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Beginner Topic 3 of 51 Concurrency & Parallelism

Amdahl's Law

Formula bounding speedup by the serial fraction.

How it works

Speedup ≤ 1/((1−p) + p/N) where p is the parallel fraction and N the processors; with p = 0.9 the max speedup is 10× no matter how many cores. Gustafson's law instead scales the problem size.

Where it is used

Predicting scaling.

Common pitfall

Ignoring that serial fractions dominate at large N.

Where it fits in the course

Amdahl's Law is topic 3 of 51 in Concurrency & Parallelism, rated Beginner. It follows Sequential Bottlenecks. The next topic is Threads in Practice.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Concurrency & Parallelism quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Amdahl's Law?
Formula bounding speedup by the serial fraction. It belongs to Concurrency & Parallelism and is a beginner-level topic.
Where do people use Amdahl's Law?
Predicting scaling.
What is the most common mistake with Amdahl's Law?
Ignoring that serial fractions dominate at large N.
What should I learn before this topic?
Start with Sequential Bottlenecks, the topic just before it in Concurrency & Parallelism.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Concurrency & Parallelism quiz.
Is Amdahl's Law hard to learn?
It is a Beginner topic, so it works as an entry point. If the terms are new, read the explanation twice and try the visualizer before the quiz.
How long does it take to learn Amdahl's Law?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Amdahl's Law?
Sequential Bottlenecks, Threads in Practice, Concurrency vs Parallelism are closely connected. The full list is in the related topics section of this page.
What should I study after Amdahl's Law?
Continue with Threads in Practice, the next topic in Concurrency & Parallelism.

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