1. January 15, 2004 Adrienne Noble
Week 2
January 15, 2004
Adrienne Noble

2. Questions? Lecture Homework Project – Due tomorrow by 2:30!
Mention names when answering questions
Office hours: Wednesday 1-2 in 006
Also, feel free to come ask me questions when you see me around the lab – if I happen to be busy, I’ll tell you
Lecture: Ed’s been going over a lot of higher level stuff, but realize that you aren’t expected to be experts on threads, critical sections, paging, file systems, etc. We’ll go over most of those in much more detail as the quarter progresses. Just a big overview right now.
Homework: probably haven’t started yet, but any questions?
Project: stress importance of writeup – almost half of total points
Shell: 15 points
System call: 15 points
Write-up: 25 points

3. Waiting for I/O
Your computer is often waiting on many devices at once:
File
Socket
Keyboard
All these devices are represented by a file descriptor
A file descriptor (fd) is given to each device so that you can perform operations like reading from that device
What are some methods you could use to read these devices when they’re ready?

4. Polling Just loop around and check each fd Advantages Disadvantages
Simple to understand, easy to implement
Disadvantages
Slow, can only read one fd at a time, doesn’t necessarily read fds in order that they have data available, busy waiting
What do I mean by saying it doesn’t read fds in the order that they have data available?
What about busy waiting?

5. Select
Give it all the fds, then it blocks and returns when a fd is ready to read
Advantages
Immediate notification that a fd is ready, no busy waiting
Disadvantages
Can still only read one at a time
You may have seen it in Networks

6. What is a process? A single instance of a running program
Every process has it’s own
memory space (static data, stack, heap, and code)
Process Control Block (PCB), which holds lots of state information
Ask someone to outline the method of creating a new process

7. Fork (multiple processes)
Create a separate process for each fd
Advantages
Can read multiple fds at once, always knows right away when something is ready, inter-process communication
Disadvantages
Lots of overhead (in creation and switching), more complicated, expensive communication
Are we really doing multiple things at once? No, but the OS switches around a lot
What happens when a process is created or switched?
Requires flushing the cache and TLB – both expensive operations

8. What is a thread? Often called a lightweight process
Shares many resources with the process
Code, heap, etc
But has it’s own stack, program counter, and registers
Much less expensive to create and switch between
Why not always use threads and just get rid of processes completely?
Might not always want to share data like threads do

9. Threads Create a separate thread for each fd Advantages Disadvantages
Much less overhead, fast, lightweight, good communication between threads
Disadvantages
Shared data, more complicated
More about project

10. Project Details Groups of 3 Start early!
First part is due two weeks from tomorrow
Implement a user thread package
Write synchronization primitives for those threads (mutex, condition variables)
This weekend might be a good time to get acquainted with the project
read the project page
looks through the code
start designed your thread implementation