1. February 5, 2004 Adrienne Noble
Week 5 Midterm Review
February 5, 2004
Adrienne Noble

2. Questions? Important dates: Midterm – Tomorrow! 
Project 2b – Due Wednesday, Feb 11
Homework 5 – Due Wednesday, Feb 11

3. Project 2a Hints Start by figuring out what’s happening in the code
Where is the process finding/handling new requests?
Understand threadpools
Make sure you leave sufficient time to do the experiment and write-up

4. What is an OS? Software layer to abstract away from hardware resources
“all the code you didn’t write” to implement your application

5. Architectural Support
Architectural features supporting the OS
Protected instructions
Kernel mode vs. user mode
Interrupts and Exceptions
Timer, set-and-set, memory protection, I/O operations, interrupts/exceptions
Access to I/O devices, page table pointers, manipulate “mode” bits
Interrupts caused by hardware devices
Exceptions are caused by software

6. OS Components Processes Memory I/O Secondary storage File systems
Protection
Accounting
Shells
GUI
Networking

7. Types of OS Monolithic design Layering Microkernels
Hardware, kernel, page manager, console manager, device managers, job managers

8. Processes A process consists of… States of a process Fork, clone, exec
Running, ready, blocked
How to get between states
Fork, clone, exec
Address space, code, data, stack/SP, PC, general purpose registers, OS resources (files, network connections, etc)
Running-> blocked = exception
Running-> ready = interrupt
Ready-> running = dispatch
Blocked-> ready = interrupt

9. Threads A thread consists of… Kernel versus User Threads
Context switching – what do you need to do?
Simplethreads package
Stack (a portion of the process’s stack), SP, PC, register values
Big difference is that kernels need to interrupt for every context switch

10. Synchronization Reasons synchronization is needed
What is a critical section?
Understand the following
Mutex
Semaphore
Monitor (Mesa and Hoare)
Condition Variable
Test-and-set instruction
Bounded buffer, reader/writers problems
Critical section solution: mutual exclusion, progress, bounded waiting (no starvation), performance

11. Sleeping Barber Problem
Barbershop Requirements
Barber sleeps if no customers waiting
Customers leave if no chairs available
Waiting customers can’t leave until haircut is done

12. Deadlock Reducing graphs Banker’s algorithm
Pretend you granted the request
Pretend all other legal requests are made
Can the graph still be reduced?
Banker’s algorithm: pretend all possibly requests have been made… is graph still reducible?

13. Scheduling Preemptive versus non-preemptive Goals of scheduling
Scheduling Algorithms
FIFO
SJF
Round Robin
Priority
What do most real systems use?
Goals: max CPU utilization, max throughput, min average response time, min average waiting time, avoid starvation
Combine algorithms!

14. Throughput vs Waiting Time
Thread
Arrival Time
Burst Time A 10 B 1 5 C 3 2
FIFO Throughput:
A: (10-0) = 10
B: (15-1) = 14
C: (17-3) = 14
( )/3 = 12.66
FIFO Waiting Time:
A: 0
B: (10-1) = 9
C: (15-3) = 12
( )/3 = 10.33