Threads

Question 1

Imagine RAM containing one piece of code and two programs sharing that piece of code. How will the CPU see this?

Answer 1

Each of the two programs has a PCB, so for the CPU, there are two different programs executing.

Question 2

Imagine RAM containing one piece of code and two programs sharing that piece of code. How will the RAM see this?

Answer 2

There is only one piece of code in memory so there is only one program.

Question 3

What is a thread?

Answer 3

In computer science, a thread of execution is the smallest sequence of programmed instructions that can be managed independently by a scheduler, which is typically a part of the operating system.

-Wikipedia

So a thread is something like: RAM containing one piece of code and two programs sharing that piece of code.

Question 4

Imagine RAM containing one piece of code and two programs sharing that piece of code.

What does this mean?

Question 5

Agnostic

Answer 5

Part of the code that is compatible with two different programs.

Question 6

What parts of this code is agnostic?

Answer 6

• Call printf

Question 7

Let P1 and P2 share the following code. What happens when P1’s quanta finishes and P2 enters?

Answer 7

All the information saved in the registers for P1 has to be stored in the PCB since P2 will use those same registers but with different values.

Question 8

*What is overhead?

Question 9

Multi-Tasking

Answer 9

Each process has only ONE execution.

Question 10

Multi-Threading

Answer 10

Each process has multiple processes running.

ex: In a browser, you can have multiple tabs.

Question 11

Spooling

Answer 11

Spooling allows programs to ”hand-off” work to be done by the peripheral and then proceed to other tasks, or to not begin until input has been transcribed.

-Wikipedia

Question 12

Is this multi-tasking or multi-threading, why?

Answer 12

Multi-tasking, since each process has its own spooler.

Question 13

Is this spooler a multi-tasking version or multi-threading version, why?

Answer 13

Multi-threading since the processes are sharing a spooler.

Question 14

In the multi-threading version of a spooler, what could we use to manage who enters the spooler first?

Answer 14

A queue

Question 15

True or false,

In the multi-thread version of the spooler quanta still applies

Answer 15

True

Question 16

What does DLL mean?

Answer 16

Dynamic Link Library.

Shared code. This is a library that is not appended to your code but rather gets pointed by your code in order to execute it.

Question 17

In threading, we don’t want threads to share the stack space why?

Answer 17

Since there is many processes using the same space in the stack, it would be hard to keep track of who is poping and pushing.

Question 18

What happens to the stack when we have threads?

Answer 18

One way of doing this is by giving the stack to each child in their PCB. That way they can each take care of their variables.

Notice, the heap will stay in RAM, this is because it is not a stack and when we malloc we get a block of memory.

Question 19

Lightweight Process (LWP)

Answer 19

It’s called lightweight since it does not take a lot of energy to the computer to do it.

This is usually done by taking a PCB in RAM and make a close of it in RAM. This is fast and easy to do.

Question 20

Say one main difference between a normal process and a LWP.

Answer 20

The run-time stack is inside the PCB. A regular PCB does not have a run-time stack section.

Question 21

Imagine a program opens a file and then creates a child.

What happens with the opened file?

Answer 21

The child will also be pointing to that same file. But if the child, later on, opens other files, this would be different from the parent.

Question 22

Threads as data structures.

Why do we like to use linked lists to represent threads?

Answer 22

Linked lists are infinite and we can connect each cell to many cells. So we can easily have a 2D linked list.

Question 23

Why is it good to limit the number of fopen() in a script?

Answer 23

fopen() will take up RAM. If you have a program that uses fopen() a lot, then it will stop the other programs from having RAM.

Question 24

Mention 4 benefits of threads

Answer 24

• Responsive (if a program gets stuck, the other programs keep running)
• Resource sharing
• Economy (cloning)
• Multiprocessor architecture advantages

Question 25

What would the complete memory look like with processes and OS PCB/Thread management?

Answer 25

Question 26

What happens to a2’s quanta?

Answer 26

It has to share it amongst the children.

Question 27

Name two ways of thinking about threading.

Answer 27

OS

Software

Question 28

What is software threading?

Answer 28

It is like a simulation of what happened in the OS when there are threads.

Question 29

What are some advantages and disadvantages of the first model?

Answer 29

Advantages:

• The LWP has its own quanta

Disadvantages:

• It could get crazy if you open many of them
• If the LWP freezes it freezes everything