What does cache do in a CPU?

[SetSakara2]

I'm having some trouble understanding what cache does in a CPU. I do not know what it does, why it exists and how much cache do you need. Could someone please simplify this for me?

[Jurrunio]

Cache saves previously processed stuff (could be machine code or other data) so it doesn't have to execute the whole way when the same command is run again. It's however a pointless metric if you're just looking into buying a CPU, benchmarks is still more reliable because cache size cannot be seen as a performance metric.

[igormp]

4 minutes ago, SetSakara2 said:

I'm having some trouble understanding what cache does in a CPU. I do not know what it does, why it exists and how much cache do you need. Could someone please simplify this for me?

It provides a way for data to be accessed faster rather than having to pick everything up from RAM, which is slow compared to CPU speeds.

 

Here's a graphical idea of how it works:

http://www.overbyte.com.au/misc/Lesson3/CacheFun.html

[ewitte]

It is just physically closer ram that is way faster and has less latency.  The CPU/GPU will try to run what it can in cache in order to speed things up.

[mariushm]

The cache keeps important data very close to the CPU cores, where it can be accessed super fast. 

The cache memory often works at super high frequencies and has super low latency but it uses a lot of silicon space therefore it's expensive. The memory sticks are fast, but much slower (lower frequency) than cache memory and with higher latency. Then mass storage (hard drives, SSDs etc) are even slower.

You could imagine yourself sitting at a desk, with a few sheet of paper spread on the desk. If you need to work with data, looking up something on the sheets of paper on the desk is super fast - that's your cache.

The memory sticks (DDR3, DDR4 etc) are like you having stacks of paper in the desk  - you have to open the door to the desk, slide out the compartment or whatever, take the stack of papers and get the sheets.

The SDD / storage is like you getting up from the desk and going to get a stack of papers from another desk or from some storage area - it's much much slower.

 

Think about playing a game, a first person shooter - the game will use some information all the time, and reuse it and update often .. coordinates of the player on the map, how many bullets each weapon has, how much life you have,  where the enemies are, and what the AI determined their behavior should be (go towards you, search for enemies etc)

Each time a frame is calculated, the game has to calculate if the player hit a wall or is hidden by a wall, if some light hits the player and makes a shadow ... the game engine can cache the calculations and redo them much faster compared to requesting the data from the actual memory sticks.

The RAM sticks will hold textures that get applied over objects in the frame, if they can't fit into the video card's ram, game level stuff , audio sounds effects, background music etc etc