The memory model guarantees that a call to the start() method on a thread happens before any actions in that thread.

The memory model does not guarantee that if two threads update the same object using a synchronized method, then both threads would see the changes occurring in the same order. 

The memory model guarantees that any changes made to a variable in one thread are immediately noticed in all other threads.

The memory model always guarantees that if two threads update the same variable then both threads would see the change in the same order.

In multi-core systems, changes of variable values in the local cache of a core are not immediately visible by threads running on other cores.

If the Java memory model made more guarantees than it does, multi-threaded programs would run faster but it would be more time-consuming for programmers to code them.