Articles: Memory

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Fast DDR2 SDRAM - only for overclockers?

The previous section may give you an impression that Core 2 Duo systems do not in fact need fast memory. Using higher-frequency memory modules makes the system costlier, yet doesn't lead to any significant performance increases. This is true, in part: memory faster than DDR2-533 can only provide a maximum of 5% performance growth in a majority of widespread applications. The problem is in the front-side bus which is only clocked at 266MHz as yet.

But it doesn't mean fast memory is completely useless for owners of Core 2 Duo systems. Although Intel has limited the frequency and bandwidth of the front-side bus, it's in our power to increase them without Intel's help. So, we'll be talking about overclocking now.

It has been proven in numerous experiments, also in our own labs, that Core 2 Duo processors are very overclocker-friendly. The frequency multiplier of such processors still being fixed at a certain value, you can only overclock them by increasing the FSB frequency. Given that the Conroe core has a huge overclocking potential and that the default multiplier of Core 2 Duo processors is usually small, you often have to lift the FSB frequency up very high at overclocking. This increases the bandwidth of the CPU-chipset thoroughfare which makes the use of fast DDR2 SDRAM more reasonable than in a non-overclocked system.

To check this out in practice we benchmarked memory subsystem performance using an overclocked Core 2 Duo and memory modules with different frequencies and latencies. Here, we didn't set it our goal to find the highest frequency Conroe-core CPUs would be capable of working at. We only wanted to see what effect on the overall system performance the memory subsystem parameters would have if the FSB was overclocked. We set the FSB frequency at a rather typical value of 400MHz (50% above the default). At that frequency the FSB bandwidth grows from 8.5GB/s to 12.8GB/s. In theory, this should make the use of dual-channel DDR2-800 worthwhile.

For our overclocking tests we took a Core 2 Duo E6300 processor with a default frequency of 1.86GHz and 7x multiplier. At a 400MHz FSB, the CPU clock rate is 2.8GHz which is easily conquerable by such processors. 

The rest of testbed components are the same as were used in the previous section of the review.

The ASUS P5W DH Deluxe mainboard has certain peculiarities as concerns support for memory at overclocking. That is, many memory frequency divisors do not work when the FSB is overclocked. Until recently we could use that mainboard at a 400MHz FSB only to test memory at either 600MHz or 800MHz (FSB:DRAM divisors of 4:3 and 1:1, respectively). Fortunately, ASUS' engineers are still working upon the mainboard and the version 1305 BIOS is stable at 1000MHz memory frequency, too (with a 4:5 divisor at 400MHz FSB). So, we can now perform a test that includes memory modes not only with step-up divisors (as at the default CPU clock rate) but also with step-down divisors.

As in the previous section, we will first run synthetic benchmarks. 

The measurements of the CPU-memory bandwidth agree with the results we've got in the previous section. When the memory frequency is higher than 800MHz, the bandwidth indeed stops to grow further which coincides with the maximum bandwidth of the FSB at 400MHz. You can see that the effective bandwidth growth is about 15% on switching from 600MHz to 800MHz memory frequency, but a mere 2% on switching from 800MHz to 1000MHz. 

The memory latency tests confirm the point: the latency almost stops to decrease after an 800MHz memory frequency. So, it is clear that DDR2-1000 SDRAM cannot bring about a significant performance gain in comparison with DDR2-800 SDRAM on Core 2 Duo platforms even when the FSB is overclocked to 400MHz. However, the increase of the memory frequency to 800MHz should be profitable; at least we've got all the necessary conditions in terms of bandwidth and latency. 

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