Course : CSC204

CSC204, Processor Management

Overview
This chapter explains how the Processor Manager manages and allocates a single central processing unit (CPU) to execute the incoming jobs. The chapter begins by presenting an overview of single-user systems and providing a foundation of terms and concepts used in processor management. Multi-core technologies are then briefly reviewed, followed by a discussion and comparison of job scheduling and process scheduling. The role of the Process Scheduler is explained in more detail, including a presentation of job and process status concepts, the process control block (PCB) data structure, and queuing. Process scheduling policies are presented next with an explanation of the advantages and disadvantages of preemptive versus nonpreemptive process scheduling algorithms. The goals of process scheduling policies are included. Six common process scheduling algorithms are explained in detail, followed by a description of four cases exemplifying how jobs move through the system. Finally, the chapter discusses the role of internal interrupts and the tasks performed by the interrupt handler.

Learning Objectives
After completing this chapter, the student should be able to describe:
• The difference between job scheduling and process scheduling, and how they relate
• The advantages and disadvantages of process scheduling algorithms that are preemptive versus those that are nonpreemptive
• The goals of process scheduling policies in single-core CPUs
• Up to six different process scheduling algorithms
• The role of internal interrupts and the tasks performed by the interrupt handler

About Multi-Core Technologies
1. Note that the term processor and core are considered equivalent and the processor(s) is located on a single silicon chip. Provide students with an introduction to multi-core technologies. Define multi-core (two or more processors) and explain what dual-core and quad-core mean.
2. Explain how multi-core engineering surfaced due to issues with nano-sized transistors and their ultra-close-placement on a computer chip. Be sure students understand that multi-core engineering helps resolve the current leakage and heat problems, in addition to allowing for the opportunity to permit multiple calculations simultaneously.
3. Note that for the Processor Manager, multi-core processing is more complex to manage than single-core processing. Multi-core processing will be covered in more detail in Chapter 6. This chapter focuses on single-core processing.

How Does Processor Manager Allocate CPU(s) to Jobs?

Process Manager performs job scheduling, process scheduling and interrupt management.

In single-user systems, processor is busy only when user is executing a job—at all other times it is idle.
Processor management is simple.

In multiprogramming environment, processor must be allocated to each job in a fair and efficient manner.
Requires scheduling policy and a scheduling algorithm

Some Important Terms

Program – inactive unit, such as a file stored on a disk.
To an operating system, a program or job is a unit of work that has been submitted by user.
“Job” is usually associated with batch systems.

Process (task) – active entity, which requires a set of resources, including a processor and special registers to perform its function.
A single instance of an executable program

Thread of control (thread) – a portion of a process that can run independently.

Processor ( CPU, central processing unit) –part of machine that performs calculations and executes programs.

Multiprogramming requires that the processor be “allocated” to each job or to each process for a period of time and “deallocated” at an appropriate moment

Job Scheduling vs. Process Scheduling

Processor Manager has 2 sub-managers:

Job Scheduler
in charge of job scheduling.

Process Scheduler
in charge of process scheduling.

Job Scheduler

High-level scheduler.
Selects jobs from a queue of incoming jobs.
Places them in process queue (batch or interactive), based on each job’s characteristics.
Goal is to put jobs in a sequence that uses all system’s resources as fully as possible.
Strives for balanced mix of jobs with large I/O interaction and jobs with lots of computation.
Tries to keep most system components busy most of time.

Process Scheduler

Low-level scheduler – assigns the CPU to execute processes of those jobs placed on ready queue by Job Scheduler.

After a job has been placed on the READY queue by Job Scheduler, Process Scheduler that takes over.
Determines which jobs will get CPU, when, and for how long.
Decides when processing should be interrupted.
Determines queues job should be moved to during execution.
Recognizes when a job has concluded and should be terminated.

CPU Cycles and I/O Cycles

To schedule CPU, Process Scheduler uses common trait among most computer programs: they alternate between CPU cycles and I/O cycles.

Poisson Distribution Curve

I/O-bound jobs (such as printing a series of documents) have many brief CPU cycles and long I/O cycles.

CPU-bound jobs (such as finding the first 300 prime numbers) have long CPU cycles and shorter I/O cycles.

Total effect of all CPU cycles, from both I/O-bound and CPU-bound jobs, approximates a Poisson distribution curve.

Processor Manager : Middle-Level Scheduler
In a highly interactive environment there’s a third layer
called middle-level scheduler.

Removes active jobs from memory to reduce degree of multiprogramming and allows jobs to be completed faster

Job and Process Status

Job status -- one of the 5 states that a job takes as it moves through the system.

HOLD
READY
WAITING
RUNNING
FINISHED

Transition Among Process States

HOLD to READY : Job Scheduler using a predefined policy.
READY to RUNNING : Process Scheduler using some predefined algorithm
RUNNING back to READY : Process Scheduler according to some predefined time limit or other criterion.
RUNNING to WAITING : Process Scheduler and is initiated by an instruction in the job.
WAITING to READY : Process Scheduler and is initiated by signal from I/O device manager that I/O request has been satisfied and job can continue.
RUNNING to FINISHED : Process Scheduler or Job Scheduler.

Process Control Block (PCB)

Process Control Block (PCB) -- data structure that contains basic info about the job
Process identification
Process status (HOLD, READY, RUNNING, WAITING)
Process state (process status word, register contents, main memory info, resources, process priority)
Accounting (CPU time, total amount of time, I/O operations, number input records read, etc.)

PCBs and Queuing

CB of job created when Job Scheduler accepts it
updated as job goes from beginning to termination.

Queues use PCBs to track jobs.
PCBs, not jobs, are linked to form queues.
E.g., PCBs for every ready job are linked on READY queue; all PCBs for jobs just entering system are linked on HOLD queue.
Queues must be managed by process scheduling policies and algorithms.

Process Scheduling Policies

Before operating system can schedule all jobs in a multiprogramming environment, it must resolve three limitations of system:

finite number of resources (such as disk drives, printers, and tape drives)
some resources can’t be shared once they’re allocated (such as printers)
some resources require operator intervention (such as tape drives).

A Good Scheduling Policy

Maximize throughput by running as many jobs as possible in a given amount of time.
Maximize CPU efficiency by keeping CPU busy 100 % of time.
Ensure fairness for all jobs by giving everyone an equal amount of CPU and I/O time.

Minimize response time by quickly turning around interactive requests.
Minimize turnaround time by moving entire jobs in/out of system quickly.
Minimize waiting time by moving jobs out of READY queue as quickly as possible.

Interrupts

There are instances when a job claims CPU for a very long time before issuing an I/O request.
builds up READY queue & empties I/O queues.
Creates an unacceptable imbalance in the system.

Process Scheduler uses a timing mechanism to periodically interrupts running processes when a predetermined slice of time has expired.
suspends all activity on the currently running job and reschedules it into the READY queue.

Preemptive & Non-preemptive Scheduling Policies

Preemptive scheduling policy interrupts processing of a job and transfers the CPU to another job.

Non-preemptive scheduling policy functions without external interrupts.
Once a job captures processor and begins execution, it remains in RUNNING state uninterrupted.
Until it issues an I/O request (natural wait) or until it is finished (exception for infinite loops).

Process Scheduling Algorithms

First Come First Served (FCFS)
Shortest Job Next (SJN)
Priority Scheduling
Shortest Remaining Time (SRT)
Round Robin
Multiple Level Queues

First Come First Served (FCFS)

Non-preemptive.
Handles jobs according to their arrival time -- the earlier they arrive, the sooner they’re served.
Simple algorithm to implement -- uses a FIFO queue.
Good for batch systems; not so good for interactive ones.
Turnaround time is unpredictable.

FCFS Example

Process    CPU Burst (Turnaround Time)
    A        15 milliseconds
    B        2 milliseconds
    C        1 millisecond

If they arrive in order of A, B, and C.
    What does the time line look like?
    What’s the average turnaround time?

Shortest Job Next (SJN)

Non-preemptive.
Handles jobs based on length of their CPU cycle time.
Use lengths to schedule process with shortest time.
Optimal – gives minimum average waiting time for a given set of processes.
optimal only when all of jobs are available at same time and the CPU estimates are available and accurate.
Doesn’t work in interactive systems because users don’t estimate in advance CPU time required to run their jobs.

Priority Scheduling

Non-preemptive.
Gives preferential treatment to important jobs.
Programs with highest priority are processed first.
Aren’t interrupted until CPU cycles are completed or a natural wait occurs.
If 2+ jobs with equal priority are in READY queue, processor is allocated to one that arrived first (first come first served within priority).
Many different methods of assigning priorities by system administrator or by Processor Manager.

Shortest Remaining Time (SRT)

Preemptive version of the SJN algorithm.
Processor allocated to job closest to completion.
This job can be preempted if a newer job in READY queue has a “time to completion” that's shorter.
Can’t be implemented in interactive system -- requires advance knowledge of CPU time required to finish each job.
SRT involves more overhead than SJN.
OS monitors CPU time for all jobs in READY queue and performs “context switching”.

Context Switching Is Required by All Preemptive Algorithms

When Job A is preempted
All of its processing information must be saved in its PCB for later (when Job A’s execution is continued).
Contents of Job B’s PCB are loaded into appropriate registers so it can start running again (context switch).
Later when Job A is once again assigned to processor another context switch is performed.
Info from preempted job is stored in its PCB.
Contents of Job A’s PCB are loaded into appropriate registers.

Round Robin

Preemptive.
Used extensively in interactive systems because it’s easy to implement.
Isn’t based on job characteristics but on a predetermined slice of time that’s given to each job.
Ensures CPU is equally shared among all active processes and isn’t monopolized by any one job.
Time slice is called a time quantum
size crucial to system performance (100 ms to 1-2 secs)

If Job’s CPU Cycle < Time Quantum

If job’s last CPU cycle & job is finished, then all resources allocated to it are released & completed job is returned to user.

If CPU cycle was interrupted by I/O request, then info about the job is saved in its PCB & it is linked at end of the appropriate I/O queue.
Later, when I/O request has been satisfied, it is returned to end of READY queue to await allocation of CPU.

Time Slices Should Be ...

Long enough to allow 80 % of CPU cycles to run to completion.

At least 100 times longer than time required to perform one context switch.

Flexible -- depends on the system.

Multiple Level Queues

Not a separate scheduling algorithm.
Works in conjunction with several other schemes where jobs can be grouped according to a common characteristic.

Examples:
Priority-based system with different queues for each priority level.
Put all CPU-bound jobs in 1 queue and all I/O-bound jobs in another. Alternately select jobs from each queue to keep system balanced.
Put batch jobs “background queue” & interactive jobs in a “foreground queue”; treat foreground queue more favorably than background queue.

Policies To Service Multi-level Queues

No movement between queues.
Move jobs from queue to queue.
Move jobs from queue to queue and increasing time quantums for “lower” queues.
Give special treatment to jobs that have been in system for a long time (aging).

Cache Memory

Cache memory -- quickly accessible memory that’s designed to alleviate speed differences between a very fast CPU and slower main memory.
Stores copy of frequently used data in an easily accessible memory area instead of main memory.

Types of Interrupts

Page interrupts to accommodate job requests.
Time quantum expiration.
I/O interrupts when issue READ or WRITE command.
Internal interrupts (synchronous interrupts) result from arithmetic operation or job instruction currently being processed.
Illegal arithmetic operations (e.g., divide by 0).
Illegal job instructions (e.g., attempts to access protected storage locations).

Interrupt Handler

Describe & store type of interrupt (passed to user as error message).
Save state of interrupted process (value of program counter, mode specification, and contents of all registers).
Process the interrupt
Send error message & state of interrupted process to user.
Halt program execution
Release any resources allocated to job are released
Job exits the system.
Processor resumes normal operation.

Key Terms

aging
cache memory
context switching
CPU-bound
first come first served
high-level scheduler
I/O-bound
indefinite postponement
internal interrupts
interrupt
interrupt handler
Job Scheduler
job status
low-level scheduler
middle-level scheduler
multiple level queues
multiprogramming
non-preemptive scheduling policy
preemptive scheduling policy
priority scheduling
process
Process Control Block
Process Scheduler
process scheduling algorithm
process scheduling policy process status
processor
queue
response time
round robin
shortest job next (SJN)
shortest remaining time
synchronous interrupts
time quantum
time slice
turnaround time