At STH, we deal with many servers. As a result, adding a lot of memory to configurations is something we do quite often. Recently, we built a server based on the AMD EPYC 4464P and a Gigabyte barebones that we tried 48GB DDR5 ECC UDIMMs on, and they worked. This is a big deal for lower-cost servers since it makes higher memory capacities available. What we noticed in going through our data was that the 192GB configuration lost a bit of performance. This makes a lot of sense since we had 4x 48GB ECC UDIMMs installed for two DIMMs per channel or 2DPC.
Why One DIMM Per Channel or 1DPC Can Be Great
During our recent Mapping Licensing for Virtualization is Cool Now piece, many focused on the low-cost single-socket server that we used, which is currently the bestseller at Newegg.
When configuring the system, we bought some low-cost generic 48GB DDR5 ECC UDIMMs, along with a two ECC DIMM set from V-Color. You can see the four DIMM set here:
When you populate both channels in the AMD EPYC 4004 series platform, the memory speed decreases from up to DDR5-5200 to DDR5-3600. That is why we see many use 2x 32GB DIMMs instead of 4x 16GB. Putting that into context, here is a Geekbench 5 run of the same system with four DIMMs (2DPC) as the baseline and two DIMMs (1DPC) as the comparison.
Geekbench 5 and Geekbench 6 behave differently, so here is the Geekbench 6 comparison as well:
2-10% is a reasonable range for most applications, but if there are extremely memory bandwidth sensitive applications, then the deltas can be much higher.
Final Words
After seeing this in our test data, we thought that many folks who do not regularly build or configure servers may not appreciate this difference. So we just wanted a quick weekend piece to show why one would want to configure 1DPC over 2DPC at the same capacity point. At the same time, 2DPC has a higher memory capacity by simply having twice as many DIMMs installed. Capacity is often worth the trade-off. This was just showing the EPYC 4004 series since we did the comparison recently. Even higher-end CPUs, and from both AMD and Intel, it is common to see 2DPC configurations with lower memory clock speeds. That is why we normally test servers with 1DPC configurations.
If you are new to DDR5 in servers, then check out Why DDR5 is Absolutely Necessary in Modern Servers.
This would probably benefit from a separate SKU of chips, as detailed in STH’s “AMD EPYC 7002 Rome CPUs with Half Memory Bandwidth” (4 channel optimized) article.
Another relevant article is STH’s “Why 2 DIMMs Per Channel Will Matter Less in Servers”.