Introduction to DDR3

Recent CPU technology has adopted the use of high speed memory module, which makes server configuration a bit tricky. I often had to figure out what kind of memory modules to use on my servers, be it a Database Server, an High Performance Computing (HPC) node or a Virtualization platform.

I hope the next series of posts will help shed some light on this matter and assist building the optimized platform for your application.

First we must understand the basics of memory technology that is currently used.  

JEDEC - Joint Electronic Device Engineering Council, is the group that set memory industry standards ,aspects such as features, functionality, AC and DC parameters, data interfaces and more.

The DDR3 SDRAM is the latest one used for servers. It is a high-speed dynamic random-access memory internally configured as an eight-bank DRAM. The DDR3 SDRAM uses a 8n prefetch architecture to achieve high-speed operation. The 8n prefetch architecture is combined with an interface designed to transfer two data words per clock cycle at the I/O pins.
Theoretically, these modules could transfer data at the effective clockrate of 800-1600MHz (for a single clock bandwidth of 400-800MHz), producing a throughput of 12800 MB/s.

Chart 1: SDRAM technologies throughput

Compatibility

DDR3 DIMMS have 240 pins, the same number as DDR2; however, since the supply voltage is different DDR3 uses 1.5V where DDR2 uses 1.8V they are incompatible. To identify between them, the DIMMS are also physically incompatible. The key notch location is different. so DO NOT try forcibly to install them. 


 DDR3 Advantages

• Higher Performance – The most important benefit is the ability to transfer 8bits of data in 1 clock cycle, which is twice than DDR2.  Higher throughput equals higher performance.
• Lower Power – DDR3 memory operate at 1.5V supply, which is 17% less than what DDR2 uses. This low power results in improving the thermal design using 30% less power.
• Larger Density – Since the DDR3 allows higher chip capacity (1GB<) it allows a maximum memory modules of 16GB.
• Data Integrity – An integrated thermal sensor refreshes the data in the device and ensures data integrity and data drainage during transfers
• Higher Speed – Twice the speed of DDR2, it has the ability to transfer I/O data eight times the speed of the memory cell it contains.

DDR3 Standard Modules

Module Name	Standard Name	Memory Clock	I/O Bus Clock	Data Rate	Peak Transfer Rate
PC3-6400	DDR3-800	100 MHz	400 MHz	800 MT/s	6400 MB/s
PC3-8500	DDR3-1066	133 MHz	533 MHz	1066 MT/s	8533 MB/s
PC3-10600	DDR3-1333	166 MHz	667 MHz	1333 MT/s	10667 MB/s
PC3-12800	DDR3-1600	200 MHz	800 MHz	1600 MT/s	12800 MB/s

Table 1: DDR3 standard memory modules

Note that Standard’s name correlates to the clockrate, not the Memory Clock or the Bus Clock, i.e DDR3-1600 offers the clockrate of 1600MHz.

Since memory configuration and capacity is effecting server performance, it is very important to understand which modules to use in which configuration. For example, populating all memory banks with the fastest modules (DDR3-1600) will cause the system to work as if DDR3-800 where used. in the next posts I’ll try to explain why.