Thread Caching and Java Memory model

Question

I'm trying to understand the Java memory model and threads. As far as I understand, each thread has a local copy of the "main" memory. So if one thread tries to change an int variable, for example, of some object, it caches the int variable and if it changes it, other thread might not see the change.

But what if threads cache some object instead of int? What threads cache it in this case? If a thread caches a reference to an object any change to the state of the object are not visible to other threads? Why?

Answer 1

CPU have different level caches L1, L2, L3. Every CPU (and also /may CPU Core) has own cache. This caches store minimal set of main memory (RAM) for performance.

  _______________    ______________    |     CPU 1     |  |     CPU 2    |    |   _________   |  |   _________  |    |  | Level 1 |  |  |  | Level 1 | |    |  |   Cache |  |  |  |  Cache  | |    |  |         |  |  |  |         | |  |  |_________|  |  |  |_________| |    |_______________|  |______________|            | |              | |            | |              | |           _|_|______________|_|__          |                       |          |      MAIN MEMORY      |           |_______________________|     Time     Command                 CPU 1 (Cache)      CPU 2 (Cache)        Main Memory      -------  ----------              ----------------    --------------       -------------   1          ---                       ---                ---                x = 10   2       Read x  (on cpu1)           x = 10              ---                x = 10   3       Write x <--20 (on cpu1)     x = 20              ---                x = 10          4       Read  x (on cpu2)           x = 20              x = 10             x = 10   5       put cache to Main mem       x = 20              x = 10             x = 20 

For example, Above execution order, x value is wrong on CPU2. x value already changed by CPU1. If x variable is defined as volatile, all write operation reflect to main memory instantly.