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Parity and ECC DIMMs

The ninth DRAM chip on one side of a DIMM is used to store parity

or ECC bits. With parity, the memory controller is capable of

detecting single-bit errors, but it is unable to correct any errors.

Also, it cannot consistently detect multiple-bit errors. With ECC, the

memory controller is capable of detecting and correcting single bit

errors and multiple-bit errors that are contiguous. Multiple-bit

contiguous errors occur when an entire x4 or x8 chip fails. The

chipset (memory controller) is also capable of detecting double-bit

errors that are not contiguous. The chipset halts the system and

logs an error when uncorrectable errors are detected. Servers use

ECC DIMMs to improve availability and reliability.

Memory ranks are not new, but their role has become more critical with the advent of new chipset

and memory technologies and growing server memory capacities. Dual-rank DIMMs improve memory

density by placing the components of two single-rank DIMMs in the space of one module. The chipset

considers each rank as an electrical load on the memory bus. At slower bus speeds, the number of

loads does not adversely affect bus signal integrity. However, for faster memory technologies such as

DDR2-667, there are a maximum number of ranks that the chipset can drive. For example, if a

memory bus on a server has four DIMM slots, the chipset may only be capable of supporting two

dual-rank DIMMs or four single rank DIMMs. If two dual-rank DIMMs are installed then, the last two

slots must not be populated. To compensate for the reduction in the number of DIMM slots on a bus at

higher speeds, modern chipsets employ multiple memory buses.

If the total number of ranks in the populated DIMM slots exceeds the maximum number of loads the

chipset can support, the server may not boot properly or it may not operate reliably. Some systems

check the memory configuration while booting to detect invalid memory bus loading. When an

invalid memory configuration is detected, the system stops the boot process, thus avoiding unreliable

operation.

To prevent this and other memory-related problems, customers should only use HP-certified DIMMs

available in the memory option kits for each ProLiant server (see the "Importance of using HP-certified

memory modules in ProLiant servers" section).

Another important difference between single-rank and dual-rank DIMMs is cost. Typically, memory

costs increase with DRAM density. For example, the cost of an advanced, high-density DRAM chip is

typically much higher (more than 2x) than that of a conventional DRAM chip. Because large capacity

single-rank DIMMs are manufactured with higher-density DRAM chips, they typically cost more than

dual-rank DIMMs of comparable capacity.

Rank interleaving

As described previously, bank interleaving allows the processor to initiate an access to one memory

bank before the previous access to a different bank has been completed, resulting in continuous data

flow. Essentially, dual-rank and quad-rank DIMMs increase the number of "banks" available for

interleaving. Rank interleaving can result in improved system performance for memory-intensive

applications such as high-performance computing and video rendering.