FB-DIMM DDR2 support appeared in Skulltrail platform for a reason. This is a heavy burden inherited from a server i5400B Express core logic set it is based on. But although FB-DIMM slots look quite justified in servers and workstations due to large memory capacities involved, in enthusiasts systems they seem kind of out of place.
Nevertheless, Intel offered us a pair of 2GB FB-DIMM memory modules from Kingston for our Skulltrail tests. These modules worked at 800MHz with 5-5-5-15 timings.
This is the fastest FB-DIMM available today, besides, it is nearly impossible to find in retail. The modules available in the market freely these days usually work at 667MHz only.
However, we decided to give up this memory, because our experiments revealed that quad-channel mode is more efficient than dual-channel mode, even if the memory is working at a lower frequency. Therefore, we performed the tests with four widely spread 667MHz 1GB FB-DIMM modules from Micron.
The modules SPD reports 5-5-5-11 timings:
However, they work just fine with even more aggressive settings, which we have certainly taken advantage of.
As you know, the main peculiarity of fully-buffered DDR2 DIMM modules is the use of serial bus for data transfer between the DDR2 SDRAM chips and memory controller instead of parallel bus. That is why they are completely incompatible with regular unbuffered and registered DDR2 modules. Each FB-DIMM module contains DDR2 SDRAM chips connected to the serial bus via the additional AMB (Advanced Memory Buffer) chip installed onto each module.
There is also another side to this approach. Memory subsystem built from FB-DIMM modules has very high latency that increases because there is an additional AMB chip in the data’s way from the memory chips to the CPU.
For example, here are the results taken from quad-channel memory subsystem built with DDR2-667 FB SDRAM modules with 4-4-4-15 timings:
You can immediately see the bottleneck of the Skulltrail platform: low read speed and high latency. Regular single-CPU desktop systems with DDR2 SDRAM (and especially DDR3 SDRAM) can boast much higher read speed and considerably lower latency.
Moreover, I would also like to say that AMB is a pretty complex device that consumes a lot of power and heats up very noticeably. That is why all FB-DIMM modules as a rule consume about 3-6W each and must be covered with heat-spreaders. During our Skulltrail platform tests we detected memory temperatures of over 70ºC.
No wonder that Intel recommends installing additional fans into Skulltrail systems specifically for memory cooling.
All in all, we can conclude that if Intel continues developing dual-socket systems for computer enthusiasts, their primary focus will be on improving the memory subsystem, no doubt.