Part iii process management cpu scheduling
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Part III: Process Management ( CPU Scheduling ). Operating Systems. CPU-I/O Burst Cycle. CPU burst - CPU executes process I/O burst - process does I/O Processes go back & forth between 2 states Frequency curve of CPU bursts is inverse exponential:

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Part III: Process Management

(CPU Scheduling)

Operating Systems

CPU-I/O Burst Cycle

CPU burst - CPU executes process

I/O burst - process does I/O

Processes go back & forth between 2 states

Frequency curve of CPU bursts is inverse exponential:

Large number of short CPU bursts, small number of long CPU bursts

CPU-bound: few but long CPU bursts

I/O-bound: many very short CPU bursts

Preemptive & Non-preemptive Scheduling

CPU decisions needed in:

switching from running to waiting (i/o request)

switching from running to ready (timer interrupt)

switching from waiting to ready (i/o done)

process termination

1st and 4th : no choice in terms of scheduling: new process must be selected for execution

2nd and 3rd: scheduling is pre-empted

Preemptive & Non-preemptive Scheduling

Non-preemptive scheduling

Once CPU is allocated to a process, it is released only by termination or waiting

Does not need special hardware (timer)

Preemptive & Non-preemptive Scheduling

Preemptive scheduling

Processes may be interrupted any time

Costly because:

shared data update by a process may be preempted

kernel process may be preempted (e.g. medium-term scheduler)

solution: wait for system call to finish -> not good for real-time computing


Gives control to process selected by short-term scheduler



Switching to user-mode

Jumping to program location to start it

Dispatch latency - time it takes the dispatcher to stop one process and start another running

Scheduling Criteria

CPU utilization - keep CPU busy (40% - 100%)

Throughput - number of processes completed per unit timeMaximize these.

Scheduling Criteria

Waiting time - sum of time spent in ready queue

Turnaround time - time it takes to execute process (from submission to completion)

Response time - time it takes to start responding (from submission to first response); does not include time to output responseMinimize the above.

Scheduling Algorithms

Scheduling: which process in the ready queue is to be run (allocated to the CPU)?

First Come First Served (FCFS)

Easily managed by a FIFO queue

Simple to write and understand

Long waiting time depending on order of process


Scheduling Algorithms

Shortest Job First (SJF)

CPU assigned to process that has shortest next CPU burst

FCFS used for processes that have same length

Can be proved to be optimal (minimum waiting time)

Difficulty is determining length or time duration of next CPU burst.

Scheduling Algorithms

Shortest Job First (continued)

Determining length or time duration of next CPU burst: approximate by using the exponential average of the job's previous CPU bursts.

tn is the length of the nth CPU burst (previous),ξn+1 the predicted value for the next CPU burst:ξn+1 = α tn + (1 – α ) ξn

Scheduling Algorithms

Shortest Job First (continued)

Long-term scheduling (batch system): length of process time limit specified by user

Short-term scheduling:

SJF cannot be implemented since there’s no way to know length of next CPU burst

Implementation relies on predicting length of next CPU burst, using the previous formula

Scheduling Algorithms

Shortest Job First (continued)

May be:

Preemptive - also called shortest-remaining-time-first

Non-preemptive - allows current process to finish

Scheduling Algorithms

Priority Scheduling

A priority is associated with each process. CPU is allocated to process with the highest priority. Equal priority processes are scheduled in FCFS order or SJF order.

SJF special case of priority scheduling algorithm. CPU is allocated to process with shortest next burst (highest priority), and longest next burst has lowest priority.

Scheduling Algorithms

Priority Scheduling (continued)

Priority defined either

Internally - measurable quantities to compute priority (memory requirements, time limits, number of open files, etc.)

Externally - set by criteria outside the O/S (importance of process, funded projects, etc.)

Scheduling is also preemptive or non-preemptive

Scheduling Algorithms

Priority Scheduling (continued)

Indefinite blocking or starvation - can leave low priority processes starving for CPU allocation

Solution: Aging - low priority process gradually increases priority until it is finally allocated to the CPU

Scheduling Algorithms

Round-Robin Scheduling

Designed specifically for time-sharing systems

Similar to FCFS but with preemption

Time quantum or time slice is defined (typically between 10 and 100 milliseconds)

CPU goes around ready queue allocating 1 time quantum to each process

Scheduling Algorithms

Round-Robin Scheduling (continued)

Two things can happen:

Process takes up 1 time quantum, or

Process gives CPU up voluntarily

Scheduling Algorithms

Round-Robin Scheduling (continued)

Performance depends heavily on size of time quantum, n

n is very small - called processor sharing (virtual processor runs at 1/n the speed) -> smaller time quantum increases context-switching -> make time-quantum >> context switch (10%)

n is large - scheduling degenerates to FCFS policy

Rule of thumb: 80% of CPU bursts should be shorter than time quantum

Scheduling Algorithms

Multilevel Queue Scheduling

Ready queue classified into several groups

Processes are permanently assigned to a queue depending on priority, memory size, etc. (e.g. separate queues used for foreground and background processes)


Absolute priority: System -> Interactive -> Batch -> Low-priority

Time-slice: 80% foreground, 20% background

Scheduling Algorithms

Multilevel Feedback Queue Scheduling

Allows processes to move between queues

Separate processes w/ different CPU burst characteristics

Too much CPU time -> move to low-priority queue

I/O-bound and interactive -> high-priority queue

Use aging : if process waits too long in low-priority queue -> move to high-priority queue to prevent starvation

Scheduling Algorithms

Multilevel Feedback Queue (continued)

General parameters:

Number of queues

Scheduling algorithm for each queue

Method to upgrade process to high-priority queue

Method to demote process to low-priority queue

Method to determine what queue a process will enter

Considered the most general (can be configured to fit a system), but also the most complex

Multiprocessor Scheduling

Scheduling process becomes more complex

Many possibilities tried, no one best solution

Heterogeneous system

Processors are different (e.g. distributed system)

Process can only run on processor it was compiled in

Multiprocessor Scheduling

Homogenous system

Identical CPUs within a multiprocessor system

Process can run in any CPU

Can have load sharing

Separate queue for each processor

May have one processor busy while another is idle (empty queue)

Remedied with a common ready queue

Multiprocessor Scheduling

Homogeneous system (continued)

Load sharing - 2 possibilities

Self-scheduling (symmetric scheduling) SMP - each processor examines queue and selects process to execute -> careful programming to ensure no two processors choose the same process

Master-slave (asymmetric scheduling) - one processor is appointed as scheduler -> in some systems, other system activities are performed by the master, slaves only execute user code

Real-Time Scheduling

Hard real-time

Resource reservation -> scheduler either admits a process w/ guaranteed response time, or rejects it

Guarantee impossible for systems with virtual memory or secondary storage

Composed of special software running hardware dedicated to critical processes

Soft real-time -> less restrictive

Requires only that critical processes receive priority

Can support multimedia & high speed graphics

Algorithm Analysis

Analytic Evaluation

Uses an algorithm and the system workload to produce a formula to evaluate performance

Deterministic Modeling

Takes a particular predetermined workload and defines performance of each algorithm for that (sample) workload

Simple, fast, and gives exact numbers (input/output)

Requires too specific and too much exact knowledge to be useful -> processes that run daily vary greatly

Algorithm Analysis

Queueing Models

CPU burst distribution is used instead of predetermined workload


Uses software to simulate major system components

Expensive -> requires hours of computer time

Design, coding, and debugging of simulator is complex

Algorithm Analysis


Simulations have limited accuracy

Only way to find out is to implement

Puts algorithm to test in real environment



Reaction of users to constantly changing O/S

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