What's the point of cache coherency?

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

There are some nuances not covered by the great responses from the other authors.

First off, consider that a CPU doesn't deal with memory byte-by-byte, but with cache lines. A line might have 64 bytes. Now, if I allocate a 2 byte piece of memory at location P, and another CPU allocates an 8 byte piece of memory at location P + 8, and both P and P + 8 live on the same cache line, observe that without cache coherence the two CPUs can't concurrently update P and P + 8 without clobbering each others changes! Because each CPU does read-modify-write on the cache line, they might both write out a copy of the line that doesn't include the other CPU's changes! The last writer would win, and one of your modifications to memory would have "disappeared"!

The other thing to bear in mind is the distinction between coherency and consistency. Because even x86 derived CPUs use store buffers, there aren't the guarantees you might expect that instructions that have already finished have modified memory in such a way that other CPUs can see those modifications, even if the compiler has decided to write the value back to memory (maybe because of volatile?). Instead the mods may be sitting around in store buffers. Pretty much all CPUs in general use are cache coherent, but very few CPUs have a consistency model that is as forgiving as the x86's. Check out, for example, http://www.cs.nmsu.edu/~pfeiffer/classes/573/notes/consistency.html for more information on this topic.

Hope this helps, and BTW, I work at Corensic, a company that's building a concurrency debugger that you may want to check out. It helps pick up the pieces when assumptions about concurrency, coherence, and consistency prove unfounded :)