Ordering

Question 1

what is asynchronous execution

Answer 1

we cannot predict the order two threads will be scheduled

Question 2

what is a race condition

Answer 2

an element that has different values based on the synchronisation of operations before it

Question 3

how can we create ordering or synchronisation

Answer 3

barrier

thread join

mutual exclusion

asynchronous message passing

synchronous message passing

Question 4

what is a thread join

Answer 4

two threads join together and proceed as a single thread

Question 5

what is asynchronous message passing

Answer 5

send the appropriate information before proceeding

Question 6

what is synchronous message passing

Answer 6

at a determined point, the first process blocks and waits for the other to reach its communication point. then a message is sent

Question 7

what is mutual exclusion

Answer 7

a mutex is a Boolean flag that prevents or allows progress.

it controls access to a critical section.

stored in main memory

Question 8

when do race conditions occur

Answer 8

when behaviour is non deterministic

Question 9

what is a semaphore

Answer 9

a means of implementing access control.

start with a value that is incremented (v) and decremented (p)

if value is 0, process must block. when exiting, signal v will increment the semaphore

Question 10

what is signal and wait

Answer 10

IPC

there are times a process should not proceed until something else has happened.

in a critical section: process must wait, then sigal once it has left

in semaphores: p and v

in queues: if a process tries to read an empty buffer it must wait until it receives a wakeup

Question 11

what is a memory barrier

Answer 11

instruction that causes all memory operations before it to be complete before starting operations after it.