Searching for the Memory Holy Grail: Part 1

Mixed Configurations

In general, Intel’s White Papers assume that you will be using two or four matched DIMMs. The reality for many users, however, is that while they may have a pair of matched DIMMs, they do not likely have four matched DIMMs at their disposal to use in their new 865/875 boards. Intel’s White Papers address mixed memory configurations only to say that they will work, but they will default to the slowest speed and SPD timings of the mixed DIMMs.

To get a better idea of what happens to performance when mixing DIMMs at default performance at DDR400/800FSB, refer to the statistics below.

DIMM Configuration	Best Memory Timing	UNBuffered Sandra 2003 Memory Test (MB/Second)	% Changed from Matched DIMM Performance
2x256MB DS + 2x512MB DS	2-7-3-3	2094 INT 2148 FLT	-25%
2x256MB DS + 2x256MB SS	2.5-7-3-3	2064 INT 2132 FLT	-22%
2x512MB DS + 2x256MB SS	2.5-7-3-3	2097 INT 2150 FLT	-23%
4x256MB Matched DS	2-7-3-3	2861 INT 2848 FLT	----

The performance declines significantly from mixing different pairs of memory, even with two pairs of matched dual-channel. A 22% to 25% drop in memory performance compared to four matched double-sided DIMMs is certainly nothing to sneeze at.

Undoubtedly, the memory modules themselves are having an impact on this drop in performance. However, the four matched DIMMs run at 2-7-2-2, which is certainly not the best score I have seen at DDR400. In fact, the 2x512MB DS pair used in the mixed tests run as a dual-channel pair at DDR400 with 2-5-2-2 timings. The SS pair of 256MB DIMMs do require CAS 2.5 for best performance, and run fine as a pair at 2.5-6-3-2 at DDR400. Yet, these slower SS DIMMs perform better mixed with the DS GOLD modules than the faster DIMMs.

There will be great variation in mixed DIMM performance, with some close DIMMs running much better than these results. However, you can clearly see from the results that the very best performance at 1:1 requires four matched DIMMs. The performance loss from running two different matched pairs of DIMMs can be enormous.