Cache Hit Rate and Average Access Time
Lesson notes
Overview
Even a small miss rate dominates average memory latency. This topic is rated beginner and is number 87 of 117 in Computer Science. Core ideas and first intuitions. No prior knowledge needed.
What you are seeing
The panel above shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
The average access time of a cache system is T = h·t_c + (1 − h)·t_m, where h is the hit rate, t_c the cache time and t_m the main memory time. Because memory is so much slower than cache, even a small miss rate dominates: with a 1 ns cache and 100 ns memory, a 95% hit rate gives an average of 6 ns, six times the cache speed. That is why improving locality of reference can speed programs up dramatically.
Reading the relationship
The expression "T = h·t_c + (1−h)·t_m" is a compact summary of the idea. It uses the symbols T, h, t_c, t_m; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Set every slider to its middle value and note the shape of the curve. 2. Move one slider at a time and describe whether the curve shifts, stretches or turns. 3. Find the value where the behaviour changes sharply, then connect it to the formula.
Try this
Start by only moving one slider at a time. Say aloud what you expect, then check.
Common pitfalls
Test small inputs by hand and then the largest ones. Off-by-one errors and forgetting how cost grows with input size are the most common traps.
How it connects
It builds on “Hash Collision Probability”, the topic before it in Computer Science. Once you are comfortable, continue with “Information Content of an Event”.
Why it matters
Computing ideas decide how fast, how safely and how well software can solve problems.
Where you might use it
- Revising “Cache Hit Rate and Average Access Time” before a class, exam or certification
- Choosing the right algorithm or data structure when writing software or apps
- Debugging programs and judging how fast or how safely code will run at scale
- Understanding how search engines, games, phones and AI systems make decisions
Frequently asked questions
What is Cache Hit Rate and Average Access Time?
Even a small miss rate dominates average memory latency. It is a beginner topic in Computer Science.
How do I use the Cache Hit Rate and Average Access Time simulation?
The panel at the top shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression T = h·t_c + (1−h)·t_m packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Core ideas and first intuitions. No prior knowledge needed. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Hash Collision Probability”, the previous topic in Computer Science. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.