Searching for the Memory Holy Grail: Part 1
by Wesley Fink on July 27, 2003 11:13 PM EST- Posted in
- Memory
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.
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Anonymous User - Tuesday, July 29, 2003 - link
Wesley Fink - Monday, July 28, 2003 - link
Yes, you read it correctly. the mixed dimms can vary from very large drops - 20 to 27% or so - to very little if any drop for closely matched pairs. Performance seems to fare best when mixed pairs are the same capacities and the same "bank" configuration.ViRGE - Monday, July 28, 2003 - link
Very good article, Wesley, but I'm a bit confused on your mixed vs. matched DIMM comparison. On the 1st configuration, 2x256DS + 2x512DS, the timings look to be the same as in the 1st table on page 4, the DDR 400 numbers. Am I reading this correctly? It seems odd(although believable) that memory bandwidth dropped 25%, even though it stayed as 4 DS DIMMs at the same timings.PrometheusN - Monday, July 28, 2003 - link
Thanks to a reader for pointing out one correction to this article. The Intel White Paper from the 865 chart reverses 1 and 2 memory positions compared to the 875 white paper. While I did not test 865 performance in the review, I did make reference to the White Paper Chart.We can also wonder, with Intel not sanctioning PAT on the 865, exactly how the 865 boards WITH PAT will behave, and if memory on an 865 with PAT changes back to the same as 875.
Anonymous User - Monday, July 28, 2003 - link
You did not close your table on page eight.3 4 DS/td> Dual Channel
Add a < to the "/td>" ;)
Otherwise... Great article, very informing, thanks for taking the time to write this up.
PrometheusN - Monday, July 28, 2003 - link
Single-Sided or Double-Sided IS a functional description - but it normally is also a visual description. BEWARE dimms that skip every other chips on both sides. These have 4 chips on each side, but are functionally Single-Sided.PrometheusN - Monday, July 28, 2003 - link
According to CPU-Z 1.18C, 4 Sticks of ram - single or double - do NOT disable "PAT" on the DFI 875PRO or the Asus P4C800-E. It depends on how the motherboard BIOS handles the 4 Dimms.Wesley Fink
Shalmanese - Monday, July 28, 2003 - link
Nice article but what are the real world consequences of these numbers? When DDR was 1st introduced, we only saw a ~10% peroformance increase from a 100% increase in memory bandwidth so I am thinking that the difference between 2 and 4 banks would be trivial.ghg - Monday, July 28, 2003 - link
Hi PromNice review, as we expected from you 8-).
Using 4 sticks of doublesided OCZ 3700 Gold disables PAT according to CPU-Z 1.18c.
Same behavior when taking 4 sticks of singlesided OCZ 4000 ?
Ciao
Gary
ghg - Monday, July 28, 2003 - link