What does flushing thread local memory to global memory mean?

Question

I am aware that the purpose of volatile variables in Java is that writes to such variables are immediately visible to other threads. I am also aware that one of the effects

Answer 1

It is really an implementation detail if the current content of the memory of an object that is not synchronized is visible to another thread.

Certainly, there are limits, in that all memory is not kept in duplicate, and not all instructions are reordered, but the point is that the underlying JVM has the option if it finds it to be a more optimized way to do that.

The thing is that the heap is really "properly" stored in main memory, but accessing main memory is slow compared to access the CPU's cache or keeping the value in a register inside the CPU. By requiring that the value be written out to memory (which is what synchronization does, at least when the lock is released) it forcing the write to main memory. If the JVM is free to ignore that, it can gain performance.

In terms of what will happen on a one CPU system, multiple threads could still keep values in a cache or register, even while executing another thread. There is no guarantee that there is any scenario where a value is visible to another thread without synchronization, although it is obviously more likely. Outside of mobile devices, of course, the single-CPU is going the way of floppy disks, so this is not going to be a very relevant consideration for long.

For more reading, I recommend Java Concurrency in Practice. It is really a great practical book on the subject.