coherent memory

Memory is memory. But different things can access that memory. The GPU can access memory, the CPU can access memory, maybe other hardware bits, whatever.

A particular thing has "coherent" access to memory if changes made by others to that memory are visible to the reader. Now, you might think this is foolishness. After all, if the memory has been changed, how could someone possibly be unable to see it?

Simply put, caches.

It turns out that changing memory is expensive. So we do everything possible to avoid changing memory unless we absolutely have to. When you write a single byte from the CPU to a pointer in memory, the CPU doesn't write that byte yet. Or at least, not to memory. It writes it to a local copy of that memory called a "cache."

The reason for this is that, generally speaking, applications do not write (or read) single bytes. They are more likely to write (and read) lots of bytes, in small chu