Help Me Choose - Memory

Help Me Choose - Memory
Dell EMC PowerEdge offers a variety of memory options to meet your workloads requirements. Memory plays a key role in your overall performance along with your processor, accelerator and storage choices.

Dell Recommends

Properly configuring a server with balanced memory is critical to ensure memory bandwidth is maximized and latency is minimized. When server memory is configured incorrectly, unwanted variables are introduced into the memory controllers’ algorithm, which inadvertently slows down overall system performance. To mitigate this risk of reducing or even bottlenecking system performance, it is important to understand what constitutes balanced, near balanced and unbalanced memory configurations.

Variables such as DIMM consistency and slot population will dictate whether a configuration is balanced or unbalanced. Follow these guidelines at the socket and server level, to achieve a balanced memory configuration:

Balanced Memory

DIMMs must be populated into a balanced configuration to yield the highest and most consistent memory bandwidth with the lowest memory access latency. Various factors dictate whether a configuration is balanced or not. For best results, follow these guidelines:

1. Each memory channel must be fully populated with one or two DIMMs for best performance. 
2. Each memory controller must be populated with at least one DIMM in a symmetrical fashion.

Symmetrical’ refers to two memory channels that are horizontally flipped.
-    Same number of DIMMs and same DIMM in each memory channel, i.e., 1 or 2 DIMMs per memory channel.
-    CPU and DIMM parts should be identical.
-    Each CPU must be identically configured with memory.

Below are the recommended Balanced Configs per generation:
15G Balanced Memory Configs Intel & AMD: 
-    2-socket systems: 16 or 32 DIMMs
-    1-socket systems: 8 or 16 DIMMs

16G Balanced Memory Configs Intel:
-    1-socket systems: 8 or 16 DIMMs
-    2-socket systems: 16 or 32 DIMMs
-    4-socket systems: 32 or 64 DIMM

16G AMD:
-    2-socket systems: 24 DIMMs (supports 1DPC)
-    1-socket systems: 12 DIMMs (supports 1DPC)
    
17G Balanced Memory Configs Intel: 
-    1-socket systems: 8 or 16 DIMMs
-    2-socket systems: 16 or 32 DIMMs

17G AMD:
-    1-socket systems: 12 DIMMs (supports 1DPC)
-    1-socket systems: 24 DIMMs (supports 2DPC)
-    2-socket systems: 24 DIMMs (supports 1DPC)

Which DIMM Do I Need?

A DIMM — or dual in-line memory module — is a series of random access memory chips mounted on a small circuit board. DIMMs are installed in sockets on your computer's motherboard.

DIMM Types

RDIMM

Registered DIMM
Provides for higher capacity options and advanced RAS features. It is the most commonly used DIMM type, and offers the best mix of frequency, capacity, and rank structure choices

LRDIMM

Load Reduced DIMM
Provides maximum capacity beyond that of an RDIMM but at a higher power consumption. Uses a buffer to reduce memory loading to a single load on all DDR signals, allowing for greater capacity.

UDIMM

Unregistered or Unbuffered DIMM
Provides low latency and low density. Used on single socket servers in the PowerEdge portfolio.

DDR5

Double Data Rate Type 5 is the newest DDR used in servers. 

DDR5 provides higher bandwidth and increased bandwidth efficiency. It offers a 50% increase in bandwidth with 4800 MT/s compared to DDR4 and supports a maximum density of 32 Gb. DDR5 also offers twice the burst length, twice the number of bank groups, and twice the number of banks.

CXL Info

CXL Info with FAQ
Compute Express Link (CXL) is an open standard for high-speed low latency interconnect between the central processing unit and expansion devices that provide cache coherent memory. CXL memory devices add memory capacity and memory bandwidth beyond DDR alone. CXL memory can be used in addition to the required DDR5 DIMM memory.

CXL is only supported on 17G servers with limited configs
Form Factors include:
Add In Card single wide FHHL (full height half length)
Total Add-in Card Capacity: 384GB
PCIE lanes: x8 & x16

Useful Resources

Intel® Optane™ Persistent Memory
2nd Generation AMD EPYC™ Processors: Balanced Memory Reference Guide
How to find the supported memory configuration guide for PowerEdge Servers.
DDR5 on AMD
Dell Next Generation PowerEdge Servers: Designed with DDR5 to Deliver Future-Ready Bandwidth
Memory Bandwidth for Next-Gen PowerEdge Servers Significantly Improved with Sapphire Rapids Architecture
Balanced Memory: Intel Xeon Scalable Processors on PowerEdge Servers
DDR5 Memory Bandwidth for Next-Generation PowerEdge Servers Featuring 4th Gen AMD EPYC Processors
Memory Channel Population