Searching for the Memory Holy Grail - Part 2

Memory Configuration (continued)

Our benchmarks in Part 1 were consistent with the configuration recommendations in the Intel White Papers for Memory Configuration of the 875 and 865 chipsets. We concluded with the following performance charts based on our testing and the information provided by Intel. We differ from the Intel White Paper charts only in the first and second positions for the Intel 865 chipset. When Intel published their White Papers, we doubt that they had any notion that most of the 865 motherboards being sold would have some version of PAT (Performance Acceleration Technology) on board. Our testing of the ASUS, AOpen, and DFI PAT-enabled boards shows that the PAT 865 boards behave more like 875 boards. We have confirmed that the first and second positions on PAT-enabled 865 boards are, as we report, below — with four DS DIMMs performing faster than two DS DIMMs.

DDR400 (1:1) Performance	DIMM Configuration	Single-Channel or Dual-Channel
1	4 DS	Dual Channel
2	2 DS or 4 SS	Dual Channel
3	2 SS	Dual Channel
4	4 SS/DS Mixed Matched Pairs	Dual Channel
5	Any DS	Single Channel
6	Any SS	Single Channel

DDR333/266 Performance	DIMM Configuration	Single-Channel or Dual-Channel
1	2 DS or 4 SS	Dual Channel
2	2 SS	Dual Channel
3	4 DS	Dual Channel
4	4 SS/DS Mixed Matched Pairs	Dual Channel
5	Any 1 or 2 DS or SS (1 DIMM or 1 in each Channel)	Single Channel
6	Any 1 SS	Single Channel

While SiSoft Sandra UNBuffered Memory Test results demonstrated real differences in performance among these memory configurations, many of you asked what we might see in real-world performance differences with these different memory configurations.

Some of the configurations were easier to test than others. Kingston had provided us with four Single-Bank (single-sided) DDR500 DIMMs that allowed us to look at performance differences in two SS DIMMs versus four SS DIMMs. We looked at performance of 1000FSB/DDR500 at 3-4-4-7-1 memory timings at 2.65V.

1000FSB (3.0GHz)/DDR500 Performance — Kingston PC4000 2 x 256 MB SS DIMMs vs. 4 x 256 MB SS DIMMs
Number of Single-Bank DIMMs	Quake3 fps	Sandra UNBuffered	Sandra Standard Buffered	Super PI 2M places (time in sec)
2	386.30	INT 2821 FLT 2786	INT 5830 FLT 5748	109
4	394.57	INT 3218 FLT 3195	INT 5841 FLT 5818	107

While the differences in Quake3 frame rate and Super PI times are small, keep in mind that the only change here is using four SS DIMMs instead of two SS DIMMs. As predicted by Sandra UNBuffered Memory Test, the four SS DIMMs perform better than two SS DIMMs.

Comparing two SS DIMMs to two DS DIMMs was done using 2 x 256 MB SS OCZ PC4000 and 2 x 512 MB DS PC4000 at timings of 2.5-3-4-6 at 2.65V. It is almost impossible to keep DIMM capacity the same, which would be the most accurate test, as none of the samples using the same memory chips would have the same exact capacity in SS and DS configurations.

1000FSB (3.0GHz)/DDR500 Performance — OCZ PC4000 2 x 256 MB SS DIMMs vs. 2 x 512 MB DS DIMMs
Number of DIMMs & Configuration	Quake3 fps	Sandra UNBuffered	Sandra Standard Buffered	Super PI 2M places (time in sec)
2 SS	392.30	INT 2918 FLT 2926	INT 5761 FLT 5868	109
2 DS	400.10	INT 3282 FLT 3324	INT 5965 FLT 5934	106

Again, the differences are small but real in Quake3 and Super PI, but with 2 DS DIMMs performing better as we had demonstrated in Part 1 with the Sandra UNBuffered Memory Test.

The fastest configuration should be 4 Double-Bank DIMMs on the Intel 875 chipset motherboard. Our first efforts to test this configuration, with 4 x 256 DS OCZ 3700 GOLD modules did not show 4 DS DIMMs any faster than 2 DS DIMMs on the ASUS P4C800-E. This was a real puzzle considering that other predicted configurations were easily verified with Quake3, Super PI and other benchmarks. When we looked deeper, the problem became clear. In every configuration that we had tested with the ASUS P4C800-E, the Performance Mode showed “enabled” but with 4 DS DIMMs at any FSB speed faster than 800, CPU-Z 1.18C shows Performance Mode “disabled”. Our trial was failing here because we were comparing 2 DS DIMMs with PAT on to 4 DS DIMMs with PAT off. We have asked ASUS for the reason behind why PAT appears to be disabled at 4 DS DIMMs above 800FSB.

Since we were mainly concerned with comparing 2 DS with 4 DS DIMMs under the same conditions, we ran 2 DS vs. 4 DS tests at 800FSB/DDR400, so that PAT was enabled in both two and four DIMM setups. We compared prototypes of high-speed PC3700 memory that we tested, looking specifically at the performance of 2 x 512 MB DS to 4 x 512 MB DS at DDR400. Timings were 2-2-4-8 at 2.75V.

800FSB (2.4GHz)/DDR400 Performance 2 x 512 DS vs. 4 x 512 DS
Number of DS DIMMs	Quake3 fps	UT2003 Flyby fps	UT2003 Botmatch fps	Sandra UNBuffered	Sandra Standard Buffered	Super PI 2M places (time in sec)
2	321.4	196.30	68.31	INT 2683 FLT 2722	INT 4704 FLT 4691	133
4	324.5	197.60	69.38	INT 2830 FLT 2923	INT 4678 FLT 4717	132

While the differences here are quite small, they still show 4 DS modules performing better than 2 DS modules. We suspect that we would see larger differences at higher speed, as we have in our other comparisons.

We have confirmed with game benchmarks and a number-crunching benchmark that the best 875/865 memory configurations can also be seen in real-world benchmarks. Because memory performance is only one component of game performance and overall system bandwidth, the effect on these benchmarks is, as expected, smaller than the variation seen in benchmarks that measure only memory performance, like Sandra UNBuffered Memory Test.