Choosing the Best Memory for Your Blade Server
With the advancement of CPU architecture, recognizing the best memory options within your server has become extra challenging. Should you handle a single DIMM or does multiple DIMMs make a difference? What about memory speed — does it actually matter? Should you buy a single rank (SR), dual rank (DR) or quad rank (QR) DIMMs. I’m not working to pretend to be a memory specialist, but hopefully, I will help explain these questions in today’s post. With modern Intel CPU architecture, there are some things to note: balancing memory is important for performance, and you’ll want to have two CPUs to get a full introduction to all memory. Although the latter may appear obvious, two CPUs may not be needed for environments with fewer workloads. As you can watch in the Intel CPU picture below, 1/2 of the server memory is attached to every CPU, limiting you from being held with 12 DIMMs in a two-socket server, populate the 2nd CPU.
More importantly, though, is supporting the memory. Every CPU has four memory channels, typically 3 DIMMs deep, support upto 12 DIMMs per CPU. Although populating a particular DIMM per CPU is allowed, you will not get the most reliable performance, and here’s why. Picture a highway, including a toll booth. There are four lanes, but just one lane is open. While the cars will not get through the unique toll booth, they can not notice through as quickly as possible if all 4 toll booths were open. This is the same as using a single DIMM. Let’s assume your application needs 32GB of RAM. If you put a 16GB DIMM on every CPU, you won’t have as excellent performance as 4 x 4GB DIMMs per CPU. In a nutshell, I continuously encourage populating in groups of 8 (4 per CPU) to maximize the memory for the channel’s performance.
Does Memory Acceleration Really Matter?
The small answer is — yes if you want performance. In a white paper addressed by John Beckett from the Dell Solutions Performance Analysis team, various memory speed is compared, which showed a substantial 13% decrease in production in 1333MT/s vs 1600MT/s. The catch, though, is that Intel has three different “bins” of CPUs, so to get the best memory frequency, you’re working to have to use a CPU in the “Advanced” bin category. An additional FYI is that the Intel-Xeon E5–2600 v3 now allows upto 2133Mhz in both 1 DIMM per channel and two DIMM per channel but drops down to 1866MHz at 3 DIMMs per channel — so if you want the best performance, buy a top bin CPU and keep the memory qty to 16 DIMMs or below. As mentioned in the previous section, for more information on populating DIMMs, check out that white paper I referenced above.
What About Memory Ranks — What is Better?
This is one question I’ll ask for your opinion on. From what I’ve researched, it appears that Dual Rank (DR) DIMMs are preferred for performance compared to Single Rank (SR) DIMMs or Quad Rank (QR) DIMMs. I know that QR DIMMs have memory chips stacked, so it runs at lower frequencies. However, I’m not sure I get why DR DIMMs perform better.
