Lecture 16 - Multitasking

Question 1

What is multitasking?

Answer 1

Multitasking means running several processes (or tasks) in parallel.
This is a more general method than background timer routines.

Question 2

What is multithreading?

Answer 2

• Multithreading is a simpler and faster version of Multitasking, which switches between parallel threads, keeping all of them in memory

Question 3

What is sequential hardware?

Answer 3

• If we only have sequential hardware, the operating system takes turns in executing each process (time slicing)

Question 4

What is the task definition?

Answer 4

Task, Process, Thread
* Similar to function/ procedure; no parameters, no return value
* Often contains (endless) loop
* Can read its own Task-ID
* Can terminate itself or others

Question 5

What is scheduling?

Answer 5

Scheduler transfers control between tasks using time slices

Question 6

What is he task model?

Answer 6

Refer to slides for diagrams

Question 7

What is Pthreads?

Answer 7

• Extension to standard Unix fork() and join()
• Previously many different implementations
• Standard IEEE POSIX 1003.1c standard (1995)
• Performance:
  – Much faster than fork() (about 10x)
  – Much faster than MPI, PVM

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▪ POSIX Threads = Pthreads
▪ A thread is best described as a “procedure that runs independently” from the main program.
▪ In Linux, a thread:
▪ Exists within a process and uses the process resources
▪ Has its own independent flow of control
▪ Terminates if the parent process terminates
▪ Because threads within the same process share resources:
▪ Changes made by one thread to shared system resources (such as closing a file) will be seen by all other threads.
▪ Two pointers having the same value point to the same data.
▪ Reading and writing to the same memory locations is possible, and therefore requires explicit synchronization by the programmer.
▪ Each thread requires a separate data stack

Question 8

What are the Pthread functions

Answer 8

Refer to slides for examples and diagrams

Question 9

What is synchronisation?

Answer 9

Problem:
* There is no way of telling when the time slice is up and the next task gets activated
* Task switch might come at a bad time
e.g. here in middle of printout
Solution:
* Task synchronization
– There are several methods, but here we only look at simple “mutual exclusive” locks = mutex
– Operations init, lock and unlock

Question 10

What are the functions for Pthreads: Mutex Functions

Question 11

What are the most important design issues for embedded systems?

Answer 11

• Small
• Cheap
• Reliable