Concurrency (L21)

Question 1

Concurrency refers to the concept of separate threads of control/execution within a program.

Answer 1

True.

Question 2

Parallelism is a synonym for concurrency.

Answer 2

False.

Question 3

What are parallel programs?

Answer 3

Programs that support simultaneous execution of distinct instruction streams.

Question 4

By definition, concurrent programs are run on multiple CPUs or cores.

Answer 4

False - parallel programs are.

Question 5

What are parallel programs typically used for?

Answer 5

Increasing the performance
of a specific algorithm.

Question 6

What is concurrency typically used for?

Answer 6

For dividing the logic of a complex programs into distinct units.

Question 7

All concurrent programs provide parallelism, but parallel programs are not necessarily concurrent.

Answer 7

False, all parallel programs provide
concurrency, but concurrent programs are not necessarily parallel.

Question 8

Controlling access to shared data is necessarily the primary issue when writing concurrent programs.

Answer 8

False - sharing write-able data is a problem, but sharing read-only data is fine.

Question 9

What is a critical region?

Answer 9

A block of code in which an update to shared data is made.

Question 10

What is the traditional approach to only allowing one thread to enter/exit the critical region at a time.

Answer 10

Locks, which are locked (1) at the start of the critical region and closed (0) at its end.

Question 11

Threads that encounter a lock perform another task and circle back later.

Answer 11

False - such threads wait/block until the data is unlocked.

Question 12

It is not possible for multiple threads to think they have the lock for some data, as setting a lock variable is done in a single line of code.

Answer 12

False - this single high level language instruction is compiled into multiple
machine level instructions.

Question 13

What is a “test and swap” instruction provided by modern CPUs?

Answer 13

An atomic assignment instruction that
always completes - allows languages to close locks “in a single instruction”.

Question 14

What C library allows concurrency that follows the lock model?

Answer 14

pthreads

Question 15

What does Java’s synchronized keyword do?

Answer 15

A synchronized method is tested such that this method can only be used by one thread at a time.

Question 16

What are two problems encountered when using many threads that share many data structures?

Answer 16

Deadlock & livelock.

Question 17

What is deadlock?

Answer 17

When separate threads are each waiting for a lock held by the other.

Question 18

What is livelock?

Answer 18

When threads are actively trying to resolve a dependency or conflict, but their actions prevent any progress from being made, leading to a perpetual “busy” state.

Question 19

Due to deadlock/livelock, low level thread management doesn’t scale very well.

Answer 19

True.

Question 20

What concurrency model is provided by Erlang?

Answer 20

Message passing.

Question 21

Messages are placed in a stack belonging to the receiving thread.

Answer 21

False - they are placed in queues.

Question 22

A thread always knows where a message comes from.

Answer 22

False - this is true only when messages have a sender ID.

Question 23

Messages are viewed as synchronous, since it allows threads to synchronize their tasks.

Answer 23

False - they are viewed as asynchronous since sending/receiving threads do not block during transfer.

Question 24

It is impossible for processes to hang using message passing.

Answer 24

False - a process may wait wait for a message from a task that has crashed.

Question 25

In message passing, the queue data structures are shared by separate tasks.

Answer 25

False.