Operating Systems{week 02}

Rensselaer Polytechnic InstituteCSCI-4210 – Operating SystemsDavid Goldschmidt, Ph.D.

from Principles of Modern Operating Systems, 1st edition by Garrido and Schlesinger, Jones and Bartlett Publishers, 2008, ISBN 0-7637-3574-4

Multiprogramming and timesharing

Multiprogramming goals: Maximize CPU utilization Maximize number of processes in

memory

Timesharing goals: Switch CPU among processes such that

users interact with each program simultaneously

Maximize fairness

almost!

Process scheduling and queues

Processes are created bythe operating system Processes initially added to

a job queue, which containsall processes waiting to enter the system

From the job queue, processes thatare ready for execution are addedto the ready queue

Schedulers

A long-term scheduler (i.e. job scheduler) selects processes from the job queue, adding those processes to the ready queue

A short-term scheduler (i.e. CPU scheduler) selects processes from the ready queueand allocates time with the CPU More on this later....

Long-term scheduling (i)

The long-term scheduler isinvoked infrequently

Long-term scheduling (ii)

The degree of multiprogramming ofan operating system is defined asthe number of processes in memory In a stable operating system,

the average process arrival rate equalsthe average process departure rate

Long-term scheduling (iii) Processes are either I/O bound or CPU

bound A CPU-bound process does little I/O and

instead makes heavy use of the CPU An I/O-bound process spends a majority of

itstime performing (i.e. waiting for) I/O

The long-term scheduler should select a good process mix of CPU-bound and I/O-bound processes

Long-term scheduling (iv)

Most modern operating systems have no long-term scheduler (e.g. Windows, UNIX) All processes are admitted to

the ready queue, regardlessof whether the operatingsystem can handle the load

Often results in userschanging their usage habits....

Medium-term scheduling

Modern operating systems implement a medium-term scheduler to swap processes to/from memory and disk

Processes (i)

A process is an active program in execution Requires CPU time, memory, file access,

network access, other I/O access

Operating system is responsible for: Creating/deleting processes Scheduling processes Allocating resources to processes Synchronizing communication between

processes

Processes (ii)

For each process, the operatingsystem manages and executesprocesses by recording: Program counter (PC) Registers Data section (global data) Stack (temporary data) Heap

(dynamically allocated memory)

Process states

As a process executes, it changes its state

Process control block

Operating system represents each process via a process control block (PCB) Process state Process ID or number Program counter (PC) CPU registers CPU-scheduling and memory

management information List of open file/resource handles

Process context switch

context switch takesa few milliseconds

Process scheduling

Short-term scheduling

The short-term scheduler frequently decides which process the CPU executes next Typical time slice (t) a process has with

the CPU is 100 milliseconds How much CPU time is wasted

if t is 100ms and it takes 10msto schedule the next processand perform the context switch?

Process creation (i)

Processes are created from other processes A parent process creates a child process,

whichin turn creates child processes of its own, etc.

A tree of processes is the result:

Process creation (ii)

Operating system resources are sometimes shared amongst processes

Possibilities: Parent and child processes share all

resources Child shares a subset of its parent’s

resources Parent and child processes share no

resources

Process creation (iii)

Each process has itsown unique processidentifier (pid)

Process creation (iv)

When a new process is created,the parent has two options: Parent process continues to execute

concurrently with its children Parent process waits for its children to

terminate their execution

The child process decides what it will do: Duplicate the parent or load a new program

Process creation (v)

In Unix, a new child process is forked viathe fork() system call

Child optionally calls the exec() system callto load a new program

What next?

Read and study Chapters 1 and 2

Do Exercises at the end of Chapter 2

Also be sure to do the programming exercise on the next slide in C

Programming exercise

Implement a program to simulate caching: Write a function called calculateAnswer() that

takes integer n as input and calculates (and returns)the sum (1 + 2 + … + n) ▪ Pretend this method is computationally costly!

Initially, the cache is empty▪ Ask the user to input a number in range 1..100 ▪ If the answer is not in the cache, call calculateAnswer() and display the resulting sum; store the result in the cache

▪ If the answer is in the cache, simply display the answer

Did it work?