Question

On CPUs like x86, which provide cache coherency, how is this useful from a practical perspective? I understand that the idea is to make memory updates done on one core immediately visible on all other cores. This is a useful property. However, one can’t rely too heavily on it if not writing in assembly language, because the compiler can store variable assignments in registers and never write them to memory. This means that one must still take explicit steps to make sure that stuff done in other threads is visible in the current thread. Therefore, from a practical perspective, what has cache coherency achieved?

Answer 1

Imagine you do this:

lock(); //some synchronization primitive e.g. a semaphore/mutex
globalint = somevalue;
unlock();

If there were no cache coherence, that last unlock() would have to assure that globalint are now visible everywhere, with cache coherance all you need to do is to write it to memory and let the hardware do the magic. A software solution would have keep tack of which memory exists in which caches, on which cores, and somehow make sure they’re atomically in sync.

You’d win an award if you can find a software solution that keeps track of all the pieces of memory that exist in the caches that needs to be keept in sync, that’s more efficient than a current hardware solution.