Parallel & Distributed Computing | AP CSP Daily Practice Day 13
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Big Idea 4: Computing Systems and Networks
Day 13 Practice - AP CSP Daily Question
[FOCUS] Parallel and Distributed Computing
Practice Question
A program needs to process 1000 images. Processing each image takes exactly 2 seconds when done sequentially. A programmer decides to use parallel computing with 4 processors, where each processor handles 250 images.
Assuming there is no overhead for distributing tasks, what is the minimum time needed to process all images?
What This Tests: Big Idea 4 covers parallel computing concepts including speedup calculations when tasks are distributed across multiple processors.
Why B is Correct
Let us work through this step by step:
- Sequential time: 1000 images x 2 seconds = 2000 seconds total
- With 4 processors: Each handles 250 images
- Time per processor: 250 images x 2 seconds = 500 seconds
- All processors work simultaneously, so total time = 500 seconds
The speedup is 2000/500 = 4x, which matches having 4 processors (ideal speedup with no overhead).
Common Mistakes
Mistake: Multiplying instead of dividing
Some students multiply the sequential time by the number of processors (2000 x 4 = 8000). Remember: more processors means LESS time, not more.
Mistake: Forgetting that processors work simultaneously
The answer is NOT 250 + 250 + 250 + 250. All four processors work at the same time, so you take the time of the slowest one (they are all equal here at 500 seconds).
[TIP] AP Exam Strategy
For parallel computing problems: (1) Calculate sequential time, (2) Divide work evenly among processors, (3) Calculate time for one processor, (4) That is your answer (assuming no overhead). Speedup = Sequential Time / Parallel Time.
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