1. Rensselaer Polytechnic Institute CSCI-4210 – Operating Systems David Goldschmidt, Ph.D.

2.  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!

3.  Processes are created by the operating system  Processes initially added to a job queue, which contains all processes waiting to enter the system  From the job queue, processes that are ready for execution are added to the ready queue

4.  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 queue and allocates time with the CPU  More on this later....

5.  The long-term scheduler is invoked infrequently

6.  The degree of multiprogramming of an operating system is defined as the number of processes in memory  In a stable operating system, the average process arrival rate equals the average process departure rate

7.  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 its time 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

8.  Most modern operating systems have no long-term scheduler (e.g. Windows, UNIX)  All processes are admitted to the ready queue, regardless of whether the operating system can handle the load  Often results in users changing their usage habits....

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

10.  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

11.  For each process, the operating system manages and executes processes by recording:  Program counter (PC)  Registers  Data section (global data)  Stack (temporary data)  Heap (dynamically allocated memory) heap stack data text/code

12.  As a process executes, it changes its state

13.  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

14. context switch takes a few milliseconds

16.  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 10ms to schedule the next process and perform the context switch?

17.  Processes are created from other processes  A parent process creates a child process, which in turn creates child processes of its own, etc.  A tree of processes is the result:

18.  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

19.  Each process has its own unique process identifier (pid)

20.  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

21.  In Unix, a new child process is forked via the fork() system call  Child optionally calls the exec() system call to load a new program