CS 3013 & CS 502 Summer 2006

Scheduling 1

Scheduling

The art and science of allocating the CPU and other

resources to processes

CS 3013 & CS 502 Summer 2006

Scheduling 2

• Bursts of CPU usage alternate with periods of I/O wait– a CPU-bound process– an I/O bound process

• Which process should have higher CPU priority?• Which process should have higher I/O or disk priority?

CS 3013 & CS 502 Summer 2006

Scheduling 3

Process Scheduling

• When to run scheduler1. Process create2. Process exit3. Process blocks4. System interrupt

• Non-preemptive – process runs until it blocks or gives up CPU (1,2,3)

• Preemptive – process runs for some time or until some asynchronous action interrupts, then scheduler selects a process to run (1-4)

CS 3013 & CS 502 Summer 2006

Scheduling 4

Scheduling – Policies

• Issues– Fairness – don’t starve process– Priorities – most important first– Deadlines – task X must be done by time t– Optimization – e.g. throughput, response time

• Reality - No universal scheduling policy– Many models– Determine what to optimize - metrics– Select an appropriate one and adjust based on

experience

CS 3013 & CS 502 Summer 2006

Scheduling 5

Process Scheduling – System Needs

CS 3013 & CS 502 Summer 2006

Scheduling 6

Scheduling – Metrics

• Simplicity – easy to implement• Job latency – time from start to completion• Interactive latency – time from action start

to expected system response• Throughput – number of jobs completed• Utilization – keep processor and/or subset

of I/O devices busy• Determinism – insure that jobs get done

before some time or event• Fairness – every job makes progress

CS 3013 & CS 502 Summer 2006

Scheduling 7

Batch System Scheduling

Note: Number of Processes in Memory determines the degree of multiprogramming

CS 3013 & CS 502 Summer 2006

Scheduling 8

Interactive System Scheduling

• First-Come, First-Served (FCFS)• Round Robin (RR)• Shortest Completion Time First

(SCTF)• Priority

CS 3013 & CS 502 Summer 2006

Scheduling 9

Scheduling Policies – FCFS

• First Come, First Served (FCFS)– Easy to implement– Non-preemptive– Minimizes context switch overhead– Favors compute bound jobs– Short jobs penalized

• I.e., once a longer job gets the CPU, it gets in the way of a bunch of shorter jobs

CS 3013 & CS 502 Summer 2006

Scheduling 10

Scheduling Policies – Round Robin

• Round Robin (RR)– Preemptive– Ready processes given a quantum of

time when scheduled– Process runs until quantum expires or

until it blocks (whichever comes first)

– Suitable for interactive (timesharing) systems

– Setting quantum is critical for efficiency

CS 3013 & CS 502 Summer 2006

Scheduling 11

Scheduling Policies – Comparison

10 jobs each take 100 seconds – look at when jobs complete

• FCFS – job 1: 100s, job 2: 200s … job 10:1000s

• RR– 1 sec quantum– Job 1: 991s, job 2 : 992s …

• RR good for short jobs – worse for long jobs

CS 3013 & CS 502 Summer 2006

Scheduling 12

Scheduling Policies – RR example

• Utilization– A and B compute bound jobs:– 100 ms @ 100%

CPU– C:– 1 ms CPU and 10 ms disk I/O– Quantum:– 100 ms

• CPU utilization = (100 + 100 +1)/210 = 95.7 %

– Quantum:– 1 ms (assume 0 overhead)• ABCAB(C-I/O)ABABAB ….AB• CPU utilization = 100%

– Smaller quantum can improve utilization – but consider overhead of context switches

CS 3013 & CS 502 Summer 2006

Scheduling 13

Scheduling Policies – STCF

• STCF – shortest time to completion first

• or shortest job first

– Can be preemptive– Good for throughput– Example

• jobs A = 100, B = 1, C = 2 msec• Done B @ 1, C @ 3 , A @ 103• Ave = 35, (for RR (1) about 37)

– Reality – knowledge of job runtime??

CS 3013 & CS 502 Summer 2006

Scheduling 14

Scheduling Policies – Priority

• Priority Scheduling– Preemptive– Process are given priorities and ranked

• Maybe done with multiple queues - multilevel

– Highest priority runs next• With multilevel queues

– Select from highest queue– Round robin within queue

– Recalculate priority – many algorithms• E.g. increase priority of I/O intensive jobs• E.g. favor processes in memory • Must still meet system goals – e.g. response time

CS 3013 & CS 502 Summer 2006

Scheduling 15

Scheduling Policy Problem

• Priority inversion – A has high priority, B has medium priority, C

has lowest priority – C acquires a resource that A needs to progress– A attempts to get resource, fails and busy

waits• C never runs to release resource!

or– A attempts to get resources, fails and blocks

• B (medium priority) enters system & hogs CPU• C never runs!

• Priority scheduling can’t be naive

CS 3013 & CS 502 Summer 2006

Scheduling 16

Solution

• Some systems increase the priority of a process/task/job to

• Match level of resourceor

• Match level of waiting process

CS 3013 & CS 502 Summer 2006

Scheduling 17

Scheduling Policies – Realtime

• Real Time System – processes have deadlines– Deadlines known– (usually) preemptive

• Static algorithm – periodic process behavior– Rate Monotonic Scheduling (RMS) – priority =

1/period• Dynamic – aperiodic

– Earliest Deadline First (EDF) – select process that must complete the soonest

More about this subject later in the term

CS 3013 & CS 502 Summer 2006

Scheduling 18

Scheduling – Examples

• Unix – multilevel - many policies and many policy changes over time

• Linux – multilevel with 3 major levels– Realtime FIFO – Realtime round robin– Timesharing

• Win/NT – multilevel– Threads scheduled – fibers not visible to

scheduler– Jobs – groups of processes are given quotas

that contribute to priorities

CS 3013 & CS 502 Summer 2006

Scheduling 19

Scheduling

• Until now:– focus on processes and CPU • Going forward:– focus on system resources

• memory, storage, I/O devices, web resources, …

• Space sharing– How can resources be split up? – e.g. disk, memory

• Time sharing – What gets to use something and for how long?– Whenever there are more requests than can be

granted• Typically resource can not be divided – I/O port, device, …

CS 3013 & CS 502 Summer 2006

Scheduling 20

Scheduling

• General theme – what is the “best way” to run n processes on k resources? ( k < n)

• Conflicting Objectives – no one “best way”– Latency vs. throughput– Speed vs. fairness

• Incomplete knowledge – E.g. – does user know how long a job will take

• Real world limitations– E.g. context switching takes CPU time– Job loads are unpredictable

CS 3013 & CS 502 Summer 2006

Scheduling 21

Scheduling (continued)

• Bottom line – scheduling is hard!– Know the models– Adjust based upon experience

CS 3013 & CS 502 Summer 2006

Scheduling 22

Break

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