At STH, we have opined on numerous occasions that to unlock true AMD EPYC “Naples” series potential you ideally want 8 DIMMs per CPU. That fills each memory channel with a DIMM. We understand that some configurations are more cost sensitive, but we still recommend at least one DIMM per die or four DIMMs per CPU as a minimum configuration. We are going to show why we make this recommendation.

Why this is Poignant for AMD EPYC Buyers

Configurators of popular server vendors allow for one or two DIMMs on AMD EPYC. This works, but it is far from ideal. Here is an example from the HPE ProLiant DL385 Gen10 QuickSpecs with pre-configured orderable models:

We were working with a system which is essentially the HPE SKU 878718-xx1 labeled a “Base Model” for HPE. This system comes with 2x 32GB DIMMs by default.

Likewise, offerings like the Dell EMC PowerEdge R7415 configurator start with a single 8GB DIMM configured. Technically, one only needs a single DIMM for these CPUs to work.

Given our guidance that one should use at least 4 sticks of DDR4-2666, one for each die, in an AMD EPYC “Naples” server, the base configurations do not conform to this.

Testing AMD EPYC “Naples” with 1 to 8 DIMMs

We decided to use an HPE ProLiant DL385 Gen10 for our testing with a single CPU. In this case, we are using the AMD EPYC 7301 16-core CPU. While we could have done every increment from 1 to 8 DIMMs, we instead focused on the four configurations we see as being the most likely: 1, 2, 4, and 8 DIMMs.

As a note: we were doing this testing for a client but the benchmark setup is not the same baseline we use for our main benchmark and review pieces. It still shows the delta, the numbers should not be a direct comparison to what we have elsewhere.

STREAM Triad

When we look at raw memory bandwidth, STREAM by John D. McCalpin is what the industry uses (learn more here.) This is the micro-benchmark we expect to have the largest performance deltas by filling memory channels. We also had AMD’s secure memory encryption enabled for this test and were using GCC, however, we did not perform any NUMA node locking as you would do with EPYC seeking a maximum throughput run.

This is an extraordinarily interesting result. We re-ran the test and even Dockerized everything to verify what we were seeing. Each time, we got slightly better scaling than just 2x from doubling the types of DIMMs. We were told that this is due to not pinning NUMA nodes which would align with what one would see on a four-socket Intel Xeon machine.

Linux Kernel Compile

We took a version of our Linux Kernel Compile benchmark and let it rip. This is a CPU benchmark, but it is representative of a common workload that many servers do every day. We

What we saw was some extreme scaling from 1 DIMM to 2 DIMMs almost doubling performance. Then a significant but more muted 13% improvement from 2 DIMMs to 4 DIMMs. 4 DIMMs to 8 DIMMs was still noticeable and repeatable with a 2.5% gain.

Taking a moment here, these improvements are as much as moving to another SKU in the abridged AMD EPYC first generation SKU stack.

Compression Tests

Here we are using our 7zip Compression benchmark. One can see that the number of DIMMs populated again had a significant impact.

Here we got slightly more muted scaling from 1 DIMM to 2 DIMMs, only about 92% better. 2-4 DIMMs gave us a 20% performance improvement. We received a 3.5% improvement moving up to 8 DIMMs.

Opposing Case C-ray

Going into this exercise, we expected our c-ray rendering benchmark results to be very close. This primarily runs in CPU cache so this is a case where there is de minimus RAM access. We are using our 4K render to add additional complexity and time hoping to tease out differences.

We use seconds here and each configuration was 206 seconds. Going down to the millisecond level would give us a 0.48s difference between 1 and 8 DIMMs. This is an extremely consistent application but we are getting into the range where 1 to 8 DIMMs is an approximately 0.2% delta which is negligible.

Summary View

Here is the summary view of our quick testing where we took the deltas of populating more DIMMs in the system.

This gives us some ballpark guideposts to have the discussion around how many DIMMs you want. The relative 4 to 8 DIMM indifference toward moving to dual memory channels shows that there is a bigger impact to populate up to 4 DIMMs and having a single channel of memory per die than having dual channel memory per die. The takeaway here for larger applications is that memory access to a different NUMA node is expensive.

Another takeaway is the cumulative impact. For example, on our compile and compression benchmarks, moving from 1 DIMM to four DIMMs was a 125-130% performance improvement. If you were using 16GB DIMMs, this means that for under $600 you can get more than twice the performance out of your AMD EPYC CPU by adding three more DIMMs to unpopulated sockets.

Final Words

This picture is going to change quite a bit with AMD EPYC Rome generation. At the same time, the current generation is an amazing value. We still strongly recommend using at least 4 DIMMs per CPU as performance otherwise will suffer greatly. Compared to some of the other workloads we run, these all fit in a relatively small RAM footprint. Running a virtualization server, for example, would show these deltas to a more extreme degree over time.

For those wondering about the AMD EPYC 7301 it is a lower clocked 16 core CPU, but it still has a full 64MB L3 cache. In initial testing, we are seeing the EPYC 7281 with only 32MB L3 cache and the higher core count 24 and 32 core count parts being more sensitive to memory population. That makes some sense as those are combinations that have less cache per core.

Our bottom line: with AMD EPYC “Naples”, get at least 4 DIMMs per socket.