Cache
- C++ course
- Expert
- Lesson 26/47
- 5 min
Learn CPU cache effects and data locality.
Cache
The cache is a small fast memory between the CPU and main memory. Data is loaded in cache line blocks (usually 64 bytes). Access to neighboring data in the same block is fast.
That is why spatial locality matters: sequential access to an array is cache-friendly; random-index access forces a new cache line load each time. Cache misses are expensive; they cost hundreds of cycles. Keeping data sequential improves performance significantly.
Locality kinds
There are two locality kinds: spatial locality — access to neighboring addresses, and temporal locality — re-access to recently used data. Both increase the cache hit rate.
Sequential array access gives spatial locality. Keeping a frequently used variable in a loop gives temporal locality. Keeping the structure small and compact (struct of arrays) is cache-friendly; more related data fits in one cache line.
Practice Sum an array sequentially
In main define int data[4] = {1, 2, 3, 4}; and int sum = 0;. Write the loop for (int i = 0; i < 4; ++i) sum += data[i];. Then write std::cout << sum;.
Starter code
#include <iostream>
int main() {
// Define the data array, sum sequentially and print
return 0;
}Sample solution
#include <iostream>
int main() {
int data[4] = {1, 2, 3, 4};
int sum = 0;
for (int i = 0; i < 4; ++i) sum += data[i];
std::cout << sum;
return 0;
}Expected output
10
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the loop bound
for (int i = 0; i < ____; ++i) sum += data[i];for (int i = 0; i < size; ++i) sum += data[i];Explanation: The loop runs up to size.
2 Which access pattern is cache-friendly?
int data[100];
for (int i = 0; i < 100; ++i) sum += data[i];- Random-index access
- Sequential-index access Correct answer
- Only even-index access
- Strided access
Explanation: Sequential access gives spatial locality; the cache line is loaded once.
3 Match each concept to its role
| Term | Meaning |
|---|---|
cache line |
64-byte block |
spatial locality |
Neighbor data access |
temporal locality |
Repeated access |
cache miss |
Load from memory |