CS Performance Optimization 3

Question 1

Which compiler optimization moves a computation out of a loop when its result never changes between iterations?

Accepted Answer

Loop-invariant code motion

Answer

Loop-invariant code motion hoists unchanging computations outside the loop so they run once.

Question 2

A multithreaded program on 8 cores shows almost no speedup over the single-threaded version. Threads frequently update the same shared counter. What is the likely cause?

Accepted Answer

Lock contention and cache-line contention on the shared counter

Answer

Serialized access to a shared counter forces threads to wait and invalidate each other's cache lines.

Question 3

What does 'false sharing' refer to in parallel programming?

Accepted Answer

Threads on different cores modifying separate variables that share a cache line

Answer

When independent variables occupy the same cache line, writes by one core invalidate the line for others.

Question 4

Which approach best reduces perceived latency when loading a large image gallery on a website?

Accepted Answer

Lazy loading images as they scroll into view

Answer

Lazy loading defers off-screen images so the visible content appears quickly.

Question 5

In Big-O terms, what improvement does switching from bubble sort to merge sort provide for large inputs?

Accepted Answer

O(n²) to O(n log n)

Answer

Bubble sort runs in O(n²) while merge sort guarantees O(n log n).

(CS) Computer Science Practice Test

(CS) Computer Science Practice Test

CS Performance Optimization 3