Ulrich Drepper's long guide from 2007 on how memory hardware really works and why memory, not the CPU, is often what makes a program slow. It covers how RAM chips work, how CPU caches are built and why they exist, and how virtual memory and NUMA change the picture, then spends a large middle section on concrete advice for writing code that uses caches well. There are plenty of diagrams and real numbers measured on real hardware, along with practical topics like data layout, cache line size, and tools that help you find memory related slowdowns. Some of the hardware details have aged, but the core ideas still hold up.