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Test 2: Overclocking

The results above can hardly be considered a motive for using high-speed memory in LGA1156 systems with dual-core processors. However, even this way of using high-speed memory, namely without increasing the base clock, is only available to select users, who are lucky to have a K-series CPU with an unlocked frequency multiplier. Most systems based on Clarkdale processors have their CPU overclocked by raising the BCLK frequency, when there is faster memory with 1333MHz+ speed. This is exactly why we decided to devote the second part of our test session to studying the effects of the memory subsystem settings on the performance of a system overclocked in this particular manner.

I have to point out right away that when you overclock your processor by raising the base clock, all busses in your system will be working at higher speeds. Besides the processor clock frequency, the speed of the QPI bus connecting the processor dies inside also increases. As a result, we expect to see more obvious connection between the performance and memory subsystem speed.

In order to make the testing conditions as realistic as possible, we overclocked our Core i5-655K CPU to 4.4 GHz. This frequency was obtained with 22x multiplier and 200 MHz BCLK clock. All dual-core Clarkdale processors support this multiplier, so the results of this test can be easily applied to majority of systems out there.

During the tests we disabled Turbo Mode, which changes the processor multiplier dynamically, because only in this case overclocking could be most fruitful.

The increase in the base clock frequency caused the available memory frequencies set with individual multipliers to change. Instead of DDR3-800, DDR3-1067 and DDR3-1333 the CPU automatically got support for DDR3-1200, DDR3-1600 and DDR3-2000 respectively. All these three modes are available not only in systems using a CPU with unlocked multiplier: they can be enabled with any Clarkdale processors. This is exactly why we used this particular BCLK frequency of 200 MHz, as this trick lets you have DDR3-2000 mode on any LGA1156 CPU.

As usual, synthetic benchmarks come first:

As we have expected, the memory frequency have much more influence over the memory subsystem practical bandwidth when the CPU is overclocked. While in nominal mode 40% increase in memory frequency boosted reading from the memory by only 7%, now the 66% higher DDR3 SDRAM frequency produces over 21% boost in memory subsystem practical bandwidth. In other words, overclocking of the internal processor QPI bus does have a positive effect on the memory subsystem performance.

There is only one catch: the results in the system with DDR3-1600 SDRAM are very low. However, there is no mistake here: the Clarkdale memory controller simply prepared another unexpected surprise for us. It is peculiar of the 8x memory frequency multiplier (which in nominal mode allows clocking the memory as DDR3-1067) to lower the practical memory subsystem bandwidth.

We see no anomalies in terms of latencies. More aggressive timings as well as increase in the memory frequency always cause adequate lowering of the practical latency.

Now let’s see how this diverse functioning of the memory controller affects the performance in other benchmarks and applications:

DDR3-1600 mode selected with the BCLK increased to 200 MHz is really far not the most optimal choice. According to the results, DDR3-1200 with lower timings almost always produces better outcome. However, we have no complaints about DDR3-2000. High-speed memory with this frequency does have a highly positive effect on overall performance during overclocking. On average, using DDR3-2000 in an overclocked system with a dual-core LGA1156 processor can improve performance by about 5%, and in applications that are sensitive to memory subsystem parameters (such as games) the performance may increase as far as by 10%.

 
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