Lecture 6 - Concurrency Intro

Question 1

What is concurrency?

Answer 1

The ability of different program parts to be executed simultanously.

Question 2

How many ways of having concurrency is there?

Answer 2

2

Question 3

What are the 2 ways of having concurrency?

Answer 3

• shared memory (one memory location used by different parts of the program running at the same time)
• message passing (different memory locations, is more robust)

Question 4

What does shared memory concurrency require?

Answer 4

Synchronisation between parts of the prgram which are to run alongside each other.

Question 5

What is parallelism?

Answer 5

Simply about making your program faster. There is usually no need to have threads, it just so happens that a program can be run in parallel when there are mutliple processors.

Question 6

How is concurrency different to parallelism?

Answer 6

Concurrency is a programming paradigm, where threads are used to get asynchronous events from the enviroment (no waiting on program, other threds can continue) or structuring program as a collection of interacting agents.

Question 7

Concurrency has?

Answer 7

different independent processes being executed

• dealing with lot of things at once

Question 8

What does parallelism have?

Answer 8

simulatnous execution of related computations

• doing lot of things at once

Question 9

What is a process?

Answer 9

Process is a program with its own memory address space.

Question 10

How many processes can be executed simulatneouly?

Answer 10

Many, CPU does it all the time.

Question 11

What is a thread?

Answer 11

independent sequence of program instructions. a process can have multiple threads, sharing the same address space.

-> can be executed simultanously
-> essentially a subset of a process

Question 12

Lifecycle of a thread:

Answer 12

• create
• start (possibly with arguments)
• each is given an identifier
  -> can be used to kill / pause / interrupt
• teminated

Question 13

In shared memory how does communication between threads happen?

Answer 13

By modifying the shared memory space.

Question 14

What are POSIX threads?

Answer 14

threading implementation in C

Question 15

What flag has to be specified with the clang compiler to compile code with POSIX threads?

Answer 15

-lpthread

Question 16

What is the posix thread create method?

Answer 16

pthread_create(thread , thread attr ,function , argument to function)

Question 17

What is the posix thread join method?

Answer 17

pthread_join(thread, NULL (result))

Question 18

What does pthread_create return when successfully creating a thread?

Answer 18

0 - refer to slide 8

Question 19

What is pthread_Join do?
What does it return when successfully

Answer 19

Waits for a another thread to terminate before terminating.
Returns 0 when successful.

Refer to slide 9

Question 20

What is assert?

Answer 20

Essentially a test, it will raise an exception when not met

Question 21

What is mutual exclusion?

Answer 21

A problem in concurrency. 2 threads accessing the same variables in shared memory, leading to inaccurate results.

Question 22

What does our code become if we do not control mutual exclusion?

Answer 22

our code becomes non-deterministic. The process is inconsistent

Question 23

What is a critical region?

Answer 23

part of the code that updates shared variables or data structures. Mutual exclusion means only one thread ever executes this part of code

Question 24

What is a race condition?

Answer 24

when the result of the program depends on the order in which the threads are executed

Question 25

What is a lock?
What is another name for a lock?

Answer 25

Lock is something that needs to be obtained in order to execute critical region.

AKA Mutex

Question 26

What must happen to the lock when thread is finished with a critical section?

Answer 26

It must be released

Question 27

What are the semantics of acquiring a lock?

Answer 27

• if another thread owns the lock , the requesting thread is blocked until owning thread releases lock
• if lock is not owned then the requesting thread is granted ownership

Question 28

What happens on released of a lock if there is blocked threads

Answer 28

The first blocked thread is automatically granted ownership

Question 29

What do producers and conumers have to do when we have a bounded buffer?

Answer 29

The producer has to wait (is blocked) when buffer is full.

The consumer has to wait (is blocked) when buffer is empty.

Consumers and producers have to both work with a a lock to ensure consistency.

When unsafe, there could be gaps left in the buffer, leaving us with an impression that it is full, when it isn’t. etc.

Question 30

Problem with having to wait for items to enter the buffer (consumer) while having the lock/

Answer 30

Since consumer is waiting for items to enter the buffer, while holding the lock, the producer is not able to add anything into the buffer. THIS IS DEADLOCK.

Question 31

What is deadlock?

Answer 31

When a thread is holding a lock, without making progress, while also blocking other threads.

Question 32

Potential solution to deadlock #1?

Answer 32

Releasing the lock when count is zero. This is via polling.

Wastes CPU time and resources.

AKA Busy Waiting

Question 33

What is a pthread_mutex_t?

Answer 33

A lock or mutex

Question 34

What is busy waiting?

Answer 34

Having a loop to continously check whether a condition has been met (keep having to unlock and lock mutexes).

Question 35

Better solution to busy waiting?

Answer 35

Condition variable

Having a variable indicating whether a condition is ready. It must be changed by the thread currently holding the lock.

Implemented with:
pthread_cond_wait(&cv, &m)
pthread_cond_signal(&cv) -> assigns ownership to one of the waiting threads

Question 36

Example of Condition Variable.

Answer 36

pthread_mutex_lock(&m);
while (!cond) {
pthread_cond_wait(&cv, &m);}

Question 37

What are condition variables analogoues to?

Answer 37

hardware interrupts

Question 38

The bounded buffer problem will have how many condition variables?

Answer 38

Two, one for add and one for remove.

Question 39

What is a monitor

Answer 39

A class that provides safe access to a shared resource.

Question 40

, Important coordination aspects:

Answer 40

• partitioning =determining which parts of the computation have to seperately evaluated
• placement = determining where threads should be executed e.g. allocated to least busy core
• communication = when to communicate and what data to send
• synchronisation = ensuring threads can cooperate without interference

Question 41

What is a semaphore?

Answer 41

-holds an integer counter which provides 2 operations:
- wait, which decrements the counter and blocks
- signal, increments the counter and wakes waiting threds

Question 42

What is a binary semaphone?

Answer 42

Semaphore holding 0 or 1.

Question 43

If we use a semaphore, then do we need locks and cvs?

Answer 43

No, all handled by a semaphore, as the locks and cvs are built in.

Question 44

What should a semaphonre structure contain?

Answer 44

mutex or lock
condition variable
current value
max value / min value

Question 45

What do we need to avoid in concurrency?

Answer 45

• deadlock
• livelock
• starvation

Question 46

What is livelock?

Answer 46

Threads run, but make no progress.

Question 47

What is starvation?

Answer 47

Some threads never making any progress.

Question 48

What do we need to do when programming concurrently?

Answer 48

must correctly lock , unlock and wait for safe access (via semaphores or condition variables)