ThinkSystem SR950 Memory DecisionsArticle

27 Sep 2017
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This paper examines the ThinkSystem SR950 memory decisions that need to be made when configuring the system or adding memory options. The key decision includes understanding the different memory types, and balancing the system memory across the server.


The ThinkSystem SR950 supports a large amount of system memory. The four-socket (4S) configuration supports up to 48 memory slots up to 6TB of memory and the eight-socket (8S) configuration supports up to 96 memory slots up to 12TB of memory. With larger system memory configurations, memory can be the most expensive component of a servers cost as well as a key element driving the server performance.

Because of these factors, it very important to understand the memory choices and build the most optimal memory configuration for your SR950 server.

Lenovo ThinkSystem SR950
Figure 1. Lenovo ThinkSystem SR950

ThinkSystem SR950 Memory

The Lenovo ThinkSystem SR950 uses Lenovo TruDDR4 memory operating at up to 2666 MHz. The server supports 12 DIMMs per processor, which corresponds to 48 DIMMs with four processors installed and 96 DIMMs when eight processors are installed. Each processor has six memory channels with two DIMMs per channel. With 128 GB 3DS RDIMMs installed, an 8-socket server supports a total of 12 TB of system memory.

All DIMMs operate at a speed of 2666 MHz, both at 1 DIMM per channel and 2 DIMMs per channel. However, if the processor selected has a lower memory bus speed (eg 2400 MHz), the DIMMs will operate at that lower speed. For the SR950, only the Gold 51xx Scalable Family CPUs operate at the lower 2400 MHz speeds

ThinkSystem SR950 Compute System Board
Figure 2. Compute system board with 2 processors and 24 total DIMMs

Lenovo TruDDR4 memory uses the highest quality components that are sourced from Tier 1 DRAM suppliers and only memory that meets the strict requirements of Lenovo is selected. It is compatibility tested and tuned to maximize performance and reliability. From a service and support standpoint, Lenovo TruDDR4 memory automatically assumes the system warranty, and Lenovo provides service and support worldwide.

Memory Options

The SR950 supports two types of 16GB DIMM and a single 32GB DIMM, 64GB DIMM and 128GB DIMMs. The SR950 supports the memory options listed in the following table.

Table 1. Memory options
Description Maximum
7X77A01302 AUNB ThinkSystem 16GB TruDDR4 2666 MHz (1Rx4 1.2V) RDIMM 96 (12 per processor)
7X77A01303 AUNC ThinkSystem 16GB TruDDR4 2666 MHz (2Rx8 1.2V) RDIMM 96 (12 per processor)
7X77A01304 AUND ThinkSystem 32GB TruDDR4 2666 MHz (2Rx4 1.2V) RDIMM 96 (12 per processor)
7X77A01305 AUNE ThinkSystem 64GB TruDDR4 2666 MHz (4Rx4 1.2V) LRDIMM 96 (12 per processor)
7X77A01307 AUNF ThinkSystem 128GB TruDDR4 2666 MHz (8Rx4 1.2V) 3DS RDIMM 96 (12 per processor)

The following rules apply when selecting the memory configuration:

  • The server supports RDIMMs, LRDIMMs and 3DS RDIMMs. UDIMMs are not supported.
  • Mixing RDIMMs and LRDIMMs is not supported.
  • Mixing 3DS RDIMMs with either RDIMMs or LRDIMMs is not supported.
  • Mixing x4 and x8 DIMMs is supported.

Choosing one Type of DIMM over the other

Hitting your exact desired system memory level might limit your DIMM choices but in some cases you can choose between two and three different DIMMs to meet the requirements.

Many end users configure server memory based on a capacity requirement. Historically, RDIMMs have been favored for lower capacity requirements and LRDIMMs for higher capacity requirements.

Choosing between the two types of 16GB DIMMs

  • Choose the 16GB 2R x8 for better performance
  • Choose the 16GB 1R x4 for better RAS as its supports Chipkill (DRAM fail support and more self-healing features)

To hit a certain memory level and allow for memory growth, it might make sense to populate the SR950 with larger DIMM sizes instead of filling up the server with smaller DIMMs. This way you have empty DIMM slots allowing expansion. A 4S example would be using 24x 32GB DIMMs instead of using 48x 16GB DIMMs for a 768 GB total system memory.

The 128 GB DIMM is a 3DS RDIMM. 3DS (three dimensional stacking) is a relatively new technology which stacks DRAM components vertically and allows for greater capacity DIMMs while not increasing the load on the memory bus.

The cost of the different memory options should also be taken into consideration:

  • The two 16GB DIMMs have essentially the same cost
  • A single 32GB DIMM is typically lower cost than two 16GB DIMMs
  • A single 64GB DIMM is currently only 1-5% higher cost than two 32GB DIMMs.

A single 128GB DIMM is currently quite a bit more expensive than two 64GB DIMMs but cost on 128GB DIMM will decrease as adoption of this new DIMM increases.

The following figure shows the 16 GB and 64 GB DIMMs.

Lenovo 16 GB and 64 GB RDIMMs
Figure 3. SR950 memory options (16 GB RDIMM and 64 GB RDIMM)

Balanced Memory for Best Performance

Configuring a server with balanced memory is important for maximizing its memory bandwidth and overall performance. The SR950 has six memory channels per processor and up to two DIMMs per channel, so it is important to understand what is considered a balanced configuration and what is not.

Balanced memory configurations enable optimal interleaving across all attached memory channels so memory bandwidth is maximized. Bandwidth can be optimized if both memory channels on the same physical processor socket are identically configured. System level memory performance can be further optimized if each physical processor socket has the same physical memory capacity.

The basic guidelines for a balanced memory subsystem are as follows:

  • Populate memory DIMMs in quantities of 6 or 12 per processor, so that all memory channels are used.
  • When mixing 16 GB and 32 GB DIMMs in the same configuration, use 16GB 2Rx8 DIMMs instead of 16 GB 1Rx4 DIMMs for better performance.
  • Populate memory channels so they all have the same total memory capacity
  • Ensure all memory controllers on a processor socket have the same DIMM configuration.
  • All processor sockets on the same physical server should have the same DIMM configuration.

If the SR950 requires a specific memory capacity, sometimes smaller DIMMs or a mix of DIMM sizes are needed to produce a balanced memory configuration.

Overall server performance is affected by the memory subsystem which can provide both high memory bandwidth and low memory access latency when properly configured. Balancing memory across the memory channels and CPUs produces memory configurations which can efficiently interleave memory references among its DIMMs producing the highest possible memory bandwidth.

For a detailed discussion of configurations with balanced memory, see the Lenovo Press paper, Intel Xeon Scalable Family Balanced Memory Configurations.

The best way to ensure a balanced memory configuration that meets your capacity and upgrade requirements, use the Memory Wizard as described in the Lenovo Memory Configurator section below.

Examples of Balanced Memory configurations for each processor:

Figure 4. 384 GB (4S) or 768 GB (8S)

Figure 5. 768 GB (4S) or 1.5 TB (8S)

Figure 6. 1 TB (4S) or 2 TB (8S)

Figure 7. 1.5 TB (4S) or 3 TB (8S)

Figure 8. 1.5 TB (4S) or 3 TB (8S)

Figure 9. 3 TB (4S) or 6 TB (8S)

Figure 10. 6 TB (4S) or 12 TB (8S)

Mirroring and Rank Sparing

If memory channel mirroring is used, then DIMMs must be installed in pairs or sets of three (minimum of one pair or set of three per processor), and all DIMMs in the pair or set of three must be identical in type and size. 50% of the installed capacity is available to the operating system.

If memory rank sparing is used, then a minimum of two single-rank or dual-rank DIMMs must be installed per populated channel (the DIMMs do not need to be identical). In rank sparing mode, one rank of a DIMM in each populated channel is reserved as spare memory. The largest rank in the channel will be automatically selected as the spare rank. The amount of memory available to the operating system depends on the number, capacity and rank counts of the DIMMs installed.

Memory System Settings

The XClarity Provisioning Manager application embedded in the Unified Extensible Firmware Interface (UEFI) of the SR950 can if requested, force memory to a speed lower than the maximum speed determined by the processor and the memory subsystem. Memory speed might be set lower to save energy in environments with little memory performance sensitivity

Memory speed options are as follows:

  • Maximum Performance (default): Memory runs at the maximum supported speed determined by processor model and memory subsystem.
  • Balanced: Memory runs at one step lower than the maximum speed.
  • Minimal Power: Memory runs at the lowest speed supported by the architecture. 1866 MHz is the lowest speed supported on SR950.

Lenovo Memory Configurator

Learning about the memory choices for the SR950 helps you optimize system memory for performance and cost. To make it easier, Lenovo has also developed a Lenovo Memory Configuration tool to optimize system memory for each ThinkSystem server.

The user can provide the amount of memory required and how they would like to optimize based on performance or by price. The memory configuration tool will then calculate the amount of DIMMs required as well as the location of the DIMMs based on the users input.

To use the tool to configure SR950 memory, do the following:

  1. Launch Lenovo Memory Configuration tool
  2. Select Mission-Critical > ThinkSystem SR950
  3. Click Specify processor and select the type and number of CPUs (2, 4 or 8)
  4. Click Next (you do not specify any existing memory on a new system)
  5. Enter the desired system memory (ie 1535 GB). The tool will round up to the next available capacity based on available DIMMs.
  6. Select the configuration priority (performance, price or custom).
  7. Review the available configuration options. Each configuration shows which memory DIMMs are installed in every DIMM slot of the server.

The tool provides an export function to save the selected configuration to an XLS spreadsheet and an XML file for use in the LESC system configurator.

Further reading

About the author

Randall Lundin is the Mission Critical Product Manager in the Lenovo Infrastructure Solutions Group. He is responsible for managing and planning Lenovo’s 4-socket and 8-socket servers. Randall has also authored and contributed to numerous Lenovo Press publications in the Mission Critical space.

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