What kind of data does L1, L2, L3 cache hold?

Question

I understand that caches are the smallest and fastest memory that is integrated into the CPU. However, what I am confused about is the information that it holds. From what I know, the capabilities of cache are:

L1 cache - 64KB L2 cache - 256KB L3 cache - 4-12 MB Ram - 4-74 GB

For example: If I open up a page on the browser, what sort of data goes into the L1, L2, and L3 cache and maybe the RAM too. Does the L1 cache try to fill in as much reusable data as possible and if it's full then the remaining goes to the L2 cache and so on? Is the data that it holds like images or words ?

I can't seem to understand so any help would be much appreciated.

Answer 1

L1 cache: Every time you access any data, either it is in L1 cache already, or one cache line worth of data (often 32 or 64 byte) is read from the next level, and stored into L1 cache. If necessary, the data that was stored in that cache line is removed, and if it was modified, it is first written to the next level of data.

Same with L2 cache; if data is not in L1 cache, then either it is already in the L2 cache, or it is read from the next level in exactly the same way. Same with L3 cache. On processors where CPUs and GPUs access the same memory, there is often another cache level between CPU / GPU and memory. So what exactly is in the various caches changes all the time.

Sometimes the programmer knows that putting data into cache is not worth it. For example, if I need to process one GB of data sequentially, which is much more than any cache, it is pointless to store the data into any higher level caches, because it will never be reused. For this reason, some processors have instructions to read data but not putting it into L2 or L3 cache.

And some processors have "streaming" instructions: If you know that you are accessing sequential data, once you read data at location x, the processor guesses that you will read data at the following cache line after this, so it starts reading data from memory and putting it into the cache before it is used. Sometimes it isn't used at all.

Answer 2

The answer is that almost all the data that the CPU works on ends up in various levels of the cache. It's not like text goes into L4, and images go into L3, audio into L2 and video into L1.

The memory controller in the CPU will manage the data in the cache, moving it between levels according to various rules of thumb that the CPU designers have decided seem to work well.