P2L2 Threads and Concurrency

Question 1

Give a metaphor for a thread: A thread is like ….

List 3 attributes:

1. ….
2. ….
3. …..

Answer 1

.. a worker in a toy shop

1. An active entity
2. Works simultaneously
3. Needs coordination

Question 2

Explain the difference between a thread and a process

Answer 2

A process has both address space and execution context information.

Threads represent multiple independent execution contexts within the same address space

address space (virtual <-> physical memory mapping)

• code
• data
• heap

execution context

• CPU registers
• stack

Question 3

State 2 benefits of multithreading

Answer 3

1. Threads can run in parallel since they are executing different instructions of the same code.
2. Specialization: For instance, we can give higher priority to tasks that handle more important tasks or service higher paying customers.

Question 4

What’s the advantage of multi-thread over multi-process? Give 2 reasons.

Answer 4

1. Each process needs its own address space.
   - -> more memory usage.
   - -> more memory swaps.
2. Inter process communication (IPC) - is more costly than inter thread communication, which consists primarily of reading/writing shared variables.

Question 5

Are threads useful on a single CPU? Explain why or why not.

Answer 5

Yes - to hide latency.

For example if a thread makes a disk request I/O - if it takes 2x as long as context switch then it’s a great deal. The thread is stuck anyway on a slow I/O request, so it pays to context switch to another thread who can continue working.