Latency vs. Frequency: Choosing Optimal DDR2 SDRAM
An improved memory controller is one of the main defining traits of the i955X and i945P chipsets. The controller officially supports DDR2-667 SDRAM, but also brings new divisors for the memory frequency, two of which set up 667MHz frequency at 800MHz and 1066MHz FSB. Besides them, the new chipsets have acquired another important divisor, 1:1. Thanks to this divisor, i955X/i945P-based mainboards can support DDR2-800 SDRAM at 800MHz FSB and DDR2-1067 SDRAM at 1066MHz FSB. The problem is what’s better for a Pentium 4 system, a high memory frequency or low memory timings? As you have seen above, modern DDR2 modules can work as DDR2-667 SDRAM at aggressive 3-2-2-8 timings or easily reach frequencies like 1GHz at 5-5-5-15.
To answer this question we performed a series of tests using DDR2 memory with different frequencies and timings on the same platform. The platform was configured like follows:
- Processor: Intel Pentium 4 670 (3.8GHz, 2 MB L2);
- Mainboard: ASUS P5WD2 Premium (LGA775, Intel 955X);
- Memory modules:
- Corsair TWIN2X1024A-5400UL;
- Mushkin XP2-6400 DDR2;
- OCZ DDR2 PC2-8000 Platinum Enhanced Latency Dual Channel;
- Graphics card: PowerColor RADEON X800 XT (PCI Express x16);
- Hard disk drive: Maxtor MaXLine III 250GB (Serial ATA-150);
- Operating system: Microsoft Windows XP Professional SP2.
The processor we use is an engineering sample so we can freely change its frequency multiplier from 14x to 19x. Thus, we have an opportunity to compare systems that differ not only in their memory subsystems but also in their FSB frequency.
First we set the FSB frequency to 1067MHz. This frequency is often used not only on computers with a Pentium 4 Extreme Edition, but also in overclocked systems based on junior Pentium 4 models. The CPU clock rate was set as 14 x 266MHz = 3724MHz.
In this case the i955/i945 chipsets allow using DDR2-533, 667, 800 or 1067 SDRAM.
Next we set the FSB frequency to near 800MHz. Since we were going to compare the results of the tests, we decided to use an 18x multiplier and raise the FSB frequency a little above the standard value, so that the resulting frequency of the CPU was close to 3.73GHz. Thus, the CPU clock rate was set as 18 x 207MHz = 3726MHz.
When using 800MHz FSB frequency, the i955/i945 chipsets allow setting the memory mode as DDR2-533, 667 or 800 SDRAM. But since we had increased the FSB frequency above the standard, the system clocked the memory as DDR2-552/690/828 SDRAM.
There’s another thing concerning the conditions of our tests that we should mention. As we showed in the previous section, even the most advanced memory cannot boast absolute stability at 1066MHz. Still we attempted to run our tests at this frequency, too, and we were successful with Corsair’s TWIN2X1024A-5400UL sticks. After increasing their voltage to 2.3V and providing additional cooling, we performed our typical tests not only at 5-5-5-15 but also at 5-3-3-15 timings.
Yet we do not claim the system was absolutely stable at such settings, even though it passed our tests. The results of DDR2-1067 are not currently of much practical use since you need to take special measures like increasing the voltage and providing more cooling to assemble a stable computer with a memory subsystem like that. On the other hand, technologies improving, we will surely see mass-produced overclocker-friendly DDR2-1067 SDRAM modules soon.