Locking

Question 1

⭐️ What is the objective of a lock?

Answer 1

To provide mutual exclusion

Question 2

⭐️ A lock is a variable (T/F)

Answer 2

True

Question 3

⭐️ What are the two states a lock can have?

Answer 3

Available/free & Locked/held

Question 4

⭐️ Give the function call: Tries to acquire the lock

Answer 4

lock()

Question 5

⭐️ Give the function call: Releases the lock that has been acquired by caller

Answer 5

unlock()

Question 6

What are the three attributes that a good lock has?

Answer 6

Mutual exclusion, fairness and performance

Question 7

What attribute of a good lock has the following description: progress & ensures no starvation

Answer 7

Fairness

Question 8

What attribute of a good lock ensures correctness?

Answer 8

Mutual exclusion

Question 9

What attribute of a good lock has the following description: overhead to grad & release lock

Answer 9

Performance

Question 10

⭐️ What is a software-based solution to locking that’s used for loads & stores?

Answer 10

A single flag variable

Question 11

⭐️ What are possible issues with the single flag variable for loads & stores solution in locking?

Answer 11

Not atomic - No MutEx
Performance overhead

Question 12

What do we call the software based solution to the critical section problem?

Answer 12

Peterson’s Solution

Question 13

⭐️ What’s the hardware based solution for locking that ensures mutual exclusion & atomic execution?

Answer 13

Test-And-Set

Question 14

⭐️ Give the definition of the instruction:
Tests and modifies the content of a memory word atomically

Answer 14

TestAndSet (TAS)

Question 15

Give the definition of the instruction:
Tests whether the value at the address specified by ptr is equal to expected, if so, updates the memory location pointed to by ptr with the new value

Answer 15

Compare-And-Swap

Question 16

⭐️ Give the definition:
A lock that spins, using CPU cycles, until the lock becomes available

Answer 16

Spinlock

Question 17

⭐️ Give the definition:
When a thread needs to endlessly check the lock value to check if the lock is held by others

Answer 17

Spin-waiting

Question 18

⭐️ What is the advantage of using a simple spinlock?

Answer 18

Mutual exclusion

Question 19

⭐️ What are the two disadvantages with using a simple spinlock?

Answer 19

No fairness & poor performance

Question 20

What lock type does the following:
Locks
1. Uses Fetch-And-Add on the ticket value
2. Return value is the threads “turn” value
Unlocks
1. Increments the turn

Answer 20

Ticket lock

Question 21

What does a ticket lock guarantee?

Answer 21

Fairness

Question 22

Ticket locks ensure that there’s no

Answer 22

Starvation

Question 23

Give the definition of the hardware primitive: Atomically increments a value while returning the old value at a particular address

Answer 23

Fetch-And-Add

Question 24

Spinlocks can be fast when there are:
- Many CPUs
- Locks are held a short time
What is the advantage that makes a spinlock fast in this case?

Answer 24

No context switch

Question 25

Spinlocks can be slow when there is: - One CPU - Locks are held a long time What is the disadvantage that makes a spinlock slow in this case?

Answer 25

Spinning is wasteful

Question 26

Give the definition of the function call: Instead of spinning, the CPU is given up to another process/thread. The yielding thread deschedules itself.

Answer 26

yield()

Question 27

Give the definition of the spinlock type: Sleep and put thread on a queue instead of spinning

Answer 27

Locks with queue

Question 28

What do locks with queue guarantee won't happen as long as all threads relinquish locks

Answer 28

Starvation

Question 29

Queue locks ensure predictable order (T/F)

Answer 29

True

Question 30

⭐️ Give the definition of the spinlock type: Hybrid approach that combines both spin-wait & sleep/block

Answer 30

Two phase lock

Question 31

⭐️ Is the lock released quickly or slowly with spin-wait?

Answer 31

Quickly

Question 32

⭐️ Is the lock released quickly or slowly with sleep/block?

Answer 32

Slowly

Question 33

⭐️ Is this the first or second phase of a two phase lock? The lock spins for a while

Answer 33

First

Question 34

⭐️ Is this the first or second phase of a two phase lock? If the lock is acquired in the first phase, put the calling thread to sleep

Answer 34

Second

Question 35

Give the definition of the spinlock type: If a thread has a locked adaptive mutex and the process/thread holding the adaptive mutex is running, the adaptive mutex executes busy waiting. Otherwise, it blocks the thread.

Answer 35

Adaptive mutex

Question 36

⭐️ What's a spinlock? 1. A lock that repeatedly checks if the lock is available 2. A lock that puts the thread to sleep until the lock is available 3. A lock that allows multiple threads to access a resource simultaneously 4. A lock that prevents deadlocks

Answer 36

1.

Question 37

⭐️ What's the main disadvantage of using a spinlock? 1. It can lead to deadlocks 2. It wastes CPU resources while waiting 3. It cannot be used in multithreaded programs 4. It's slower than a mutex

Answer 37

2.

Question 38

⭐️ Spinlocks are starvation free. (T/F)

Answer 38

False

Question 39

What are the three metrics to evaluate locks?

Answer 39

Correctness Fairness Performance

Question 40

⭐️ Ticket locks ensure that threads acquiring the lock grab it in FIFO order (T/F)

Answer 40

True

Question 41

⭐️ Which hardware primitive is used to implement the ticket lock? TestAndSet CompareandSwap FetchAndAdd

Answer 41

FetchAndAdd

Question 42

⭐️ Spinlocks are always slow because of the busy-waiting (T/F)

Answer 42

False; they can be fast if the critical sections are short

Question 43

⭐️ What's the purpose of calling yield() inside a spinlock implementation? 1. To release the spinlock and allow another thread to acquire it 2. To wake up another thread that's waiting for acquiring the spinlock 3. To voluntarily give up the CPU, mitigating busy-waiting 4. To add the calling thread to the waiting queue

Answer 43

3.