What Every Programmer Should Know About Memory

What Every Programmer Should Know About Memory Ulrich Drepper Red Hat, Inc. drepper@redhat.com November 21, 2007 Abstract As CPU cores become both faster and more numerous, the limiting factor for most programs is now, and will be for some time, memory access. Hardware designers have come up with ever more sophisticated memory handling and acceleration techniques–such as CPU caches–but these cannot work optimally without some help from the programmer. Unfortunately, neither the structure nor the cost of using the memory subsystem of a computer or the caches on CPUs is well understood by most programmers. This paper explains the structure of memory subsys- tems in use on modern commodity hardware, illustrating why CPU caches were developed, how they work, and what programs should do to achieve optimal performance by utilizing them. day these changes mainly come in the following forms:1 Introduction In the early days computers were much simpler. The var- • RAM hardware design (speed and parallelism). ious components of a system, such as the CPU, memory, mass storage, and network interfaces, were developed to- • Memory controller designs. gether and, as a result, were quite balanced in their per- • CPU caches. formance. For example, the memory and network inter- faces were not (much) faster than the CPU at providing • Direct memory access (DMA) for devices. data.