Row-major vs Column-major confusion

Question

I\'ve been reading a lot about this, the more I read the more confused I get.

My understanding: In row-major rows are stored contiguously in memory, in column-major colu

Answer 1

You are right. it doesn't matter if a system stored the data in a row-major structure or a column-major one. It is just like a protocol. Computer : "Hey, human. I'm going to store your array this way. No prob. Huh?" However, when it comes to performance, it matters. consider the following three things.

1. most arrays are accessed in row-major order.

2. When you access memory, it is not directly read from memory. You first store some blocks of data from memory to cache, then you read data from cache to your processor.

3. If the data you want does not exist in cache, cache should re-fetch the data from the memory

When a cache fetches data from memory, locality is important. That is, if you store data sparsely in memory, your cache should fetch data from memory more often. This action corrupts your programs performance because accessing memory is far slower(over 100times!) then accessing cache. The less you access memory, the faster your program. So, this row-major array is more efficient because accessing its data is more likely to be local.