What is a memory model, anyway?
A memory model describes the relationship between variables in a program (instance fields, static fields, and array elements) and the low-level details of storing them to and retrieving them from memory in a real computer system. Theoretically, all variables are stored in memory, but the most current value for a given variable may not always be visible to all other threads. This could happen because of actions taken by the compiler (for example, optimizing a loop index variable by storing it in a register), the runtime, or the hardware (the cache may delay flushing a new value of a variable to main memory until a more opportune time).
In multiprocessor systems, processors generally have one or more layers of memory cache, which improves performance both by speeding access to data (because the data is closer to the processor) and reducing traffic on the shared memory bus (because many memory operations can be satisfied by local caches.) Memory caches can improve performance tremendously, but they present a host of new challenges. What, for example, happens when two processors examine the same memory location at the same time? Under what conditions will they see the same value?At the processor level, a memory model defines necessary and sufficient conditions for knowing that writes to memory by other processors are visible to the current processor, and writes by the current processor are visible to other processors. Some processors exhibit a strong memory model, where all processors see exactly the same value for any given memory location at all times.
