university of pennsylvania 10/5/00cse 3801 cpu scheduling cse 380 lecture note 9 insup lee

10/5/00 CSE 380 1

University of Pennsylvania

CPU Scheduling

CSE 380

Lecture Note 9

Insup Lee

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CPU SCHEDULING

• The basic problem is as follows: How can OS schedule the allocation of CPU cycles to processes in system, to achieve “good performance”?

• Components of CPU scheduling subsystem of OS:

– Dispatcher - performs context switches

– Scheduler - selects next process from those in main memory (short-term scheduler)

– Swapper - manages transfer of processes between main memory and secondary storage (medium-term scheduler)

– Long-Term Scheduler - in a batch system, determines which and how many jobs to admit into system.

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Behavior of Processes in Execution

• Useful concept in context of scheduling:

– CPU Burst - period of CPU time process is executing between I/O requests. The text also talks about “I/O burst”.

• Processes alternate between CPU & I/O bursts:

– CPU I/O CPU I/O CPU ... I/O CPU

• CPU-bound process uses CPU for long periods of time.

• I/O-bound process uses CPU briefly before it issues another I/O request.

• In interactive system, usually important to have fast response time. Accomplished by having scheduler favor I/O-bound processes.

• Necessary to determine as quickly as possible the nature (CPU-bound or I/O-bound) of a process, since usually not known in advance.

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General Organization for CPU Scheduling

• Processes ready to use CPU are kept in a ready queue.

• Processes waiting for I/O are kept in wait queue associated with device on which they are performing I/O.

• Dispatcher: called from timer interrupt or when current process blocks for I/O performs a context switch to first process in ready queue

• Scheduler: called periodically, usually less often than dispatcher. Reorders ready queue to reflect changing priorities.

• Swapper: keeps track of “process mix'' in main memory. If CPU utilization not high enough, processes can be “swapped” between main memory and a swap area on secondary storage.

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Types of Scheduling Algorithms

• Preemptive: process may have CPU taken away before completion of current CPU burst

• Non-preemptive: processes always run until CPU burst completes

• Static Priority

• Dynamic Priority

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Performance Criteria for Scheduling

• Scheduling (as an Optimization task): How to best order the ready queue for efficiency purposes.

• CPU utilization: % of time CPU in use

• Throughput: # of jobs completed per time unit

• Turnaround Time: wall clock time required to complete a job

• Waiting Time: amount of time process is ready but waiting to run

• Response Time: in interactive systems, time until system responds to a command

• Response Ratio: (Turnaround Time)/(Execution Time) -- long jobs should wait longer

• The overhead of a scheduling algorithm (e.g., data kept about execution activity, queue management, context switches) should also be taken into account.

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Kleinrock's Conservation Law

No matter what scheduling algorithm is used, you cannot help one class of jobs without hurting the other ones.

Example: A minor improvement for short jobs (say, on waiting time) causes a disproportionate degradation for long jobs.

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Basic Scheduling Algorithm

• FCFS - First-Come, First-Served

– Non-preemptive

– Ready queue is a FIFO queue

– Jobs arriving are placed at the end of queue

– Dispatcher selects first job in queue and runs to completion of CPU burst

• Advantages: simple, low overhead

• Disadvantages: inappropriate for interactive systems, large fluctuations in average turnaround time are possible.

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FCFS Example

Pid Arr CPU Start Finish Turna Wait Ratio---+---+---+-----+------+-----+----+----- A 0 3 0 3 3 0 1.0 B 1 5 3 8 7 2 1.4 C 3 2 8 10 7 5 3.5 D 9 5 10 15 6 1 1.2 E 12 5 15 20 8 3 1.6

---+---+---+-----+------+-----+----+----- A 0 1 0 1 1 0 1.00 B 0 100 1 101 101 1 1.01 C 0 1 101 102 102 101 102.00 D 0 100 102 202 202 102 2.02

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RR - Round Robin

• Preemptive version FCFS

• Treat ready queue as circular

– arriving jobs are placed at end

– dispatcher selects first job in queue and runs until completion of CPU burst, or until time quantum expires

– if quantum expires, job is again placed at end

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Properties of RR

Advantages: simple, low overhead, works for interactive systems

Disadvantages: if quantum too small, too much time wasted in context switching; if too large, approaches FCFS.Typical value: 10 - 100 msecRule of thumb: choose quantum so that large majority (80-90%) of jobs finish CPU burst in one quantum

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SJF - Shortest Job First

• non-preemptive

• ready queue treated as a priority queue based on smallest CPU-time requirement

– arriving jobs inserted at proper position in queue

– dispatcher selects shortest job (1st in queue) and runs to completion

Advantages: provably optimal w.r.t. average turnaround time

Disadvantages: in general, unimplementable. Also, starvation possible!

Can do it approximately: use exponential averaging to predict length of next CPU burst ==> pick shortest predicted burst next!

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Exponential Averaging

n+1 tn n

n+1 : predicted length of next CPU burst

tn : actual length of last CPU burst

n : previous prediction

= 0 implies make no use of recent history

n+1 n

= 1 implies n+1 = tn (past prediction not used).

= 1/2 implies weighted (older bursts get less and less weight).

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SRTF - Shortest Remaining Time First

• Preemptive version of SJF

• Ready queue ordered on length of time till completion (shortest first)

• Arriving jobs inserted at proper position

• Dispatcher selects shortest job and runs to completion or until a job with a shorter remaining time arrives in the system.

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Performance Evaluation

• Deterministic Modeling (vs. Probabilistic) Look at behavior of algorithm on a particular workload, and compute various performance criteria

Example:

workload - Job 1: 24 units Job 2: 3 units Job 3: 3 units

• Gantt chart for FCFS:

| Job 1 | Job 2 | Job 3 | 0 24 27 30

Total waiting time: 0 + 24 + 27 = 51

Average waiting time: 51/3 = 17

Total turnaround time: 24 + 27 + 30 = 81

Average turnaround time: 81/3 = 9

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RR and SJF

• Chart for RR with quantum of 3:

| Job 1 | Job 2 | Job 3 | Job 1 | 0 3 6 9 30


Avg. waiting time: 15 / 3 = 5

• Chart for SJF:

| Job 2 | Job 3 | Job 1 | 0 3 6 30


Avg. waiting time: 9 / 3 = 3

• Can see that SJF gives minimum waiting time. RR is intermediate. (This can be proved in general.)

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HPF - Highest Priority First

• general class of algorithms

• each job assigned a priority which may change dynamically

• may be preemptive or non-preemptive

Problem: how to compute priorities?

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Multi-Level Feedback (FB)

· process enters (next) lower queue with each timer interrupt (penalized)

· bottom queue is standard Round Robin

· process in a given queue are not scheduled until all higher queues are empty

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FB Discussion

I/O-bound processes tend to congregate in higher-level queues. (Why?)

Nice, since obtain greater device utilization

Quantum in top queue should be large enough to satisfy majority of I/O-bound processes

Variations – Priority can assign a process a lower priority by starting it at a lower-level

queue

can raise priority by moving process to a higher queue can use in conjunction with aging

Adaptive to adjust priority of a process changing from CPU-bound to I/O-

bound, can move process to a higher queue each time it voluntarily relinquishes CPU.

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University of PennsylvaniaShort-term Process Scheduling in 4.3 BSD• based on multi-level feedback queues

• priorities range from 0 to 127

• 0-49 reserved for processes executing in kernel mode

• 50-127 reserved for scheduling processes in user mode

• number of queues 127 in theory; 32 in VAX

• time quantum = 1/10 sec (empirically found to be the longest quantum that could be used without loss of the desired response for interactive jobs such as editors)

– short time quantum means better interactive response

– long time quantum means higher overall system throughput since less context switch overhead and less processor cache flush.

• priority dynamically adjusted to reflect

– resource requirement (e.g., blocked awaiting an event)

– resource consumption (e.g., CPU time)

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Unix CPU scheduler (cont.)

• two values in the PCBp_cpu: an estimate of the recent CPU usep_nice: a user settable weighting factor (-20..20); default = 0; negative increases priority.

• A process' priority calculated every four clock ticks (40 msec)p_usrpi = PUSER + p_cpu/4 + 2 p_nice

where PUSER = 50.

• CPU utilization, p_cpu, is incremented each time the system clock ticks and the process is found to be executing.

• p_cpu is adjusted once every second (using a digital decay filter)p_cpu = [(2 load)/(2 load + 1)] p_cpu + p_nice

where load is an average number of ready processes over the previous 1 minute.

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Examples

Example 1. single compute-bound process which monopolizes CPU.

p_cpu = 0.66 p_cpu + p_nice

Example 2. this process accumulates Ti clock ticks every 1 second and p_nice is zero.

p_cpu = 0.66 T0

p_cpu = 0.66 (T1 + .66 T0) = .66 T1 + .44 T0

p_cpu = 0.66 T2 + .44 T1 + .30 T0

p_cpu = 0.66 T3 + ... + .20 T0

p_cpu = 0.66 T4 + ... + .13 T0

About 90 % of the CPU utilization is forgotten after 5 seconds.

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For blocked process,

the system recomputes the priority of a process when the process is awakened and has been sleeping for longer than 1 second.

p_slptime is set to 0 when a process starts sleep and is incremented once every second.

when the process is awakened,

p_cpu = [2 load/(2 load +1)]p_slptime p_cpu

Then, computes p_usrpri as above.

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Real-time Systems

• On-line transaction systems and interaction systems

• Real-time monitoring systems

• Signal processing systems

o typical computations

o timing requirements

o typical architectures

• Control systems

o computational and timing requirements of direct computer control

o hierarchical structure

o intelligent control

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Desired characteristics of RTOS

Predictability, not speed, fairness, etc.

• Under normal load, all deterministic (hard deadline) tasks meet their timing constraints

• under overload conditions, failures in meeting timing constrains occur in a predictable manner.

Interrupt handling and context switching take bounded times

Application- directed resource management

• scheduling mechanisms allow different policies

• resolution of resource contention can be under explicit direction of the application.

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Real-Time System Scheduling

• Processes have timing constraints such as period, deadline

• Schedulability analysis

• Cyclic executive

• Priority based scheduling algorithms

• earliest deadline first

• rate monotonic algorithm

• Priority inversion problem; e.g., Mars’ Pathfinder reset problem

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The Priority Inversion Problem

T1

T2

T3

blocked attempt lock R lock(R)

lock(R)

unlock(R)

unlock(R)

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The priority inheritance protocol

T1

T2

T3

lock R fails lock(R)

lock(R)

unlock(R)

unlock(R)

T3 blocks T2

T3 directly blocks T1T3’s priority is

university of pennsylvania 10/5/00cse 3801 cpu scheduling cse 380 lecture note 9 insup lee

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