第5章アルゴリズムのハードウェアによる実現：コンピューターシステム設計

Throughout our discussion of the memory hierarchy, we have determined the following:

• Memory is read in a chunk of consecutive 64 bits which is 8 bytes.

• Whenever there is a cache miss, the cache works to bring a cache block, not the few bytes the processor needs. Most caches now have a cache block of 64 bytes. Those 64 bytes are consecutive bytes brought from the lower-level cache, that come from the even lower cache (i.e., from L3 to L2 to L1). The L3 cache brought the block from memory.

When a processor repeatedly visits the same memory locales, it has locality. We can surmise that if a program accesses the data in a consecutive manner, it gets better performance, called spatial locality. Also, if we reuse the data as much as possible, we get better performance because the data is available in the cache, and we can increase the cache hits. We call this second criteria temporal locality.

We can get even better performance if the programmer writes efficient code that makes the best use of the underlying hardware. An efficient program has a very important characteristic called locality. For example, if you want to add two arrays together (i.e., A[0] + B[0], A[1] + B[1]), then you get a good performance if you access these two arrays sequentially from element 0 until the end of the arrays which is an example of spatial locality. So, whenever you are writing a program, pay close attention to how you access the data.

Instructions also reside in memory. If we have a for-loop that is executed several thousand times, the instructions in the loop body are reused in every iteration which is an example of temporal locality.