DDR5 RAM price history

Every DDR5 chip has on-die ECC, but it only fixes errors inside the chip. Data leaving the chip and traveling across the stick and motherboard to the CPU is not protected. Real ECC memory adds 8 check bits (marked EC8) or 4 (marked EC4) to each 32-bit subchannel, and every server RDIMM has it. On a desktop, error correction only works if both the CPU and the motherboard support it. You don't need to buy ECC memory specifically to run local LLMs.

Desktops use unbuffered memory (UDIMM); servers use registered memory (RDIMM). An RDIMM has an extra registering clock driver chip (RCD), so each channel can carry more chips. Sticks of 32GB and 64GB are common, and all RDIMMs have ECC. The trade-off is lower speeds and looser timings: server sticks are commonly DDR5-4800 or 5600, desktop sticks 6000 or 6400. DDR5 sticks carry their own power management chip, and desktop and server sticks take different supply voltages, so they are not interchangeable.

The number before R is the rank count. A rank is a group of chips on the stick that are read and written together to make up 64 bits of data: 1R is single-rank, 2R is dual-rank. The number after x is the data width of each chip in bits. It takes 8 x8 chips or 16 x4 chips to make 64 bits, so x4 sticks can reach higher capacities, and most high-capacity server sticks are 2Rx4. The more ranks in a channel, the more likely the motherboard is to lower the memory speed when all slots are filled.

CL (CAS latency) is the number of clock cycles between a read command and the data coming out. Lower is faster. To get the real latency, convert it using the speed: nanoseconds = CL × 2000 ÷ speed in MT/s. DDR5-6000 CL30, common on desktops, works out to 10 ns; DDR5-4800 CL40, common on servers, to about 17 ns. When an LLM runs, every generated token means reading several to tens of GB of weights from memory. Bandwidth is what counts, and timings have very little effect.

Sticks with different speeds will work together, but they all run at the speed of the slowest stick, with the loosest timings. With different capacities, only the capacity that matches on both channels is interleaved in dual-channel mode, so bandwidth drops. Different brands usually work, but the safest choice is the same model from the same batch. Desktop and server sticks can't be mixed, and neither can DDR4 and DDR5 (the notch is in a different place, so they don't fit). With 4 sticks in a DDR5 desktop, most platforms need a big drop in speed to stay stable, so if speed is the goal, use just 2 sticks. On a server, every channel needs identical sticks to reach full bandwidth.

A DDR5 stick is split into two 32-bit subchannels, which together still transfer 64 bits, or 8 bytes, at a time, so bandwidth per stick = speed × 8. DDR5-6000 gives 48 GB/s, which is what PC5-48000 on the label refers to. Platform bandwidth = bandwidth per stick × number of channels. A desktop has only 2 channels, so 4 sticks still give 96 GB/s: extra sticks add capacity, not bandwidth. EPYC 9004 has 12 channels, which reach 460.8 GB/s when all are filled with DDR5-4800.

The number is the speed, and the letter after it is the JEDEC timing grade: DDR5-4800B, common on servers, is CL40, and 5600B is CL46. PC5-38400 is the same speed written as bandwidth (4800 × 8 = 38400 MB/s). An R on the label means RDIMM; U would be a desktop stick and S a laptop stick. EC8 means each subchannel carries 8 ECC check bits. Desktop sticks without this marking only have on-die ECC inside the chips.