When configuring the RAM for a computer system, one of the first decisions is whether to use registered (RDIMM) or unbuffered (UDIMM) dual in-line memory modules. RDIMMs and UDIMMs utilize different architectures that impact performance, reliability, scalability, and cost. Understanding the key differences between RDIMMs and UDIMMs can help IT professionals select the best memory solution for their specific system requirements.
What is a Dual In-Line Memory Module (DIMM)?
Before diving into RDIMMs and UDIMMs, it helps to understand what a DIMM is. DIMM stands for dual in-line memory module. This term refers to the physical design of the memory module, which has contacts on both sides to connect to the motherboard. As the standard memory module form factor, DIMMs provide several advantages over older single in-line memory modules (SIMMs):
- Higher memory bandwidth from a 64-bit data interface compared to 32-bit SIMMs
- Support for higher memory densities per module
- Takes up less space on the motherboard
While DIMMs have a common physical layout, there are several subcategories based on the internal memory buffering and registering, including:
- UDIMM – Unbuffered with no registers
- RDIMM – Registered with onboard buffers
- LRDIMM – Load reduced with additional memory buffers
The focus of this article will be contrasting UDIMMs and RDIMMs.
UDIMM Overview
UDIMM stands for unbuffered DIMM. This term reflects that UDIMMs lack any buffering or registering logic between the DIMM‘s DRAM memory chips and the system memory controller. With a UDIMM, all signals pass directly between the DRAM and memory controller.
Some key characteristics of UDIMMs include:
- Lower cost compared to RDIMMs
- Moderate capacities from 2GB to 32GB currently
- Limited scalability with a maximum of 1-2 UDIMMs recommended per memory channel
- Higher latency when multiple UDIMMs installed per channel
- Commonly used for desktops, laptops, and entry servers
Without an onboard register, UDIMMs minimize delays allowing for excellent single module performance. However, the lack of buffering means installing multiple UDIMMs per channel adds capacitive loading. This additional electrical load causes signal degradation which limits the number of UDIMMs per channel.
RDIMM Overview
In contrast to UDIMMs, RDIMM stands for registered DIMM. RDIMM modules include an onboard register positioned between the DRAM chips and system memory controller. Often referred to as a buffer, this register stores and redrives command and address signals sent to the DRAMs.
Some key traits of RDIMM memory include:
- Higher cost versus UDIMMs
- Superior scalability, supporting dozens of RDIMMs per channel
- Excellent signal integrity for improved reliability
- Higher capacities up to 128GB
- Primarily used in high-end servers and workstations
The register on the RDIMM isolates electrical loading from multiple modules away from the memory controller. This enables large RDIMM configurations with minimal performance impact. The register also redrives command and address signals which maintains signal integrity even for heavily loaded channels.
Architectural Differences
Now that you have a high level overview of UDIMMs and RDIMMs, it helps take a closer look at the architectural variations between the two memory types:
No Register vs Integrated Register
The lack of a register is the most significant difference between UDIMMs and RDIMMS. With a UDIMM, the DRAM chips directly connect to the memory controller. This provides excellent performance with a single module, but quickly degrades as additional UDIMMs overload the memory channel.
In contrast, RDIMMs integrate a register chip placed between the DRAM and memory controller. Often called a buffer, this register electrically isolates DRAM loads allowing for excellent scalability and signal integrity with dozens of RDIMMs installed.
Direct vs Multi-Drop Topology
UDIMMs utilize a direct topology with the memory controller signaling each DRAM chip individually. This topology contributes to the electrical loading challenges seen when using multiple UDIMMs per channel.
For RDIMMs, the register chip enables what is known as multi-drop bus topology. Here, the memory controller only connects directly to the buffer chip. The buffer then controls the individual DRAM chips using a shared command, address and data bus.
No Detection vs ECC Support
Most UDIMMs lack any advanced error detection or correction capabilities. A small number of UDIMMs support ECC (error correction code) which enables detection and correction of single-bit memory errors. However, most UDIMMs have no safeguards for data integrity.
In comparison, RDIMMs routinely utilize more advanced error detection through parity bits or ECC. Any transmission errors can be reported back to the memory controller thanks to the RDIMM‘s register chip.
Low Density vs High Density Support
Current UDIMMs support memory densities from 2GB up to 32GB per module, although 8GB UDIMMs are most common. These modest densities combined with UDIMM‘s limited scalability make them best suited for desktops and general purpose servers.
RDIMMsRANGE support from 4GB and can reach as high as 128GB for leading edge server systems. This massive memory capacity combines with excellent scalability are why top-tier servers utilize RDIMMs.
Performance Impact
The architectural differences between UDIMMs and RDIMMs directly influence memory performance. Key metrics like module capacity, channel scalability, bandwidth, and latency compare as follows:
Module Capacity
RDIMMs support markedly higher memory capacity per module, reaching up to 128GB from leading vendors. UDIMM capacities top out at 32GB currently. This 4x higher module density better utilizes available motherboard DIMM slots.
Channel Scalability
The limited electrical loading tolerance of UDIMMs restricts configurations to 1-2 UDIMMs per memory channel. In comparison, RDIMM‘s registers allow reliably running over a dozen modules per channel.
Bandwidth
With only one module installed per channel, UDIMMs offer higher memory bandwidth thanks to lower latency. However, when utilizing multiple modules per channel, RDIMMs pull ahead bandwidth-wise. The latency penalty from RDIMM buffering gets offset by UDIMM electrical loading.
Latency
UDIMMs demonstrate better latency with light loading thanks to no buffers or registers to traverse. Heavily loaded UDIMM configurations see this advantage diminish or reverse as electrical loading on unbuffered modules reduces performance.
RDIMMs see slightly higher latency empty or lightly loaded due to the register chip. But heavier configurations favor RDIMM latency as electrical loading gets isolated to the register instead of bogging down the memory controller.
Reliability
Beyond just performance metrics, RDIMM and UDIMM memory also varies regarding reliability:
Signal Integrity
Without registers, UDIMMs are vulnerable to degradation from electrical loading as more modules get added. At some point, the memory channel becomes unreliable leading to errors or even failure.
Conversely, RDIMM buffering ensures excellent signal integrity and electrical isolation regardless of configuration size. This enables much more reliable performance under heavy loads.
Error Detection
Standard UDIMMs have no safeguards against data corruption making silent memory errors a real concern. While ECC UDIMMs add error detection, this comes at a capacity cost while only covering single-bit flips.
RDIMMs integrate more robust error detection out of the box. Parity or ECC provides multi-bit detection allowing memory errors to get flagged before propagating.
Thermal Stresses
Dense, high performing servers generate substantial thermal output which applies physical stress on components. RDIMM registers help reduce this stress on DRAM chips improving lifespan and reliability.
Cost Comparison
One area where UDIMMs maintain a clear advantage is pricing. Based on online retail pricing, we can compare cost for a sample 32GB configuration:
- 2x 16GB UDIMMs = $60-$75
- 1x 32GB RDIMM = $185-$250
As this illustrates, RDIMMs carry a 3-4x cost premium over UDIMMs. This pricing delta mirrors the density advantages from RDIMM buffering requiring fewer modules for a given configuration.
UDIMM vs RDIMM Use Cases
Given the technical and economic variations between UDIMMs and RDIMMs, certain usage scenarios lend themselves towards one memory module or the other:
UDIMM Use Cases
Thanks to their lower cost and power needs, UDIMMs work well for mainstream computing applications including:
- Desktop computers
- Laptop, mobile, and embedded devices
- Entry level servers
- Networking/communications gear
UDIMM limitations around density and scalability make them less ideal for systems requiring hundreds of gigabytes of memory or complex memory configurations.
RDIMM Use Cases
On the flip side, RDIMM capabilities including density, scalability, and reliability perfectly match up with high-end server requirements:
- Database servers
- Virtualized environments
- High performance computing clusters
- Data analytics/data warehousing
- In-memory computing
The enhanced error detection of RDIMMs also suits customer applications demanding excellent data integrity for transactions, scientific computing, or financial data.
Can You Mix UDIMMs and RDIMMs?
With motherboards supporting either UDIMMs or RDIMMs, one common question is whether you can utilize both memory types together. Unfortunately, mixing UDIMMs and RDIMMs within the same system won‘t work. The different electrical characteristics, signaling behavior, and timing parameters makes this impossible.
However, for newer server platforms that partition memory channels across CPU sockets, you CAN run UDIMMs and RDIMMs simultaneously. By utilizing UDIMMs on some memory channels tied to one socket and RDIMMs on other channels connected through another socket, this heterogeneous mode provides some flexibility.
Choosing UDIMM vs RDIMM: Key Considerations
When deciding between utilizing UDIMM or RDIMM memory, keep the following guidelines in mind:
- Density needs – RDIMMs support 3-4x higher capacities.
- Channel loading – Heavily loaded channels favor RDIMMs.
- Reliability requirements – RDIMMs offer more protection.
- Budget – UDIMMs costs far less for a given memory configuration.
- Supported module types – Motherboard dictates memory support.
Matching your workload profile and server requirements against UDIMM and RDIMM technical tradeoff helps guide the right memory selection.
Conclusion
UDIMM and RDIMM present two distinct memory module architectures optimized for different use cases. UDIMMs minimize cost while RDIMMs maximize performance and reliability under heavy configurations. Selecting the best DIMM comes down to the memory capacity, channel scalability, data integrity, and budgetary needs for the server or system under consideration.