Lecture 8: Synchronisation in Parallel Programming - Locks and Barriers, Condition Variables, Lecture 9: More about locks

Question 1

What are barriers, locks and condition variables used for?

Answer 1

Barriers: simple mechanism letting threads wait for each other (pthread barrier)

Locks: allowing threads to safely access shared data (prevent concurrent access)

Condition variables: for event signalling between threads

Question 2

When are barriers used?

Answer 2

1. Want the whole answer from this step before proceeding to the next step
2. Would also use when data dependence limits loop parallelisation

Question 3

What is the term for

Concurrent operations on shared state should not happen at the same time

Answer 3

Race condition

Question 4

What is the term for

Bits of code in our program where shared data is accessed/updated

Answer 4

Critical sections

Question 5

What do locks ensure?

Answer 5

Serialisation: amount of threads that can concurrently execute it (generally 1)
Atomicity: when a thread T starts to run a critical section S, T must first finish S before another thread can enter S

Question 6

What is the term for

Synchronisation primitive enforcing limits on the execution of a critical section

Answer 6

Lock

Question 7

True or False

Multiple threads can hold a given lock at a time

Answer 7

False

Question 8

What is a mutex?

Answer 8

Mutual exclusion lock

pthread_mutex

Question 9

What is an example of lock usage? Why is it important?

Answer 9

Bounded buffer: circular FIFO producer-consumer buffer.

If locks are not used:
1. Could have two threads writing (deposit) to or reading (extract) from the same element of buffer, and one value is lost. 2. Could then either increment in_index
Once: whole call of deposit() lost
Twice: spurious (old) value apparently deposited

Question 10

Benefits and drawbacks of usleep?

Answer 10

With the usleep set to an arbitrary time, may sleep for a much longer time than needed

Without usleep, keep trying non-stop, monopolising the CPU and wasting cycles

Question 11

What are condition variables?

Answer 11

Used to signal threads (event signalling) waiting for a conjunction of:
1. A lock becoming free and
2. An arbitrary event (e.g. buffer becoming non-full)

pthread_cond_wait, pthread_cond_signal

Question 12

What are some issues with locks?

Answer 12

1. Deadlock
2. Lost wakeup issue

Question 13

How can lost wakeup occur? How can it be solved?

Answer 13

Implementation details, timing, or race conditions in the signaling mechanism

Wake up all threads (vs. a single thread) waiting on a condition variable

Question 14

Why is it important to get your synchronisation strategy right from the beginning?

Answer 14

Concurrency issues are hard to debug

Question 15

What determines how big a chunk of code which depends on obtaining a lock should you write?

Answer 15

Granularity

Question 16

What are the types of granularity?

Answer 16

1. Fine-grained: divides the shared resource or data structure into multiple smaller segments, and each segment is protected by its own lock
2. Coars-grained: applying a single lock to encompass larger sections of code or the entire shared resource

Question 17

A ____ lock can be taken by a thread that already holds it

Answer 17

reentrant

Question 18

What is a rentrant lock used for?

Answer 18

A thread locking a lock it already holds results in undefined behaviour, so using a renentrant lock avoids a thread deadlocking with itself

Question 19

Match the following

1. Semaphores
2. Spinlocks
3. Read-write locks

A. Threads attempting to hold an unavailable lock will busy-wait, instead of sleep (mutexes, monopolises CPU, lower wakeup latency)
B. Mutexes that can be hold by multiple threads, useful to coordinate access to a fixed number of resources
C. Allows concurrent reads and exclusive writes

Answer 19

1. B
2. A
3. C