97 Things Every Java Programmer Should Know

Chapter 36. Inline Thinking

Patricia Aas

Computers changed. They changed in many ways, but for the purpose of this text they changed in one significant way: the relative cost of reading from RAM became extremely high. 

This was something that happened gradually, until RAM accesses could completely dominate the performance metrics of an application. The CPU was constantly waiting for memory accesses to finish. And as the cost of going to RAM, relative to registers, grew and grew, chip manufacturers introduced more and more levels of cache and made them bigger and bigger.

And caches are great! If what you need is in them…

Caches are complex, but as a rule they will predict that a subsequent memory access will be close to, or preferably adjacent to, a recent, previous access. This is done by fetching a bit more than needed from memory and storing this excess in the cache, often called prefetching. If a later access can get its value from the cache instead of RAM, it is referred to as a “cache-friendly” access.

Imagine that you need to iterate through a big array of relatively small objects, maybe a bunch of triangles. In Java today, you don’t really have an array of triangles; you have an array of pointers to triangle objects because regular objects in Java are “reference types,” meaning you access them through Java pointers/references. So even though the array is probably a contiguous ...