W9 - Intro to Threads

Question 1

Two parts of a traditional process with a single thread of execution

Answer 1

Sequential program execution stream
- this is the code executed as a sequence. includes state of CPU registers, stack, …

Protected resources
- main memory state, I/O state, …

Question 2

Thread

Answer 2

A sequential execution stream within a process.

Question 3

Is there protection between threads?

Answer 3

No. Threads share the process’s resources.

Question 4

Multithreading

Answer 4

A single program made up of a number of different concurrent activites (threads of execution).

Question 5

Creating process vs creating thread

Answer 5

New thread will use less memory, takes less time to create, and terminate faster.
Threads naturally share memory for communication, for processes the kernel needs to provide protection and mechanisms for communiation.

Question 6

Benefits of multi threading

Answer 6

• Exploit multiple cores
• If one task blocks on I/O other tasks in the program should still be able to execute (increased responsiveness)
• Some problems are best structured as communicating tasks (gui should be responsive)

Question 7

What state is shared by all threads in process

Answer 7

Content of memory (global variables, heap)
I/O state (file descriptors, network connections)

Question 8

How are threads useful in a web server as an example

Answer 8

Threads are created to process requests and issue responses. In the meanwhile the server continues to listen for other requests. Other threads to read data, access DB, etc, can also exist.

Question 9

Why choose multi threading over multiple processes

Answer 9

Context switch expensive compared to thread switch.
Thread creation/termination is faster than process creation/termination.
Threads naturally share memory

Question 10

What does our single threaded process model look like?

Answer 10

Process control block
User address space
User stack + kernel stack (just this one pair)

Question 11

What does our multi threaded process model look like?

Answer 11

Process control block
User address space (shared between all threads)
Multiple threads, each made up of:
- Thread control block
- User stack + Kernel stack

Question 12

What state remains private to each thread (typically kept in kernel memory)

Answer 12

TCB (Thread control block)
Thread ID
CPU registers (including PC)
Execution stack pointer
Scheduling info (e.g. priority)
Pointer to the PCB

Question 13

Reminder: What’s a PCB

Answer 13

Process Control Block is a data structure used by the OS to keep track of processes

It has:
- PID
- Process State
- PC
- CPU Registers
- Mem management info
- Scheduling info
- I/O Status
- If multithreaded, thread list or TCBs.

Question 14

Benefits of using processes over threads

Answer 14

Much simpler, since theres no uncontrolled access to shared variables and you can avoid all sorts of errors.

Question 15

Thread Control Block

Answer 15

Private info individual to the thread stored here.
- TID
- CPU registers (including PC)
- Execution stack ptr
- Scheduling info
- Pointer to PCB