Few questions about Cache memory

Question

I have few questions about Cache Memory

In my system "free -h" command gave me below output,

      total        used        free      shared  buff/cache   available
Mem:   7.6G        2.1G        1.5G         46M        4.0G        4.8G
Swap:  1.6G         28M        1.6G

1. Why "Cache Memory" is required inside the main memory?

As far as i know "Cache Memory" is different than the main memory(RAM), It is very costly and it is very faster, Please Correct me if i am wrong.

2. Before 1 hour this cache memory was 3 GB and then it is increased by 1 GB?

Please note that I din't start/run any new processes

3. Till what limit i can start new processes/application?

I mean as per free -h command output,

free memory = 1.5 GB, Cache memory = 4 GB and Swap memory = 1.6 GB

can i start new application up to (1.5 + 1.6) GB or (1.5 + 1.6 + 4) GB`?

4. Can i set/configure this cache memory value?

If yes, then how to?

Comments

Thought to answer this, but Manikandan J has nicely explained this :)

Answer 1

1. Why "Cache Memory" is required inside the main memory?

I think when referring to "Cache Memory" you're thinking of memory used for disk caching, which is RAM used to store information from a hard disk to speed up future processes. It will be freed when future processes have to use information that is not already present in memory and then be used by those new processes.

2. Before 1 hour this cache memory was 3 GB and then it is increased by 1 GB?

This is nothing bad and/or unusual, as unused memory is wasted memory. The system probably preloads code it deems usable in the future.

3. Till what limit i can start new processes/application?

Basically until the available memory after buffers/cached is zero. When this happens, your system will start using swap memory on disk (note this is not RAM but space on your hard disk). This is undesirable as disk speeds are considerably lower than RAM speeds and will result in your system becoming really slow.

4. Can i set/configure this cache memory value?

It's Linux/Unix, so of course it is configurable :)