Memory

When selecting memory, one first has to check which memory types are supported by the selected processor. Not all support the latest memory speeds. And even if they do, the maximum memory speed might not be supported beyond a certain amount of memory modules.

Each processor supports a limited number of memory channels. In order to maximize the available memory bandwidth, the number of RAM modules should always be a multiple of the available channels. E.g., Intel Skylake processors support up to 6 channels, that means the number of RAM DIMMS should be either 6, 12, 18, 24, 30, and so on to maximize utilization of the available bandwidth.

Error correction code (ECC) memory

One critical feature for long running calculations is ECC memory. It supports an automatic error correction that detects when individual bits flip. Given the amount of RAM nowadays and the density of the system, the probability of individual bits in memory to flip due to electromagnetic interferce, cosmic radiation is high. It does come with a premium price, which makes these RAM modules a bit more expensive compared to comsumer RAM modules.

Memory Bandwidth / Latency

When comparing different RAM modules, a lot of information can be obtained from their name.

In a DDR4-2400 module, the 2400 stands for the data rate in MegaTransfers/s, which multiplied by 8 is the bandwidth of 19200MB/s. Since DDR stands for double data rate, the command rate is 1200 MHz, so half of the data rate value.

Modules not only differ in their data rate, but also their latency. The technical specifications of RAM modules specify a Column Address Strobe (CAS) latency, which stands for the amount of cycles it takes for the first word of a column in memory to be accesses.

A CAS of 15 means that it takes 15 cyles to access the first word. With a command rate of 1200 MHz, this is 15 times 1 divided by 1200Mhz, which is 12.5ns. In comparison, a CAS 17 module this access time increases to 14.17ns. So lower CAS values mean faster access times.