Java Synchronization: volatile

What does it mean by "volatile"?

New JMM (1.5+)

1) Volatile variable reads from memory and write to memory instead of local cache

Picture below shows example of a multi core system. Each CPU  has cache, which is super fast memory locally for that CPU. For optimization purpose, many information stored in cache. But in multi threaded application which shared variable used, if each CPU update and fetch info from its own cache, value of the shared variable is wrong. Example: a counter
By using volatile, every time volatile variable read, it is fetched from memory. Every time volatile variable written, it is pushed from L1 to L2 to memory.

2) Instruction which use volatile variable cannot be reordered

3) Volatile variable observes what happened 

For example, old value of w = 0 , x = 0 , f = true. f is a volatile variable.
Now CPU1 updates x = 2 and f = false. As f is volatile, it is flushed to RAM. x is not volatile, however f observes that x changes as well, so x is also flushed to RAM. There is nothing change in w so w does not get flushed on purpose. However, if w is in the same cache line with x or f it may get flushed too.
Now it is sure that new value of x and f is in memory. However, we can only guarantee CPU2 has updated value for f. x is not volatile so it does not guarantee that CPU2 will fetch new value for x. However, if x is same cache line with f then may be it is also loaded.

Old JMM (before 1.5)

1) Volatile variable reads from memory and write to memory instead of local cache
2) Instruction which use volatile variable cannot be reordered with another instruction using volatile variable but may get reordered with instruction using non-volatile variable

Note

1) Volatile is very expensive

2) Volatile  on reference object means any get(), set() is from memory which means it is super expensive