Memory Configuration in the new HP G6

I had a conference call with our guys from HP this past week about the optimal memory configuration for the new HP G6 servers.  The architecture of the G6 is much different from the G5 which really changes the way memory is configured in the new servers.

In the G5 servers everything was configured in pairs and shared between processors if you had more than one.  So if you wanted 32GB or RAM in your servers you would have 8 x 4GB DIMMs.  The new G6 is configured in sets of 3 DIMMs and memory is dedicated per processor if you have more than one.  In the case of an VMware ESX server we would usually recommend a DL380 with 32GB of RAM.  For a G5 the memory would be configured in the 8 x 4GB DIMM configuration mentioned above.  Now with the new G6 architecture you could still use this configuration but the performance would be significantly affected.  The optimal configuration would be:

 Processor 1:    3 x 4GB DIMMs + 3 x 2GB (6 memory slots total) = 18GB RAM

Processor 2:    3 x 4GB DIMMs + 3 x 2GB (6 memory slots total) = 18GB RAM

Total RAM = 36GB

The new DDR3 Memory chips come in two flavors, Unregistered (U) and Registered (R), the configuration above uses registered memory or RDIMMs.  See details under “DDR-3 memory technology” below.

 Here is some more detailed info that I pulled from the HP website.

 Integrated Memory Controller

One of the biggest improvements in Intel Xeon 5500 series processors is the integrated memory controller. The memory controller uses three channels (up to 1333-MHz each) to access dedicated DDR-3 memory sockets. This delivers a big performance improvement over previous architectures that provide only two memory channels and require processors to share a single pool of system memory. The three memory channels have a total bandwidth of 32 GB/s.

 Each Intel Xeon 5500 series processor has a three-level cache hierarchy that consists of an on-core 64-KB L1 cache, a separate 256-KB L2 cache for each core, and a new inclusive, shared L3 cache of up to 8 MB. The L3 cache duplicates the data stored in the L1 and L2 caches of each core. This data duplication eliminates unnecessary searches, or snoops, to those caches and minimizes latency. Additional data tracking technology in the L3 cache ensures inter-core cache coherency. If one processor needs to access the cache or DDR-3 memory of the other processor, it uses the high-speed QPI between the two processors.

 DDR-3 memory technology

ProLiant G6 servers based on the Intel Xeon 5500 series processor support DDR-3 memory technology–DDR3-800, DDR3-1066, or DDR3-1333. DDR-3 dual in-line memory modules (DIMMs) provide the same reliability, availability, and serviceability as DDR-2 DIMMs; however, DDR3 DIMMs use less power, have lower latency, and deliver higher bandwidth. DDR-3 DIMMs operate at 1.5V, compared to 1.8V for DDR-2 DIMMs. This translates into more than 25% in power savings comparing the fastest DDR-2 DIMM (DDR2-800) to the slowest DDR-3 DIMM (DDR3-800). The power savings increase to almost 35% comparing the most commonly used DIMMs, DDR2-667 and DDR3-1066. It‘s important to note that there are two types of DDR-3 DIMMs—registered (RDIMMs) and unbuffered (UDIMMs)—and they cannot be used together in a system. ProLiant G6 servers support up to three RDIMMs per channel or up to two UDIMMs per channel. RDIMMs have larger capacity (up to 8 GB each) than UDIMMs (up to 2 GB each). Higher-end ProLiant G6 servers support up to 18 sockets. In these servers, RDIMMs enable total memory capacity of up to 144 GB, compared to 24 GB for UDIMMs. This makes RDIMMs the ideal choice for virtualization, while UDIMMs provide cost and power savings for less memory-intensive applications.

 The memory channels can operate at up to 1333 MHz, but the actual speed depends on the number and type of DIMMs populating the slots. For example, in a fully-populated system using DDR3-1333 5 DIMMs, the memory bus speed drops to 800 MHz to maintain signal integrity. Therefore, the type of workload dictates the optimum number and type of DIMMs to use. Memory capacity may be of primary importance in virtualization environments, while memory channel speed may be more critical for high performance computing applications.

Because there are several memory options, HP simplifies memory selection by providing two helpful resources. First, the on-line ProLiant Memory Configuration Tool ( will walk you through the steps to configure your server‘s memory and provide an orderable parts list. Second, the DDR-3 Memory for Dummies booklet provides information and tips about populating the system memory of ProLiant G6 servers.

 Here are some links that may help if you want more details:

 Proliant G6 Technology Overview (whitepaper)

 Memory Technology Evolution: An Overview of System Memory Technologies (whitepaper)

HP ProLiant G6: A Backstage Tour (24 minute video)


One Response to “Memory Configuration in the new HP G6”

  1. Why the Nehalem CPUs Are Hot (No, that’s not a pun) « Jason Nash’s Blog Says:

    […] of people have seen some of the differences.  For example, my counterpart Brian recently wrote a post on the new way memory is configured and accessed.  But what else is new and different?  Is this a […]

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