheduler (or job scheduler)Q Which processes should be brought into the

ready queueQ Invoked very infrequently (seconds, minutes) ⇒

(may be slow)Q Controls the degree of multiprogramming

q Short-term scheduler (or CPU scheduler)Q Which process should execute next (allocates

CPU)Q Invoked very frequently (milliseconds) ⇒ (must

be fast)

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Schedulers (Cont.)

q Processes can be described as either:Q I/O-bound processv spends more time doing I/O than computations, v many short CPU bursts

Q CPU-bound processv spends more time doing computationsv few very long CPU bursts

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Example Scheduling in XINU

q Each process assigned a priorityQ Non-negative integer valueQ Initialized when process createdQ Can be changed

q Scheduler chooses process with highest priorityQ Processes with the same priority are scheduled

in a round-robin fashionq Policy enforced as a system-wide invariant

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The XINU Scheduling Invariant

q At any time, the CPU must run the highest priority eligible process. Among processes with equal priority, scheduling is round robin.

q Invariant enforced duringQ System callQ Interrupt

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Implementation of Scheduling

q Process eligible if state isQ ready or current

q To avoid searching process tableQ Keep ready processes on linked list called ready

listQ Order ready list by priorityQ Selection in constant time

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Example Scheduler Code

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Puzzle #1

q Invariant says that at any time, one process must be executing

q Context switch code moves from one process to another

q Question: which process executes the context switch code?

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Solution to Puzzle #1

q ‘‘Old’’ processQ Executes first half of context switchQ Is suspended

q ‘‘New’’ processQ Continues executing where previously suspendedQ Usually runs second half of context switch

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Puzzle #2

q Invariant says that at any time, one process must be executing

q All user processes may be idle (e.g., applications all wait for input)

q Which process executes?

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Solution to Puzzle #2

q OS needs an extra processQ Called NULL processQ Never terminatesQ Cannot make a system call that takes it out of

ready or current stateQ Typically an infinite loop

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Lab 1 – Process Scheduling

q Part IQ The chprio() function contains a bug (sys/chprio.c)

q Part IIQ The resched() function contains a limitation

(sys/resched.c), i.e., a process with a lower priority could suffer from starvation

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Lab 1 – Process Scheduling

q Part IQ The chprio() function contains a bug (sys/chprio.c)

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Lab 1 – Process Scheduling

q Part IIQ The process scheduling policy has a limitation,

namely process starvationQ You are asked to implement two different policiesv Random schedulerv Proportional sharing scheduler

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Random Scheduler

q Total N processes Pi (i=0..N-1) in the ready queue

q Each PiQ Priority: PRIOiQ Probability: PRIOi/sum(PRIOj) (j=0..N-1)

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Proportional Sharing Scheduler

q Two factors:Q The priority and execution time

q Timer interrupt handlerQ Related files: sys/clkint.S sys/clkinit.cQ Interrupt rate – based on clock timer v ctr1000: 1ms

Q Scheduling rate: v Interrupt rate * QUANTUM

Q Othersv preempt: preemption counter

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Lab 1 – Process Scheduling

q Read relevant source code in XinuQ Process queue managementv h/q.h sys/queue.c sys/insert.c, …

Q Proc. creation/suspension/resumption/termination: v sys/create.c, sys/suspend.c sys/resume.c, sys/kill.c

Q Priority changev sys/chprio.c

Q Process schedulingv sys/resched.c

Q Other initialization codev sys/initialize.c

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Process Control Block (proc.h)

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Next Lecture

q Process Synchronization

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