I have a GA-P35-DS3R motherboard with a Q6600 CPU. I'm using two pairs of DIMMs, G.SKILL F2-6400CL5D-4GBPQ 2*2GB, and OCZ2G8002GK 2*1GB. Both are DDR2-800 5-5-5-15, and the only secondary timing difference I see is 52 tRFC on the 2*2GB vs. 42 tRFC on the 2*1GB. I'm using dual channel mode, "interleaved" according to the BIOS, "symmetric" according to CPU-z and "128-bit Mode" according to Memtest86+.
With either the 2*1GB or 2*2GB kits by themselves, Memtest86+ reports 3846 MB/s memory bandwidth. If I put in both, I get 2727 MB/s. If I put in only 3 DIMMs, which uses "flex memory mode" according to the BIOS, I also get 2727 MB/s. The frequency, timings, and secondary timings are exactly the same using the 2*2GB alone and using both sets.
If I instead run the various Passmark memory and CPU benchmarks, there may be a slight drop, but it's 1% or less. Passmark runs a lot of different tests, and I also don't notice a performance decrease in regular use. The Memtest86+ results seem to be the result of some real effect with a specific workload, but I guess that kind of workload is very rare, even in memory benchmarks.
(BTW. I fixed my
earlier problem by getting this new 2*2GB kit. One DIMM in the previous set seems defective due to not working in the same channel with any other DIMM. The other seems fine and works with any other DIMM. I'm not using either of those old DIMMs, so that shouldn't be relevant to this issue.)
