1. ICS 143 - Principles of Operating Systems
Discussion 3 - HW1 review & CPU Scheduling
Joe Nash

2. Outline Homework 1 Solutions - will be posted
Lecture 6 - CPU Scheduling Review

3. General Strategies
Ctrl-f lecture notes
Google question
Piazza

4. Don’t do this

5. Homework 1 Q1 Polling/Interrupt definition - Lecture 1 page 66
How many machine instructions can be executed in the time it takes a character to be typed?
Arithmetic
Interrupts are Asynchronous
Traps are software generated interrupts.
Lecture 1 page 89

6. Homework 1 Q2
Multiprogramming vs timesharing - Definition Lecture 1 page 67/68
Distributed computing systems
Resources distributed over multiple machines
Cloud computation, distributed databases, sensor nets

7. Homework 1 Q2 continued C. Discussion 1 page 20
Hard real-time = deadline missed -> failure
Soft real-time = deadline missed -> performance lost
Non-Real time = deadline missed -> nothing lost
Spotify = soft
Android UI = soft
Cruise control = hard
HF trading = hard

8. Homework 1 Q3
For each of the brief description about the following system calls used in Unix, write the name of the system call and its parameters. (Hint: Use the man pages.)
Read from a file descriptor --- ssize_t read(int fd, void *buf, size_t count);
Wait for process termination --- pid_t wait(int *status);
Create a child process (fork) --- pid_t fork(void);
Suppose you want to write a program that opens the file passed to it on the command line (argv[1]) and writes another file back out (with the same contents). List 3 system calls that will be used in this program. Any three of the following would be fine: open, read, write, close
For each of the following operations, indicate whether they should be privileged or not by checking ☒ the appropriate boxes.
Yes No
Searching a string for a particular sequence X
Generate an interrupt
Process hardware interrupt
Check list of currently running processes
Access network card buffer

9. Homework 1 Q4 Lecture ¾ page 14-15 Lecture ¾ page 7 PCB Yes No
CPU registers X
Number of processes
Program counter
Process id
File permissions

10. Homework 1 Q5 Why is that threads are faster to create than processes?
Create process -> allocate memory for the process and create a process control block.
Create thread -> create a much smaller thread control block.
Suggests one application that benefit from the use of threads
Multiple tasks at once
Web crawler, FTP, matrix multiplication
Distinguish between user-level threads and kernel-level threads in terms of cost of switching. Which of these is more expensive to switch?
Switching between user-level threads is cheaper because there is no context switch.
Indicate which of these resources are shared among the threads
Shared
Private
Memory (Heap) X
Program counter
Execution stack
Global variables

11. Interprocess Communication
Message passing
Send/Receive
Shared memory
Producer/Consumer Buffer

12. CPU Scheduling First Come First Serve Round Robin
Shortest Job First (SJF)
preemptive & non-preemptive
When do they perform well?
Run them from inputs

13. When do the scheduler algos perform badly?
First Come First Serve
Badly - long jobs first
Round Robin
Badly - many jobs same length
Shortest Job First (SJF)
Badly - many short jobs, runtime unpredictable
Not exhaustive

14. Running the Schedulers
Given Create

15. Finished
Questions?